Julie Gullstrand

Nicolas Claidière

Joël Fagot
email: joel.fagot@univ-amu.fr

Lauriane Faraut

Aurélien Frick

Manon Genet

Marion Serre

Computerized assessment of dominance hierarchy in baboons (Papio papio)

Keywords: monkey, supplanting behaviours, dominance hierarchy, automatized testing, social cognition

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Agonistic behaviours are widely present in the animal kingdom. These behaviours usually occur between group members in a context of competition for limited resources (e.g., [START_REF] King | The ecology of aggressive behavior[END_REF] and are relevant to understand a variety of phenomena such as adaptive fitness (e.g., [START_REF] Neat | Proximate costs of fighting in male cichlid fish: the role of injuries and energy metabolism[END_REF] or dominance style (e.g., [START_REF] Flack | Dominance style, social power, and conflict management: a conceptual framework[END_REF]. The structure of dominance hierarchies and their mode of computation has been amply discussed in the literature [START_REF] Bang | Choosing an appropriate index to construct dominance hierarchies in animal societies: A comparison of three indices[END_REF][START_REF] Balasubramaniam | Consistency of dominance rank order: A comparison of David's scores with I&SI and Bayesian methods in macaques[END_REF][START_REF] Hemelrijk | The construction of dominance order: comparing performance of five methods using an individual-based model[END_REF]De Vries, 1998;De Vries, Stevens, & Vervaecke, 2006). However, this domain of research has witnessed very little evolution regarding the methodology used for data recording. Agonistic behaviours can differ depending on the dominance style of the considered species (e.g., [START_REF] Flack | Dominance style, social power, and conflict management: a conceptual framework[END_REF] and their outcome and frequency can both potentially be used to infer group dominance hierarchies. According to [START_REF] David | Ranking from unbalanced paired-comparison data[END_REF], in linear dominance hierarchies the most dominant individuals emit more agonistic behaviours than they receive, increasing their proportion of contests won. In such situations, dominance hierarchies can therefore be inferred from the difference in frequencies of agonistic behaviours emitted and received by each individual within the group [START_REF] Dugatkin | Winner and loser effects and the structure of dominance hierarchies[END_REF]. In animals with a less linear social organization, the analysis of dominance hierarchies is often more difficult to establish due to a smaller number of agonistic behaviours emitted and received, especially in species with more egalitarian styles (Watts, 1994). [START_REF] Fischer | Charting the neglected West: The social system of Guinea baboons[END_REF] for instance reported in the male-male tolerant Guinea baboons (Papio papio) that they were not able to establish a significant linear rank hierarchy because of the low number of agonistic interactions, and despite the fact that agonistic interactions were mostly unidirectional (see also Patzelt, 2013).

Another difficulty encountered in previous studies is that agonistic behaviours require serious efforts to be recorded. In most studies on dominance hierarchies, data are collected by effortful and time-consuming observations, either directly (Barton, Byrne & Whiten,1996) or based on video recordings [START_REF] Foris | Comprehensive analysis of affiliative and agonistic social networks in lactating dairy cattle groups[END_REF]. With the development of modern technologies, new methods have emerged for automatic identifications of animals (e.g., GPS tracking in baboons, Strandburg-Peshkin et al., 2015; bar code tracking in birds, [START_REF] Alarcón-Nieto | An automated barcode tracking system for behavioural studies in birds[END_REF]deep learning in birds, : Ferreira et al., 2019), but the use of automated identification methods remains limited in social studies on dominance hierarchy. In rare noticeable exceptions, authors inferred dominance hierarchies from the combination of video tracking with (1) RFID data (in mice: [START_REF] Weissbrod | Automated long-term tracking and social behavioural phenotyping of animal colonies within a semi-natural environment[END_REF], (2) identity data obtained from automated reading of ear-tags (in cows [START_REF] Foris | Automatic detection of feeding-and drinking-related agonistic behavior and dominance in dairy cows[END_REF], or (3) automated barcode or GPS reading (in pigeons: [START_REF] Nagy | Contextdependent hierarchies in pigeons[END_REF], but these methods remain time consuming because of video processing. In another study with olive baboons (Papio anubis) the author used supplanting behaviours to assess dominance hierarchy to overcome the lack of the other agonistic behaviours [START_REF] Packer | Male dominance and reproductive activity in Papio anubis[END_REF].

The main goal of the current research was to explore a new method to infer dominance hierarchies automatically. [START_REF] Fagot | Automated testing of cognitive performance in monkeys: Use of a battery of computerized test systems by a troop of semi-free ranging baboons[END_REF] have implemented novel test systems to study cognition in semi-free baboons. In this study, a colony of Guinea baboons has free access to a battery of ten ALDM (for Automated Learning Devices for Monkeys) test systems. The ALDM test systems are automatized operant conditioning systems equipped with touchscreen on which the cognitive tasks are presented, as well as a RFID tag reader for the identification of the subject. With this technology, each test system reads the identifying RFID microchip implanted in each arm of the baboon when it enters a workstation. This allows the self-paced presentation of the cognitive tasks to the members of the colony, without any social isolation or physical capture. [START_REF] Claidière | Using Automated Learning Devices for Monkeys (ALDM) to study social networks[END_REF] reported that the spatial use of these ten ALDM test systems provides information on the social structure of the baboon colony: the social network inferred from the co-presence of individuals within the spatially adjacent workstation correlated strongly and reliably with the social network inferred from the affiliative behaviours (e.g. grooming or greeting behaviours) observed inside the enclosure where the group of baboons lived. In this study, we assessed if the use of ALDM systems can also provide information on dominance hierarchy, therefore complementing our previous studies on affiliative network [START_REF] Claidière | Using Automated Learning Devices for Monkeys (ALDM) to study social networks[END_REF][START_REF] Gelardi | Detecting social (in) stability in primates from their temporal co-presence network[END_REF].

Following [START_REF] Packer | Male dominance and reproductive activity in Papio anubis[END_REF], we considered in our research that supplanting behaviours are interesting behavioural markers of the dominance hierarchy. A supplanting behaviour can be defined as a sequence of behaviour in which one animal A approaches another individual B who then leaves. Supplanting behaviours have the advantage over many other agonistic behaviours to have a clear winner (individual A). According to [START_REF] Johnson | Supplanting by olive baboons: dominance rank difference and resource value[END_REF], supplanting behaviours lead to the establishment and maintenance of dominance hierarchies in olive baboons (Papio anubis), and these behaviours are observed in this species regardless of food quality and rarity, between individuals closer in ranks, avoiding greater risks in escalation of more aggressive displays. To the best of our knowledge, our study is the first study in which dominance hierarchy was inferred from supplanting behaviours in Guinea baboons.

Given the proposed relation between supplanting behaviours and dominance hierarchy, our study investigated the dominance hierarchy in Guinea baboons considering supplanting behaviours within the ALDM test systems. In Experiment 1, we first assessed dominance hierarchy from conventional observations of agonistic behaviours expressed within the enclosure where the baboons lived. Secondly, we extracted hierarchy from ALDM dataset, considering supplanting behaviours within the testing workstations and compared the hierarchies obtained with the two methods. Using three different datasets, we found that the dominance hierarchy converged with the two methods. Thirdly, experiment 2 aimed at documenting the experimental potentials of inferring dominance hierarchy from automatically recorded ALDM supplanting behaviours. Supplanting behaviours were extracted from a previous data set already published in [START_REF] Goujon | Learning of spatial statistics in nonhuman primates: contextual cueing in baboons (Papio papio)[END_REF] and used as a factor for reanalysing the cognitive performance of the baboons in this study. We found that supplanting events lead to a transient improvement of cognitive performance for the baboon supplanting its partner, confirming the potential usefulness of inferring dominance hierarchies to study cognitive processes.

Experiment 1: Assessing dominance hierarchy from ALDM testing

Experiment 1 aimed at comparing dominance hierarchies inferred from the ALDM supplanting behaviours with those obtained from more naturalistic observations of the social behaviours spontaneously expressed in the baboon's enclosure.

Methods

Participants and living conditions

All participants belonged to the same group of Guinea baboons (Papio papio) from the CNRS primate Centre, Rousset-sur-Arc, France. The group consisted of males and females from different families living with their offspring in a large enclosure. The entire group was housed inside a 25 x 30 m 2 outdoor enclosure connected to an indoor area and to two experimental trailers providing access to computerized (ALDM) test systems (see below). The group had ad libitum access to water, and feeding was provided daily at 5 pm.

Individuals from this group were studied during three study periods, between 2013 and 2015. The size of the group varied among these periods, from 21 to 22 individuals, due to births or deaths from natural causes. The subjects retained for our research where those who were old enough to participate in computerized testing. Supplementary table 1 reports the name, sex, year of birth and participation of the individuals in each test period.

Ethics statements

This research was carried out in accordance with European Union and French ethical standards and received approval from the French Ministère de l'Education Nationale et de la Recherche (approval no. APAFIS-2717-2015111708173794-V3).

Behavioural observations in the enclosure

The baboons' spontaneous behaviour in their outdoor enclosure were recorded during three observation periods, which were approximately evenly interspersed within a 3-year time window (from 2013 to 2015, see supplementary table 1). These three periods will be referred to below as the main-, control-1 and control-2 study periods. The method used for behavioural recording during each period is described below.

Observational methods: main study period. Data were recorded during the main study period from the 21 st of September to the 21 st of October 2015. Behavioural observations in that period used a focal sampling method [START_REF] Altmann | Observational study of behaviour: Sampling methods[END_REF]. The coding scheme contained a long list of affiliative and agonistic behaviours but the current paper will only consider the following six behaviours which are all known to occur in agonistic contexts: "aggression", "chase", "threat", "supplanting behaviour", "avoidance" and "yakking", see supplementary table 2 for definitions). Every day of observation was composed of three sessions during which two experimenters observed the baboons. In every other observation session one experimenter was assigned the role of observing the baboons (in order to identify the behaviour performed, its producer and receiver) while the other simultaneously monitored timing and entered the data in the Behavioural Observation Research Interactive Software (BORIS) program (Friard & Gamba, 2006). The observer had to observe each baboon during five minutes in a predefined randomized order varying from sessions to sessions, for a total of 120.8 hours of observation.

We recorded a total of 934 agonistic events during this observation period.

Inter-observer reliability was assessed using two hours of video recordings of the baboons' behaviours. These videos contained sequences of five minutes of observation per subject and were recorded at the beginning (one hour) and end (one hour) of the observation periods. Each experimenter coded independently the behaviours observed in the video using the same protocol as defined above. The concordance between the two observers was coded using Kappa-Cohen's method [START_REF] Cohen | A Coefficient of Agreement for Nominal Scales[END_REF][START_REF] Landis | The Measurement of Observer Agreement for Categorical Data[END_REF]. Inter-observer agreement was high regarding the category of the behaviour (Ksession1=0.68; Ksession2=0.85), and the identity of the baboons (Ksession1=0.88; Ksession2=0.85).

Observational method: control periods 1 and 2. The database from control period-1 (from the 2 nd to the 29 th of July 2014) contained a total of 300 hours of behavioural observation which were recorded in the context of [START_REF] Claidière | Using Automated Learning Devices for Monkeys (ALDM) to study social networks[END_REF]. The data published in [START_REF] Claidière | Using Automated Learning Devices for Monkeys (ALDM) to study social networks[END_REF] only concerned affiliative behaviours, and therefore used a scanning procedure (Altman, 1974) adapted for that purpose. Information on the agonistic behaviours collected in this study has remained thus far unpublished. The dataset from control period-2 (from the 2 nd to the 16 th of May 2013) used a total of 180 hours of observation (unpublished data) previously collected for the same purpose and with the same method as in period-1. In control periods 1 and 2, the observers used the same coding scheme as in the main study period, the only difference being therefore that the data were collected with a scan instead of the focal sampling method. The datasets for control periods 1 and 2 contained a total of 600 and 351 agonistic events respectively, with known agonistic dyads. The difference in the number of agonistic events in periods 1 and 2 is explained by the difference in duration of these periods.

ALDM testing

In our laboratory, the baboons have a permanent access to ten Automated Learning Device for Monkeys (ALDM) systems, and this was also true during the three study periods of this research. The ALDM test systems are described in detail in [START_REF] Fagot | Automatic testing of cognitive performance in baboons maintained in social groups[END_REF] and [START_REF] Fagot | Automated testing of cognitive performance in monkeys: Use of a battery of computerized test systems by a troop of semi-free ranging baboons[END_REF]. In short, each ALDM workstation consisted of a 70 cm × 70 cm × 80 cm test chamber equipped with (1) a RFID microchip scanner aimed at reading subjects' identity from RFID microchips implanted in their arms; (2) a 19 inches' computer touch screen displaying the task, and (3) a food dispenser delivering a reward inside the test chamber when the subject responded correctly to the task. In practice, the baboons could freely enter and leave each experimental chamber via opened doors installed in the wire mesh of their enclosure.

When a baboon voluntarily entered an ALDM test system, it was identified via the forearms' microchip and this identification triggered the initiation of the test program. All ALDM trials were recorded with detailed information regarding the baboon and its performance, including the date and time of the trial, as well as the workstation used. This information was used to detect supplanting behaviours within the ALDM test systems, and to infer the dominance hierarchy from these behaviours (see below). In practice, the baboons received several different computerised cognitive tasks (e.g., Wisconsin task) of varying durations during the three study periods. However, for Experiment 1 we will consider that the type of cognitive test is unimportant for our current scientific purpose, because our primarily goal is to determine if we can use ALDM computerised data to determine the group hierarchy.

Assessing dominance hierarchy

To assess the dominance hierarchy from behaviours observed in the enclosure, we built three "winner/loser" files from the original data, one per study period. Whenever an agonistic behaviour was observed, the individuals displaying aggression, chase, threat, or supplanting behaviour was considered "winner" in each dyad, while the baboons receiving these behaviours were considered "losers". The coding was reversed for the avoidance and yakking behaviours, which correspond to social submission.

To assess dominance from the data collected in the ALDM test systems, we reasoned that supplanting behaviours in the ALDM test systems should be indicated by a very short interval between the last trial of the supplanted subject ("loser") and the first trial of the supplanting subject ("winner"). Based on this reasoning, we analysed the ALDM data in two steps. Firstly, we defined a "run" as a succession of trials by the same individual within the same workstation, and with inter-trial interval of less than 30 seconds (i.e. not exceeding twice the average delay between two consecutive trials). A run therefore represents a succession of trials corresponding to a single foraging bout. Secondly, to identify supplanting behaviours within the workstations (i.e. cases in which the foraging bout was disrupted), we computed the time interval between the last trial of one baboon within a given workstation and the first trial of the next individual in the same workstation. We considered that this behavioural sequence corresponded to a supplanting behaviour when this interval between two individuals was smaller than 30 seconds (i.e. the foraging bout had been disrupted). Using this procedure, we obtained an ALDM winner/loser files for each observation period, which had the same structure as the winner/loser file from the spontaneous behaviour in the enclosure. This procedure allowed the identification of 7844 cases of supplanting behaviours (for 22132 runs considered) during the main study period (in 2015), and 7700 (22769 runs) and 7479 (17634 runs) during the control periods 1 and 2 respectively. The Elo-rating method [START_REF] Elo | The rating of chessplayers, past and present[END_REF] was used to calculate dominance hierarchy from the sets of the winner/loser files described above. Interested readers are referred to [START_REF] Elo | The rating of chessplayers, past and present[END_REF] for a detailed description of this method. In short, each individual started with the same score with this method, and at each new contest the score of the winner increased by a certain amount while it decreased for the loser by the same amount. To calculate the dominance hierarchy, we used RandomElo function from the Elo-rating package [START_REF] Neumann | Assessing dominance hierarchies: Validation and advantages of progressive evaluation with Elorating[END_REF] that ran on the statistical environment R (R version 4.0.2 ; R Core Team, 2015), and followed [START_REF] Neumann | Elo-rating-a brief tutorial[END_REF]'s procedure, with starting value of 1000 and a constant k optimized using maximum log-likelihood (provided in supplemental table 4). For each inferred dominance hierarchy, through the 3 sampling periods and both types of data sets, we assessed triangle transitivity and linearity index using Elo-rating package viewable in supplemental table 4.

Finally, we ensured the repeatability of the hierarchies by computing repeatability scores using AniDom package (Sánchez-Tójar, Schroeder & Farine, 2018), using the same constant k optimized and 1000 randomisations.

Statistical analyses

Correlational analyses. A first way to validate our approach is to directly compare the correlation between the dominance hierarchies obtained through behavioural observations and through computerised testing. We used two different correlation tests on Elo-scores: Spearman and Kendall correlations. The non-parametric Spearman correlation test compares the relative order of dominance hierarchies (for example if A is above B in one hierarchy and B above A in the other) and is widely used in the literature [START_REF] Balasubramaniam | Consistency of dominance rank order: A comparison of David's scores with I&SI and Bayesian methods in macaques[END_REF][START_REF] Hunter | Social hierarchy and feeder access in a group of 20 sows using a computer-controlled feeder[END_REF]Williamson, Lee & Curley, 2006). The Kendall correlation test is less frequently used [START_REF] Bang | Choosing an appropriate index to construct dominance hierarchies in animal societies: A comparison of three indices[END_REF][START_REF] Hemelrijk | The construction of dominance order: comparing performance of five methods using an individual-based model[END_REF] but compares the strict value of the ranks of individuals in the hierarchy (for example if A is +3 ranks above B in one hierarchy and -1 rank in another) and therefore seems more precise. For the purpose of exhaustivity, we report below the results of both tests.

Modelling. A high correlation between the two hierarchies would show that both hierarchies are similar but would not inform us on the cause of this similarity. For instance, it is possible that dominant individuals are more frequently involved in cognitive testing than subordinate individuals, and that a higher number of comings and goings in the workstations increases the rank of individuals in the hierarchy calculated from ALDM testing. Exact permutation tests (Fisher, 1954) were used to assess the contribution of such variables to our measurement of dominance hierarchies. In an exact permutation test, we perform a large number of permutations of the variables that may not account for the observed correlation while maintaining the variable of interest constant, every time calculating the correlation between the hierarchy obtained from the randomly permutated computerised data and the one obtained from observed data. This generates a distribution of correlation coefficients obtained under the null hypothesis that the factors that have been permutated do not account for the correlation. If the observed correlation falls outside the 95% confidence interval of the distribution of simulated coefficients, the null hypothesis can be rejected, and we can conclude that the factors that have been permutated are important in explaining the correlation.

Two main parameters can influence the position of the individuals in the dominance hierarchy, they include the frequency of agonistic interactions (i.e. supplanting behaviours) each baboon has in the workstation (which could be linked to proportion of contest in traditional hierarchy assessment; [START_REF] David | Ranking from unbalanced paired-comparison data[END_REF] and the identity of the winner-loser dyads involved in each agonistic interaction. We therefore used two random models, one testing each variable. This modelling approach was applied to the analysis of the data collected during the main study period only, because this dataset was the largest.

Random model 1. In the first model, the identity of individuals between two recordings was randomly permutated before supplanting behaviours computation (we performed 50 000 permutations for 22132 runs for each of the 500 simulations). Each monkey maintained in random model 1 the same frequency of participation in cognitive tasks, but the permutation of the subjects' identity in the dataset altered the proportion of supplanting behaviours performed by each participant as well as the identity of the winner-loser dyads. If the dominance hierarchy obtained from the ALDM is determined mostly by the baboons' frequency of participation in experiments, then we expect the correlation obtained from the simulations to be similar to that obtained with the untransformed ALDM data.

Random model 2. The aim of the second model was to examine if the hierarchy can depend on the frequency of contests won, irrespective of the identity of the dyad involved in the supplanting behaviour. We performed 500 simulations in which we performed 50 000 permutations of the identity of the winners of two different supplanting behaviours (making sure the same individual did not appear both as winner and loser). Each individual therefore maintained the same number of contests won but with different losers. If the hierarchy is mostly determined by the number of contests won and not by the winner-loser identity, then we expect the correlation between the hierarchy obtained from the simulations to be similar to that obtained with untransformed data.

Results

Reliability and repeatability of the hierarchies

The hierarchies obtained for the three periods and two types of data are highly repeatable and reliable (Repeatability index were very high: rObs = 0.92 and rALDM = 0.96 for the main study period; rObs = 0.88 and rALDM = 0.97 for the Control period-1 and rObs = 0.94 and rALDM = 0.95 for the Control period-2). Summary table in the appendix present the sets of Elo-scores and ranks for each baboon (supplemental table 3), as well as the linearity and transitivity indices of the triangles for each hierarchy (supplemental table 4).

Main study period

The correlation between the hierarchies obtained with observational and computerised data is shown on the left-hand panel of Correlational analyses: all three study periods Figure 1 illustrates the correlations obtained between the observational and ALDM dominance hierarchies in the three study periods. As indicated above, that correlation was high and significant for the main study period (Figure 1, left panel). This was confirmed by the analysis of the two other study periods as shown in the middle and right panel of Figure 1 (control period-1: Kendall correlation: c1 = 0.76, CI = [0.57; 0.91], N = 22, p < 0.001 and Spearman correlation: rsc1 = 0.9, CI = [0.71; 0.97], N = 22, p < 0.001; control period-2: Kendall correlation:c2 = 0.76, CI = [0.57; 0.91], N = 21, p < 0.001 and Spearman correlation: rsc2 = 0.9, CI = [0.70; 0.98], N = 21, p < 0.001). On average, the absolute difference in ranking remained relatively low in both control periods, it was equal to 2.0 ranks (max = 6) and 2.1 ranks (max = 8) for the control periods 1 and 2, respectively (Individual Elo-scores and ranks are presented in supplementary table3).

Discussion of Experiment 1

Experiment 1 employed two methods to assess dominance hierarchy, conventional observations on the one hand and the analysis of supplanting behaviours during ALDM testing on the other hand. These two methods presented very similar rankings in all three datasets covering a period of 3 years, considering both Kendall and Spearman correlations and similarly high repeatability indices. Simulations further demonstrate that the hierarchies obtained with the ALDM method reflect the social dominance hierarchy, independently of variations in participation to cognitive testing. Both the frequency of supplanting behaviours and the identity of the individuals involved in the supplantation are crucial factors determining the dominance hierarchy. We believe that such results, at the group and individual levels, demonstrate that dominance hierarchy can be inferred reliably from the analysis of supplanting behaviours within the ALDM workstations Experiment 2: Effect of supplanting behaviours on cognitive performance

The main goal of Experiment 2 is to illustrate the possible use of ALDM hierarchies obtained from supplanting behaviours in the context of cognitive studies. [START_REF] Goujon | Learning of spatial statistics in nonhuman primates: contextual cueing in baboons (Papio papio)[END_REF] have previously published data on the cognitive performance of our group of baboons in the ALDM test system. This research used the so called "contextual cueing" task [START_REF] Chun | Contextual cueing: Implicit learning and memory of visual context guides spatial attention[END_REF], in which the baboons had to detect a target in two types of visual displays, first on non-predictive backgrounds providing no information on target location, and second on predictive backgrounds which have been associated by training to specific target location on the computer screen. Results indicated a strong effect of the background on the speed of target detection, with faster response times (RTs) in the predictive compared to the non-predictive condition. In Experiment 2, we reanalysed [START_REF] Goujon | Learning of spatial statistics in nonhuman primates: contextual cueing in baboons (Papio papio)[END_REF] data in order to assess if the ALDM supplanting behaviours recorded during that research had subtle (but previously undetected) effects on the baboon's cognitive performance.

Methods

Participants

The study used the same baboon colony as in Experiment 1. However, because this study was conducted in 2012, and therefore one year before the control period 2 of Experiment 1, this colony had a slightly different composition. The data set therefore contained information on a total 20 baboons (5 males and 15 females) whose name and biographical information are indicated in the supplementary section.

Experimental setup, computerized task and database

Experiment 2 used the same ALDM workstations as in Experiment 1. The experimental task is already presented in detail in [START_REF] Goujon | Learning of spatial statistics in nonhuman primates: contextual cueing in baboons (Papio papio)[END_REF] and will therefore only be briefly described here. In short, the trials started with the display of a fixation stimulus on the screen.

Touching this stimulus triggered the display of 8 items, including one target (the capital letter "T") and 7 distractors (capital "L" with different orientations). To be rewarded, the baboons had to detect the target on the screen and to touch it. In the predictive condition (P), six patterns of distractors were each associated with a fixed target position. In the non-predictive condition (NP), six different configurations of distractors were presented independently of the target position. The P and NP trials were presented for one month (July 3 rd to August 3 rd 2012) within a balanced design. Due to a few technical problems, a small number of ALDM test boxes were temporally stopped during testing. Because a too small number of boxes may have affected the monkeys' turnover and the detection of supplanting behaviours, we only retained the data corresponding to the days when a minimum of 8 boxes were accessible. This criterion lead to a very small rate of rejection (only 3 days), while keeping high the likelihood of detecting supplanting behaviours. The database analysed here contains a total of 411534 trials (mean = 20576, median = 21 134, SD = 6355 per baboon).

Data analysis

Investigating the effect of supplanting behaviours on loser of each dyad is difficult, because the subjects either stop temporarily after a supplanting event, or move to a different workstation, with a high variability in time and behaviour between the last trial after the supplantation and the first trial of the next working bout. The analysis therefore focussed on the winner who performed trials immediately after the supplanting event. We considered two types of trials for that subject: those made after the baboon had supplanted another baboon (named "Supplant" trial), and control trials in which it arrived in an empty workstation (baseline trial).

Data analysis therefore proceeded in two steps (1) identification of supplanting and baseline trials, and (2) analysis of the baboons' RT depending on whether the trials were recorded or not after a supplanting event. We used the same procedure as in Experiment 1 to identify runs and instances of supplanting behaviours. A total of 20556 runs were identified with 9310 runs starting after a supplanting event. The dominance hierarchy of the group was computed based on these ALDM supplanting behaviours using the same procedure as in Experiment 1.

Because of a very low rate of errors in cognitive test (range: 96%-99%) statistical analyses focused on RTs. Social challenges can induce extremely rapid biological effects in animals (e.g., neuropeptides release, [START_REF] Kelly | Aggression: Perspectives from social and systems neuroscience[END_REF], with potential consequences on behaviour and cognition (see the discussion of Experiment 2 for additional information). We therefore conducted a preliminary analysis of the data and found that RT differences across conditions were mostly found on the very first trials following supplanting behaviour. On that basis, our next analyses only considered the first 6 trials of each run. The RT of each trial was considered for statistical analyses only if (1) the subject gave a correct response in the trial, and

(2) the RT did not exceed three standard deviations from the individual mean RT. This standard filtering procedure avoided consideration of outlier trials resulting from transient inattention (e.g., [START_REF] Lacreuse | Sex differences in inhibitory control in sociallyhoused baboons (Papio papio)[END_REF]. The final dataset contained a total of 99 619 trials. Linear mixed-effect models (LMM) were computed to analyse the results, using the lme4 package in R [START_REF] Bates | Lme4: Linear mixed-effects models using S4 classes. R package version 0[END_REF]. [START_REF] Goujon | Learning of spatial statistics in nonhuman primates: contextual cueing in baboons (Papio papio)[END_REF] have reported that the RTs decreased in this task with the number of days of testing. We therefore included as random factors the "Name" of the subjects with a random intercept and slope depending on the number of "Days" of the experiment. This random factor controlled for repeated measurements and the evolution of RT with learning. We also included a simple random intercept for the "Age" of the subjects, because [START_REF] Goujon | Learning of spatial statistics in nonhuman primates: contextual cueing in baboons (Papio papio)[END_REF] reported inter-individual differences between age groups. Based on the above considerations, we report the results of the three way interaction between the following fixed factors: (1) the type of trial (baseline vs supplant trial, referred to as the "Behavioural condition" in this analysis), ( 2) the type of contextual background (P or NP, referred to as the "Experimental condition") and ( 3) the position of the trial in the run (from 1 to 6, referred to as "Ntrial" variable).

Finally, a last model investigated if the effect on RTs was modulated by the difference of hierarchical position of both protagonists of the supplanting behaviour. In this model, we only considered as dependent variable the RT of the very first trial performed by the winner after a supplanting behaviour in the non-predictive condition, because the effect of supplantation is stronger on the very first trial and then progressively fades. The explanatory variable was the difference of rank between the winner and the loser of the ALDM supplanting behaviour. With 20 individuals in the group, difference in ranks could vary between -19 (if the most subordinated individual supplanted the most dominant) and +19 (if the most dominant individual supplanted the most subordinated).

Results

The results of the three-way interaction model are shown in Table 1. Our results confirm the main effect of experimental condition (P vs NP) previously described, with faster RTs in the predicted condition (on average by 54 ms; RT mean = 541 ms, SD = 214 ms). In addition, there was a small effect of the position of the trial in a run with RTs decreasing on average by 1.36 ms with the number of trials performed. Finally, RTs were shorter in trials following a supplanting behaviour compared to trials performed without supplanting behaviours, on average by 20 ms on the first trial. We found no evidence of interactions between these different variables.

Insert Table 1 about here Regarding the effect of the difference of hierarchical position (between winner and loser of the supplanting behaviour), we found significant RT differences depending on difference in ranks between the winner and the loser: The most challenging supplanting behaviours, corresponding to the supplanting of a higher-ranking individual by a lower-ranking individual gave rise to the shortest RTs during the ALDM trials immediately following that behaviour, with an estimated difference in RTs of about 1.75 ms (SE = 0.56 ms, t value = 3.12, p < 0.001) for each difference in ranks. For instance, when Ewine (10th rank) supplants Angele (4th rank) the estimated RT is 26 ms lower than if she supplants Dream (19 th rank), because there is a difference of 15 ranks between Angele and Dream.

Discussion of Experiment 2

Experiment 2 investigated the effect of supplanting behaviours on cognitive performance. We analysed the first six trials of each run performed in the ALDM system, distinguishing those performed after a supplanting behaviour from those without. The results showed a significant effect on baboons' RTs: RTs were shorter in trials following supplanting behaviour, compared to baseline trials. This difference was significant in both predictive and non-predictive conditions. We also found that RTs decreased when the winner supplanted an individual of a higher social rank suggesting that the response speed can be affected by social challenges.

Since the goal of the current paper is mostly methodological, we will not discuss at length the behavioural and brain mechanisms by which the supplanting behaviours can affect RTs but will only briefly suggest three hypotheses that may account for our findings. A first potential mechanism is a release of steroid hormones which are known to occur in animals after social challenges (e.g., for a review see [START_REF] Kelly | Aggression: Perspectives from social and systems neuroscience[END_REF]. Androgens and especially testosterone are known to affect cognitive performance (e.g., [START_REF] Janowsky | Thinking with your gonads: testosterone and cognition[END_REF] and to enhance spatial abilities. Alternatively, the release of neuropeptides such as oxytocin could explain our results, knowing their contribution to prosocial and aggressive behaviours. The action of oxytocin has been reported on various types of social behaviours, including social approach (Thompson and Walton, 2004) and aggression [START_REF] Bosch | Maternal aggression in rodents: Brain oxytocin and vasopressin mediate pup defence[END_REF]. Note that oxytocin releases have rapid effects on the behavioural response (e.g., [START_REF] Knobloch | Evoked axonal oxytocin release in the central amygdala attenuates fear response[END_REF]. This may account for the transient effects of supplanting behaviour in our task. Finally, faster RTs after a supplanting behaviour can be accounted for by a winner effect [START_REF] Chase | Aggressive interactions and intercontest interval: How long do winners keep winning[END_REF].

Reeve, Olson & Cole (1985) argued that winning enhances intrinsic motivation in the task relative to loosing. According to this view, the baboon winning the contest would have an enhanced motivation to perform well in the computerized task, even more so if subordinates were supplanting dominants, gaining a time advantage on the task. Independently of the validity of these different hypotheses, which remains to be tested, Experiment 2 has revealed that the dominance hierarchy can have subtle effects on cognitive performance that would probably remain undetectable with more traditional experimental approaches. We believe that the analysis of such effects can help answering questions regarding the interaction between social and non-social cognition.

General discussion

The main goal of Experiment 1 was to demonstrate that the baboons' supplanting behaviours within the ALDM test system can be used reliably to determine the dominance hierarchy of the group. We showed that the dominance hierarchy inferred from untransformed ALDM data differed from those obtained after a random permutation of the individuals performing the trial, suggesting that the ALDM dominance hierarchies are independent of the frequency of baboon's participation to ALDM testing. Secondly, the dominance hierarchies inferred from untransformed ALDM data also differed from those obtained after random permutation between winners, showing that the dominance hierarchy crucially depends on the identity of the individuals interacting. Thirdly, and most importantly, strong correlations were found between the dominance hierarchies obtained from conventional observations and those obtained from ALDM testing for three independent study periods spanning a total of three years (all correlations' r > 0.7). Earlier studies with sows [START_REF] Hunter | Social hierarchy and feeder access in a group of 20 sows using a computer-controlled feeder[END_REF], birds [START_REF] Evans | Inferring dominance interactions from automatically recorded temporal data[END_REF] and cows [START_REF] Foris | Automatic detection of feeding-and drinking-related agonistic behavior and dominance in dairy cows[END_REF] have already shown the interest of computer-controlled identification systems to infer dominance hierarchies.

Dominance hierarchies are often difficult to measure in non-human primates, such as in the male-male tolerant Guinea baboons [START_REF] Fischer | Charting the neglected West: The social system of Guinea baboons[END_REF]Patzel, 2013). It might be the case that agonistic behaviour are more frequent in captivity (such as in our study) than in the wild, and this could explain why we found a linear dominance hierarchy in our group, while [START_REF] Fischer | Charting the neglected West: The social system of Guinea baboons[END_REF] did not. Nevertheless, the present study on the same baboon species as in [START_REF] Fischer | Charting the neglected West: The social system of Guinea baboons[END_REF] highlights the usefulness and reliability of supplanting behaviours in the assessment of the dominance hierarchy. Its findings match perfectly with the previous studies on baboons 'supplanting behaviours [START_REF] Packer | Male dominance and reproductive activity in Papio anubis[END_REF][START_REF] Johnson | Supplanting by olive baboons: dominance rank difference and resource value[END_REF] that reported that baboons were using supplanting behaviours to reinforce existing social relationships. The consistency of our results, across test periods and methods, opens new perspectives for future studies on dominance hierarchy, and demonstrates that such hierarchies can be inferred from the analysis of supplanting behaviours during ALDM testing.

The current study showing that dominance hierarchies can be calculated from automatized ALDM testing also converges with earlier studies demonstrating that ad-libitum computerized cognitive testing allows the extraction of social information. Previous studies from our research group have already demonstrated that ALDM cognitive testing permits studies on the static social network [START_REF] Claidière | Using Automated Learning Devices for Monkeys (ALDM) to study social networks[END_REF], on the dynamic of social networks and their instability [START_REF] Gelardi | Detecting social (in) stability in primates from their temporal co-presence network[END_REF], as well as on audience [START_REF] Huguet | Cognitive control under social influence in baboons[END_REF] and social comparison effects [START_REF] Dumas | Other better versus self better in baboons: an evolutionary approach of social comparison[END_REF]. It was also previously found that the cognitive performance measured during ALDM cognitive testing is modulated by socially affiliative and agonistic events in the enclosure [START_REF] Marzouki | Baboons' response speed is biased by their moods[END_REF]. Studies from other laboratories have further shown that the ordering of the interactions with such automatized test systems depends on social factors in Rhesus monkeys (Macaca mulatta): monkeys of different ranks tended not to work at the same times to avoid social conflicts (Gaze, Lutz, Meyer, Hassetts & Hampton, 2019).

Social effects of freely accessible test systems were also reported in crested macaques (Macaca nigra) who increased their association with others during the testing days (Whitehouse, Michelatta, Powell, Border & Waller, 2013). From all these studies, we conclude that the spontaneous interactions within automatized test systems presented to groups of individuals should be interpreted as social events, and not only as purely cognitive events. Automatized test systems therefore offer a unique opportunity to conduct experiments on either physical or social cognition in non-human primates, as well as on the interactions between the two. The transient effect of the supplanting behavior on the RTs recorded in the cognitive task of Experiment 2 is a new illustration of the possibility offered by ALDM testing to work at the interface between social and non-social cognition.

Modern analyses of behavior tend to be more and more sophisticated and call for the development of new systems to collect social information automatically. The current study shows that the provision of automatized operant conditioning test systems to monkeys maintained in social groups can fulfil that need. One of the most obvious advantage of this method is that extremely large datasets can be collected efficiently to infer dominance hierarchy, in sharp contrast to the more effortful traditional observational methods. Of course, this approach also has some limitations. One of them is that it requires sophisticated technologies that can hardly be accessible to every laboratory working in comparative cognition. The last few years have witnessed the development of freely accessible test systems comparable to the ALDM: in monkeys (e.g., Gazes, Brown, Basile & Hampton, 2012;[START_REF] Fizet | An autonomous, automated and mobile device to concurrently assess several cognitive functions in group-living non-human primates[END_REF]Tulip, Zimmermann, Farningham & Jackson, 2017;Butler & Kennerley, 2018), as well as in other groups such as birds (Huber, Heise, Zenan & Palmers, 2015;[START_REF] Morand-Ferron | Taking the operant paradigm into the field: associative learning in wild great tits[END_REF]. We are therefore convinced that such technology will develop even more in the future, opening unprecedented opportunities to grasp the full complexity of animal's social and non-social cognition.

Open Practices Statements:

The data analysed in this study are available here: https://osf.io/w39fr/ Patzelt, A. ( 2013). The social system of Guinea baboons (Papio papio) with a focus on malemale relationships (Doctoral dissertation, Niedersächsische Staats-und Universitätsbibliothek Göttingen).

R Core Team (2015). R: A language and environment for statistical computing. R Foundation

Figure 1 .

 1 Figure 2, mean Kendall correlation: sim = 0.15, 95% CI = [-0.16; 0.43]; mean Spearman

  The average number of trials per individuals were as follows for each type of trial: Baseline-NP: mean = 1351, median = 1201, SD = 730; Baseline-P: mean = 1368, median = 1213, SD = 743; Supplant-NP: mean = 1129, median = 890, SD = 674; Supplant-P: mean=1133, median = 893, SD = 667.

Figure 1 :

 1 Figure 1: Correlation between the dominance ranks of each individual (labelled by the 3 first

Table 1 :

 1 Results of the linear mixed effects model described in the text.

	Random effects:
	Groups:	Name	Variance	SD		Corr
	Name	(Intercept)	620.4	24.91
		Day	1523.5	39.03		-0.93
	Age	(Intercept) 10395.8	101.96
	Residual		31242.1	176.75
	Fixed effets:
				Estimate SE	df	t-value p-value
	Intercept			562.00 23.61 1.92E+01 23.80 <0.001
	Experimental condition: Predictive	-54.22	3.32 9.96E+04 -16.31 <0.001
	Ntrial			-1.36	0.63 9.96E+04 -2.14	0.03
	Behavioural condition: Supplant	-19.73	3.52 9.96E+04 -5.60 <0.001
	Beh. cond. by Exp. cond.		8.45	4.93 9.96E+04	1.71	0.09
	Beh. cond. by Ntrial		1.70	0.94 9.96E+04	1.82	0.07
	Exp. Cond. by Ntrial		0.29	0.89	9.96E+04	0.32	0.75
	Beh. Cond. by Exp. Cond. by Ntrial	-0.07	1.32	9.96E+04	-0.05	0.96
	Note. Number of observations: 99619, number of inidividuals:20

Table 2 :

 2 Results of the linear mixed effects model described in the text. Number of observations: 4411, nmber of individuals: 20 Supplementary table 4: Hierarchical indexes through the three periods for both ALDM and observation data. This table shows OptK: the optimal K used in Elorating procedure of dominance hierarchy assessment, Triangle transitivity (Pt: the proportion of triangles that are transitive, and ttri: this proportion scaled so that it is 0 for the random expectation and 1 for maximum transitivity) and Linearity indices (de Vries linearity index h(Appleby, 1983; de Vries, 1995) and Landau corrected index h'(de Vries, 1998)) from Elorating package and r: the repeatability index(Shizuka & McDonald, 2012) from AniDom package for 1000 randomisations.Appleby, M. C.(1983). The probability of linearity in hierarchies. Animal Behaviour, 31, 600-608.DeVries, H. (1995). An improved test of linearity in dominance hierarchies containing unknown or tied relationships. AnimalBehaviour, 50, 1375-1389. Shizuka, D., & McDonald, D. B. (2012). A social network perspective on measurements of dominance hierarchies. Animal Behaviour, 83, 925-934.

	Random effects:
	Groups:	Name	Variance	SD	Corr
	Name	(Intercept) 3402.9	58.334
		Day	15.5	3.937	0.17
	Age	(Intercept) 4966.9	70.476
	Residual		30685.2	175.172
	Fixed effects:
		Estimate	SE	df	t-value	p-value
	(Intercept) 633.1636 22.5610	18.5753	28.064	<0,001
	DiffRank 1.7580	0.5639	4277.7434 3.118	0.00183
	Note.

This work was supported by the Labex BLRI (ANR-11-LABX-0036) and the Institut Convergence ILCB (ANR-16-CONV-0002).


Paul Estevenon

Éléonore Welcomme

Christelle Tamain

Gauthier Jouan

Stephanie Szenknect

Adel Mesbah

Christophe Poinssot

Philippe Moisy

Nicolas Dacheux

Eleonore Welcomme

The formation of PuSiO4 under hydrothermal conditions

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Silica and silicate phases are by far among the main components of the earth crust and are omnipresent in most natural environments.

2 Among them, one can note natural actinide silicates ThSiO 4 (thorite), USiO 4 (coffinite) and associated uranothorite solid solutions. 4 The potential impact of actinide silicate species onto the actinide mobility [5][6][START_REF] Riglet-Martial | Competition carbonate-silicate sur la complexation des actinides en milieu argileux riche en silice[END_REF][START_REF] Zänker | [END_REF][9][10][11][12][13] imply to take into account the chemical interactions between actinide elements and silica. Understanding their behavior appears as a crucial issue especially in the case of direct disposal of spent nuclear fuel (SNF) in deep geological repository conditions. The synthesis of plutonium silicate, PuSiO 4 , was reported once by Keller 3, 14 in 1963. This silicate was obtained by performing hydrothermal treatments (7 days at 230°C, argon atmosphere) starting from mixtures of PuO 2 and SiO 2 co-precipitated in carbonate ions rich reactive media (C NaHCO3 = 1M). PXRD measurements performed on this sample allowed to identify that PuSiO 4 crystallized in a zircon type structure (tetragonal, space group I4 1 /amd), which is shared by M IV SiO 4 silicates (with M IV = Zr, Hf, Ce, Th, Pa, U, Np and Am). 15 This study also established that dry chemistry routes performed at high temperatures were inadequate to obtain this plutonium bearing silicate phase. However, to our knowledge, PuSiO 4 has never been obtained again since. The yields of the actinide silicate syntheses under hydrothermal conditions were unclear and previous transpositions of this protocol to the USiO 4 system showed a poor repeatability. More recently, ab initio calculations determined that PuSiO 4 could be metastable 16-22 compared to the mixture of crystalline PuO 2 and SiO 2 . However, it was also suggested that this phase may be stable at low temperature compared to amorphous plutonium oxide and silica. 17 According to the literature, Pu(III) and Pu(VI) silicate species have been reported. [23][24][START_REF] Uchida | IOP Conference Series: Materials Science and Engineering[END_REF][START_REF] Bessonov | [END_REF][27][28] It is also the case for plutonium(IV) silicate aqueous species [29][30][31] , especially for Pu(OSi(OH) 3 ) 3+ .

Since zircon type silicate, ZrSiO 4 and HfSiO 4 , have been identified for long as potential specific waste forms for highlevel radioactive waste, [START_REF] Lutze | Radioactive waste forms for the future[END_REF] the synthesis of (Zr,Pu)SiO 4 and

(Hf,Pu)SiO 4 solid solution [START_REF] Burakov | Proceedings of International Conference NUCEF'98[END_REF][START_REF] Weber | [END_REF][35][36][37][START_REF] Burakov | Materials Research Society, Symposium Proceedings[END_REF][START_REF] Weber | [END_REF][40][START_REF] Fortner | Materials Research Society, Symposium Proceedings[END_REF][START_REF] Weber | [END_REF][43][START_REF] Burakov | Global 2001 International Conference on: ''back-end of the fuel cycle: from research to solutions[END_REF][START_REF] Burakov | Materials Research Society, Symposium Proceedings[END_REF][START_REF] Burakov | [END_REF][47][START_REF] Hanchar | Materials Research Society, Symposium Proceedings[END_REF][START_REF] Hanchar | Materials Research Society, Symposium Proceedings[END_REF][START_REF] Geisler | [END_REF] became a key issue to prepare materials which could incorporate large amounts of plutonium. However, these two solid solutions are not ideal and the solubility limit of plutonium in such materials was limited to only 7 -10 mol.%. 45, 49 Th(IV), U(IV) and Ce(IV) are often used as Pu(IV) surrogates due to their ionic radii and chemical behavior (such as hydrolysis properties) close to those observed for plutonium. These elements are also known to form silicate-based phases, which crystallize in the zircon-type structure. Moreover, all these silicate-based phases could be obtained by hydrothermal methods.

3, 51 Therefore, ThSiO 4 , USiO 4 and CeSiO 4 may be considered as potential analogues in order to study the hydrothermal synthesis of PuSiO 4 .

Since the first synthesis proposed by Frondel and Collette in 1957, 52 the hydrothermal synthesis of ThSiO 4 has been widely studied.

3, 14, 53-61 Two ways of synthesis were identified, from these results. The first was performed in carbonate ions rich reactive media (carbonate ions playing the role of ligand with regard to thorium) and the second, which was developed in ligand free reactive media. We recently studied both of these ways in order to identify the key parameters which may impact the synthesis of ThSiO 4 , particularly the concentrations of the reactants, the pH of the reactive media or the temperature and duration of the hydrothermal treatment.

62, 63

Hydrothermal synthesis of USiO 4 was also extensively studied. 3, 14, 53, 61, 64-73 However, from a general point of view, this synthesis remained poorly reproducible. Recently, Mesbah et al. [START_REF] Mesbah | [END_REF] identified an efficient way to prepare coffinite, allowing its formation with a good repeatability. This protocol consisted in preparing a mixture of uranium (IV) and sodium metasilicate under inert atmosphere, in alkaline media (10 ≤ pH ≤ 12). Finally, NaHCO 3 was added in order to set the pH at 8.7. This mixture was then treated through hydrothermal conditions (150°C ≤ T ≤ 250°C) under inert atmosphere and then purified by specific dissolution of UO 2 and SiO 2 .

The synthesis of CeSiO 4 was based on the hydrothermal oxidation of Ce(III) species. This silicate was obtained from Ce(III) aqueous species in weakly basic reactive media. However, the synthesis of CeSiO 4 was strongly limited by the very restrictive conditions in terms of reactants, pH and working atmosphere. 76 Recently, we developed a very efficient way of synthesis based on the hydrothermal oxidation of Ce(III) silicate solid compounds. 77 In these conditions, CeSiO 4 was obtained in a wide pH range under aerated atmosphere.

Materials and methods

Reagents

Samples preparation

Transposition of the Keller's protocol. Experiments were first adapted from the protocol described by Keller for the preparation of AnSiO 4 (An IV = Th, Pa, U, Np, Pu and Am).

3, 14

Nanometric PuO 2 was prepared by precipitation with the help of ammonia according to the protocol developed for U(IV) and a silicate excess of 3 mol.% in the reacting mixture (synthesis pathway B1 in Table 1). The mixture was transferred into 23 mL Teflon lined reactors in air atmosphere and then heated in Parr autoclaves under hydrothermal conditions for 7 days at 237°C or for 15 days at 150°C under autogenous pressure. Thus, the obtained precipitates were separated from the supernatant by centrifugation for 1 min at 14 000 rpm, washed twice with deionized water and once with ethanol, and then finally dried overnight under the glovebox atmosphere (air). This transposition did not produce any trace of PuSiO 4 while the formation of PuO 2 was always observed. More generally, as described below, all the attempts to prepare PuSiO 4 from Pu(IV) reactant under hydrothermal conditions based on the optimized protocols reported for U [START_REF] Mesbah | [END_REF]

25

, respectively. This route of synthesis has been recently described for the preparation of Ce(III) silicate precursors. 77 Nanometric hydrated PuO 2 was prepared by ammonia precipitation according to the protocol developed for U(IV). 78 The as-precipitated hydrated oxo/hydroxo was mixed with nanometric silica powder then the powders were milled together in ethanol in an agate mortar in order to improve the mixture homogeneity). As previously described, the sample was dried at room temperature under the glovebox atmosphere (air). The resulting mixture was pelletized and heated during 1 hour at 800°C in Ar -4% H 2 atmosphere in order to prepare dense pellets of PuO 2 and SiO 2 mixture. The resulting pellets were finally heated for 9 hours at 1350°C in Ar -4% H 2 atmosphere in a 90% Pt-10% Ir crucible, allowing the formation of both Pu 4.67 ( ). However, the experimental systems led to polyphase samples containing both plutonium (III) silicates, free of PuO 2 and crystalline SiO 2 . A 1:1 molar ratio led to mixtures richer in Pu 2 Si 2 O 7 (Figure S2) while the samples prepared with a 8:6 molar ratio led to mixtures richer in Pu 4.67 (SiO 4 ) 3 O (Figure S3). These mixtures were used to evaluate the behavior of both Pu(III) silicates during dissolution tests and will be subsequently identified as 1). The pH of this suspension was then adjusted to the expected value, ranging between pH = 1 and pH = 8 by the means of 8 mol•L -1 and 0.1 mol•L -1 NaOH solutions. All of the mixtures were put into Teflon lined reactors in Parr autoclaves and then treated thermally between 60°C and 150°C under autogenous pressure for 7 to 21 days. The precipitates were separated from the supernatant by centrifugation for 1 min at 14 000 rpm, washed twice with deionized water and once with ethanol, and then finally dried overnight under the glovebox atmosphere.

Characterization

All the spectrophotometric measurements were performed on a Varian Cary 6000i spectrometer. The spectrophotometer was installed outside the glovebox and the signal was collected via optical fiber. Measurements were performed in PMMA cuvettes between  = 350 nm and 900 nm. The plutonium concentrations were determined by deconvolution of the UV-Visible spectra, using a set of reference spectra containing actinide solutions with known concentrations previously recorded in the same conditions. PXRD data were recorded on the resulting powders using the Bruker D8 advance diffractometer equipped with a lynxeye detector and using the Cu K α radiation (λ = 1.54184 Å) in a Bragg Brentano geometry. The data were acquired using adapted sample holders to avoid any potential radioactive contamination. Gold (99.96% pure grade, Alpha Aesar) was added to the sample as an internal standard in order to calibrate the angular positions of the observed XRD lines (PDF 00-004-0784 79 ). The XRD lines of this internal standard and those of sample holders have been putted in grey in the following XRD patterns. PXRD patterns were recorded between 5° and 100° (2θ) with steps of 0.02° and counting times of 2 s•step -1

. All the collected data were then refined by the Rietveld method using the Fullprof suite package. 80 During the refinements, different profile and structure parameters were allowed to vary such as the zero shift, unit-cell parameters, scale factor, and overall displacement factor. However, the occupancy of each site was fixed to the calculated values. µ-Raman spectra were recorded with a Horiba-Jobin Yvon LabRam device used in conjunction with a nuclearized optical microscope (Optique Peter, Lyon, France) with a  20 objective. A Nd:YAG LASER (532 nm) with a output power adjustable from 20 to 120 mW was used with a variable filter to provide low-excitation-beam power levels, to avoid the LASER beam damage. The microscope was mounted in a glovebox, while the Raman spectrometer and the laser were installed outside the gloves box with a fiber-optic signal transmission line. For each spectrum, a measurement time of 100 to 3600 s was chosen. FTIR spectra were recorded in the 600 -4000 cm -1 range using a dual channel Vertex 70 FTIR spectrophotometer from Bruker Optics equipped with an ATR module which enabled solid samples to be examined directly inside the glovebox without any prior preparation. The spectra collected in such operating conditions exhibited a resolution of 1 cm -1 . Sixty scans were performed for each sample in order to average the measurement error. SEM observations were conducted on powder samples deposited on carbon adhesive sticks and metallized with gold, using a Zeiss SUPRA-55 electronic microscope, equipped either with an Everhart-Thornley Detector (ETD) or a Back-Scattered Electron Detector (BSED) under high vacuum conditions with accelerating voltage between 5 and 30 kV. These conditions were chosen in order to obtain high resolution images.

Results

Syntheses based on the actinide protocols for An IV SiO 4 (An = Th, U and Pu)

Several attempts to prepare PuSiO 4 were made using the protocols reported in the literature for ThSiO 4 in ligand free reactive media and for ThSiO 4 , USiO 4 and PuSiO 4 in carbonate ions rich reactive media.

Syntheses in ligand free reactive media (synthesis pathway A1).

According to the results obtained for the synthesis of ThSiO (determined from visible spectrum) in the final supernatant solution. At higher pH (pH ≥ 1.9), the plutonium precipitation was quantitative. Whatever the pH considered, the synthesis of PuSiO 4 was strongly limited by the plutonium(IV) hydrolysis which results in the quick formation of plutonium hydroxide and its evolution to PuO 2 .

According to the conditions identified for the synthesis of thorium silicate 62 an additional hydrothermal synthesis was performed at a higher temperature (237°C, pH initial = 0.8). However, the results of this synthesis were similar to those obtained at 150°C, with the precipitation of PuO However, no proof of plutonium oxidation was evidenced in the solution or the solid phase, therefore redox reactions involving plutonium were not sufficient to explain these differences. Very similar statements have been made for experiments conducted in the conditions which allowed to form ThSiO 4 in carbonate ions rich media, starting from Pu(IV) in aqueous solution, at 237°C for 15 days in aerated atmosphere (synthesis pathway B2). 63 The PXRD diagram corresponds to the precipitation of nanometric or poorly crystallized PuO 2 (Figure S7). Experiments based on the protocol developed by Mesbah et al. [START_REF] Mesbah | [END_REF] for coffinite, starting from Pu(IV) in aqueous solution, were performed at 150°C and 237°C for 10 to 15 days in air (synthesis pathway B3). This protocol did not allow to prepare PuSiO 4 either. The results were similar to those obtained for the two protocols mentioned above, leading to the formation of PuO 2 as the sole crystalline phase identified by PXRD (Figure S8).

It might be inferred that the precipitation of PuO 2 (due to its use as a reactant or to the pH adjustment by NaOH which would quickly lead to the formation of Pu(OH) 4 and then to PuO 2 ) and the fact that the pH of the reactive media strongly disfavors PuO 2 dissolution, are very limitative to form PuSiO 4 . Therefore, in order to avoid the formation of PuO 2 , complementary experiments were performed with a Pu(IV) solution stabilized in carbonate ions rich media (2.

1 mol•L -1 instead of 1.0 mol•L -1
, synthesis pathway B4). Spectrophotometric analyses of the reactive media before the hydrothermal treatment did not exhibit any evidence of the Pu(IV) complexation by silicate ligands but suggested that plutonium speciation was dominated by a limiting plutonium(IV) carbonate complex, Pu(CO 3 ) 5

6-(Figure S9).

Hydrothermal treatments were performed at 150°C and 237°C for 11 to 15 days. On the one hand, the hydrothermal treatment at 237°C led to the formation of nanometric or poorly crystallized PuO 2 identified by PXRD (Figure S10). On the other hand, the synthesis performed at 150°C led to the formation of a mixture of crystallized Pu(V) sodium carbonate, Na 3 (PuO 2 )(CO 3 ) These species result from the hydrothermal oxidation of stabilized plutonium carbonate species and their precipitation. Moreover, spectrophotometric characterizations performed on the supernatant after hydrothermal treatment exhibited the quantitative precipitation of plutonium as PuO 2 .

Nevertheless, no precipitation of PuSiO 4 was observed in these conditions.

Syntheses by hydrothermal oxidation of Pu(III) silicate precursors

Since the attempts to prepare PuSiO 4 using the protocols reported for ThSiO 4 and USiO 4 failed, complementary experiments were performed using the method optimized for CeSiO 4 , i.e. starting from cerium (III) silicate reactants.

77 Indeed, as reported for cerium, the use of Pu(III) precursors could prevent the hydrolysis of Pu(IV) and then lead to the formation of plutonium silicate.

Syntheses in nitric acid reactive media (synthesis pathway C1).

Based on the protocols developed for the preparation of CeSiO ). The fact that these residues obtained after using alkaline conditions had the same chemical form (according to PXRD) as the starting precursor, however, to be taken with caution. Indeed, the color of these samples changed from blue to greenish-brown. This change in color might be explained by the oxidation of the plutonium (III) silicate at the solid/solution interface or by the formation of an amorphous Pu(IV) phase. However, this oxidation seems to be limited due to the very slow dissolution process of the precursor in these experimental conditions and was not clearly evidenced by PXRD, Raman or infrared characterization (amorphous secondary phase).

It may be inferred that the absence of Pu(III) silicate dissolution for 2 < pH < 8 may be associated to the low dissolution rate of these silicate based species in these conditions compared to more acidic media. Another explanation of this phenomenon might result from the formation of a silica based passivative layer onto the surface of the plutonium (III) silicate precursor, due to the very low solubility of SiO 2 in this pH range. [START_REF] Iler | The chemistry of silica: solubility, polymerization, colloid and surface properties, and biochemistry[END_REF] However, even if these two mechanisms could be complementary, the apparent absence of Ce(III) silicate dissolution was also observed in alkaline conditions (pH > 8) which would promote the solubility of silica. 77 Therefore, it might be supposed that the same behavior occurred for the Pu(III) silicate analogs. For the lowest plutonium content (i.e. 0.84 mmol of Pu) using Pu 2 Si 2 O 7 as precursor and for hydrothermal treatment performed with an initial pH of 3.6, the formation of PuSiO 4 as a minor phase was observed (mixed with PuO 2 as the major phase) (Figure 2). The synthesis performed at lower pH values (i.e. pH = 2) led to the formation of mixtures of PuO 2 (observed by PXRD and Raman spectroscopy) and SiO 2 (identified by IR spectroscopy). As previously stated in nitric reactive media, no apparent degradation of the Pu(III) silicate precursor was observed when working with higher pH values (i.e. pH = 7.6).

In order to compare the two Pu(III) silicate precursors, an additional experiment was conducted at pH = 4.0, with 0.84 mmol of Pu, T = 150°C and in air atmosphere, starting with Pu 4.67 (SiO 4 ) 3 O. These conditions allowed the formation of PuSiO 4 as minor phase of a two-phased system also composed by PuO 2 (Figure 3). In order to evidence the impact of the Pu(III) silicate relative concentration on the formation of PuSiO The results obtained by PXRD analyses highlighted that the increase of the reactants amounts in the starting media increased the yield of formation of PuSiO 4 (Figure 4). Indeed, the fraction of PuSiO 4 was estimated by Rietveld refinement to 30 wt.% in the later conditions while it was found to be under 5 wt.% for the experiments performed with 0. . It could also result from the misestimation of the reference parameters. Indeed, the lattice parameters reported by Keller were obtained on diffuse PXRD peaks and no yield of synthesis was reported.

3, 14

Additionally, one cannot completely exclude the occurrence of PuSiO 4 metamictization (i.e. amorphization under radiation), which might lead to the increase of the a parameter and to the decrease of the c parameter. The characterization of the samples by Raman spectroscopy (Figure S15) allowed to identify some of the characteristic bands of PuSiO 4 . Indeed, the symmetric and antisymmetric stretching modes of SiO 4 were observed at 910 cm -1 and 929 cm -1 , respectively, whereas the symmetric bending mode was located 432 cm -1 . The bands associated to the antisymmetric bending mode of SiO 4 were not evidenced because of their intensity. These results are in good agreement with the data reported for isostructural silicatebased phases (Table 2).

MSiO4

Ionic radius of VIII M 4+ (Å) 83 ν2 ν4 ν1 ν3

HfSiO4 [84][85][START_REF] Estevenon | Crystal Growth & Design[END_REF] 0.83 448 cm -1 620 cm -1 984 cm -1 1018 cm -1 ZrSiO4 [START_REF] Dawson | [END_REF]88 0.84 439 cm -1 608 cm -1 974 cm -1 1008 cm -1 PuSiO4 (this study) 0.96 432 cm -1 -910 cm -1 929 cm -1

CeSiO4 76 0.97 416 cm -1 592 cm -1 903 cm -1 919 cm -1 USiO4 55 1.00 424 cm -1 591 cm -1 906 cm -1 919 cm -1 ThSiO4 55 1.05 438 cm -1 592 cm -1 887 cm -1 914 cm -1 SEM observations performed on both Pu(III) bearing silicate precursors and PuSiO 4 -containing samples allowed to clearly observe a change in the material morphology consequently to the hydrothermal treatment (Figure 5). Indeed, from the typical morphology of the high-temperature sintered Pu slow dissolution, oxidation and precipitation process at the solid-liquid interface.

Discussion

Statements on the attempts to synthesize PuSiO 4 from Pu(IV) reactants

The impossibility to prepare PuSiO 4 in acidic, carbonate ions free, reactive media could be explained by the weak interactions between plutonium and silicate ions in acidic solution. Indeed, the coexistence of free plutonium (IV) ions and silicate ions is very limited in terms of pH range because of the low solubilities of plutonium(IV) hydroxide over pH = 1 89 and of silica below pH = 8. [START_REF] Iler | The chemistry of silica: solubility, polymerization, colloid and surface properties, and biochemistry[END_REF] Moreover, due to the very refractory character of plutonium dioxide, the remobilization of plutonium to enable the formation of PuSiO 4 is strongly disadvantaged from kinetic and thermodynamic points of view according to the ab-initio calculations which showed that PuSiO 4 could be metastable [16][17][START_REF] Williamson | Fundamental thermodynamics of actinide-bearing mineral waste forms[END_REF][START_REF] Ferriss | [END_REF][20][START_REF] Lemire | Chemical thermodynamics of neptunium and plutonium[END_REF][START_REF] Robie | [END_REF] compared to the mixture of crystalline PuO 2 and SiO 2 .

Considering the results reported for actinide analogs, 3, 63, 74 it might be inferred that performing hydrothermal synthesis in carbonate rich media would favor the formation of plutonium silicate due to the pH conditions which could promote the simultaneous presence in solution of silicate and tetravalent plutonium (through carbonate ions assisted dissolution of PuO 2 ). However, none of the synthesis conditions explored enabled the formation of PuSiO 4 , probably because of the strong competition between hydroxide, silicate and carbonate complexations with tetravalent plutonium. Thermodynamic data allowing the evaluation of these competitive effects under hydrothermal conditions are lacking. Nevertheless, since plutonium oxide was formed, the use of silicate and carbonate ligands were insufficient to counterbalance its precipitation or to enhance its dissolution. When plutonium carbonate complexes are formed, the competitive effects between carbonate and silicate ligands did not seem to promote the formation of silicate-based species. At low temperatures (i.e. 150°C), hydrothermal treatments led to the formation of plutonium carbonate solid phases, while at higher temperatures (i.e. 237°C) carbonate ions complexation was probably disfavored and plutonium reactivity with hydroxide species led to the formation of PuO 2 . We considered, therefore, the formation of PuSiO 4 in these conditions to be quite uncertain even if its formation has been reported in the literature by this path of synthesis. [START_REF] Keller | [END_REF] These results suggest that at least one unidentified key parameter was involved in the synthesis reported by Keller.

Statements on the synthesis of PuSiO 4 from Pu(III) silicate reactants

The development of syntheses starting from Pu(III) silicate precursors proved that the formation of PuSiO 4 was possible using such reactants. The pH of the reactive media was found to play a key role for the formation of the Pu(IV) silicate. Indeed, the dissolution of Pu(III) silicate was promoted in acidic media (pH < 3), leading to the precipitation of PuO 2 , whereas the low dissolution rate of the starting precursors in alkaline media only led to the oxidation of plutonium at the solid/solution interface. However, the formation of PuSiO 4 was observed in an intermediate pH window (typically between pH 3 and 4) which was sufficient to provide the dissolution of the Pu(III) based precursor, slow oxidation of Pu(III) into Pu(IV) (or its oxidative dissolution) followed by the precipitation of PuSiO 4 .

The nature of the reactive media used also impacted the formation of PuSiO 4 . Indeed, working in nitric media did not enable the formation of PuSiO 4 , probably due to the too rapid oxidation of the Pu(III) by nitrous acid (from nitric acid) and/or dioxygen present in the system. Therefore, it may be inferred that the formation of PuSiO 4 could be favored by the slow Ln(IV)/An(IV) Ionic radii VIII M 4+ (Å) 83 Hydrolysis thermodynamic constants M(OH)n (4-n)+ (logβ°n) 89,90 logKsp°(MO2•xH2O) Moreover, it might also be inferred that the complexation of tetravalent elements by silicates ligands could decrease the +IV/+III redox potential according to the Nernst law. This phenomenon could explain the easy oxidation to +IV oxidation state, and the formation of both PuSiO 4 and CeSiO 4 . However, to the best of our knowledge, no quantification of this modification has been reported yet.

All the experiments performed from a Pu(IV) reactant did not allow to form Pu(IV) silicate and only led to the precipitation of plutonium oxide or plutonium carbonate. The different behaviors observed between plutonium (IV) and the surrogate elements studied could be explained by the competition between hydrolysis and silicate complexation. Indeed, the protocol of synthesis adapted from the ThSiO 4 synthesis in carbonate ions free acidic reactive media 62 was not efficient to form USiO 4 , CeSiO 4 76 or PuSiO 4 , and only led to mixtures of corresponding metal dioxide and silica. This difference was explained by the stronger trend in hydrolysis, and to precipitate as oxide phases, of U 4+ , Ce 4+ and Pu 4+ compared to Th 4+ (Figure 6 and Table 3). The transposition of the protocols of synthesis reported in carbonate ions rich reactive media, which allowed the formation of pure ThSiO 4 63 and USiO 4 (in mixture with UO 2 ), [START_REF] Mesbah | [END_REF] didn't allow the formation of CeSiO 4 76 and PuSiO 4 but only gave a metal dioxide and silica mixture. These results might also be explained by the higher trend in hydrolysis and precipitation as oxide phases of Ce 4+ and Pu 4+ compared to Th 4+ and U 4+ (Figure 6 and Table 3 Comparison of PuSiO4 and its surrogates according to their ways of synthesis.

form Pu(IV) bearing silicate species, which could precipitate as PuSiO 4 . The presence of strong oxidants in the solution, such as nitrous acid, might be limitative because they would trigger a rapid surface oxidation of the Pu(III) silicate precursors, leading to the formation of PuO 2 or a Pu(IV) silicate passivating layer. Due to the similar limitation encountered for Ce(IV) and Pu(IV) and the closeness between Ce(III) silicate and Pu(III) silicate reactivity when forming CeSiO 4 and PuSiO 4 , it is worth noting that cerium (IV) can be considered as the best surrogate element among those considered to mimic the behavior of plutonium (IV) in reductive and silicate rich media. Since CeSiO 4 was prepared from Ce(III) species at temperatures as low as 40°C, 76 it also raises crucial questions on the potential behavior of plutonium in the environment [5][6][START_REF] Riglet-Martial | Competition carbonate-silicate sur la complexation des actinides en milieu argileux riche en silice[END_REF][START_REF] Zänker | [END_REF][9][10][11][12][13] in the presence of silicates. A particular impact could be found when storing Pu(III) based radioactive waste forms in reductive and silicate rich environments. Moreover, the in-situ formation of plutonium (III) by chemical reduction or by radiolysis might also lead to the formation of PuSiO 4 .

Conclusions

Studying the hydrothermal synthesis of PuSiO 4 , by application of the ways of synthesis optimized for MSiO 4 analogs (Th, U, Ce) allowed to determine a method to form this silicate-based phase. It consisted in the preparation of Pu(III) silicate phases and their slow hydrothermal oxidation in hydrochloric reactive media at pH = 3-4. From a general point of view, our experiments suggested that the properties of plutonium (IV) in the presence of silicate ions were closer to those of cerium (IV) than those of uranium (IV) and thorium. Therefore, cerium (IV) may be considered as the best surrogate to mimic the behavior of plutonium (IV) in silicate rich media. Our experiments did not confirm the results reported by Keller 3, 14 and, more generally, did not allow to form PuSiO 4 from Pu(IV) containing mixtures. However, the identification of the intermediate species and the understanding of the formation of PuSiO 4 mechanism still remain very important issues. Determining the role of the Pu(III) bearing silicate species in solution during the synthesis of PuSiO 4 and their electrochemical behavior may be really challenging. Additionally, the identification of the oxidant species involved in the Pu(III) silicate oxidation is also a crucial point to be clarified in order to understand the relevance of these species under environmental conditions. The formation of PuSiO 4 starting from Pu(III) silicate phases also shew a new light on the potential reactivity of americium and curium bearing silicate complexes 96, 97 and its consequences on the speciation of these actinides in the environment.

Even if this study did not allow the formation of pure PuSiO 4 , it shows that the formation of the plutonium (IV) silicate is possible in very specific conditions. Based on the results already obtained on the cerium-silicate system, it might be supposed that the reaction yield could be improved by working under inert atmosphere, increasing the amount of reactant, the holding time under hydrothermal conditions or by modifying the working temperature. More specifically, the experiments performed on the CeSiO 4 surrogate have evidenced that the formation of cerium (IV) silicate was very limited at temperatures higher than 150°C but was possible at temperatures as low as 60°C starting with solid Ce(III) silicates. 77 Some other ways of synthesis involving complexing species such as carbonate ions (in order to determine the unidentified parameter in Keller's protocol) or fluoride ions (which were successfully employed to prepare ZrSiO 4 , HfSiO 4 and ThSiO 4 syntheses 52, 98-100 ) might also allow the remobilization of the plutonium which would precipitate as PuO 2 and improve the synthesis yield of PuSiO 4 . However, the plutonium complexation by strong ligands such as fluoride might also disadvantage the formation of the silicate phase compared to more stable phases. Nevertheless, this study constitutes a first step in the formation of pure PuSiO 4 which is a crucial point in order to get solubility data on this phase and more importantly, to evaluate its importance in the study of plutonium environmental chemistry.

78 and

 78 mixed together with commercial nanometric SiO 2 powder. This mixture was then put in a 1 mol•L -1 NaHCO 3 aqueous solution in order to obtain a final solution of C Pu = 0.21 mol•L -1

Fig 1 .

 1 Fig 1.PXRD patterns obtained after hydrothermal treatment (T = 150°C, t = 7 days, air atmosphere) of Pu4.67(SiO4)3O (1) starting with 0.84 mmol of Pu in nitric media (4mL) for starting pH values of 7.2 (3), 3.9 (4), 1.8 (5) and 1.1(6). Characteristic XRD lines of PuO2 and PuSiO4 were extracted from references 1 and 3 , respectively.

Fig 2 .

 2 Fig 2. PXRD patterns obtained after hydrothermal treatment (T = 150°C, t = 20 days, air atmosphere) of Pu2Si2O7 reactant (2) starting with 0.84 mmol of Pu in nitric media (4mL) for starting pH values of 7.6 (7), 3.6 (8) and 2.0(9). Characteristic XRD lines of PuO2 and PuSiO4 were extracted from references 1 and 3 , respectively.

Fig 4 .

 4 Fig 4.PXRD patterns obtained after hydrothermal treatment (T = 150°C, t = 14 days, air atmosphere) of Pu2Si2O7 reactant (2) starting with 1.7 mmol of Pu in hydrochloric medium (4mL) and with pH = 3.4(11). Characteristic XRD lines of PuO2 and PuSiO4 were extracted from references 1 and 3 , respectively.

Fig 5 .

 5 Fig 5. SEM micrographs obtained for sintered Pu2Si2O7 precursor (9 hours, T = 1350°C, Ar -4% H2) (a) and for PuSiO4-containing sample (PuSiO4 on the bottom, right and top left) obtained after hydrothermal treatment of Pu2Si2O7 for 14 days at 150°C in air (hydrochloric acid and pH = 3.4) (b).

  4 could be associated to a multistep mechanism. The dissolution of the silicate based precursors could occur through the hydrolysis of Si-O-Si bounds at the surface of the plutonium (III) silicate, leading to the release of Pu(III) bearing silicate complexes in solution. These complexes might be slowly oxidized (probably by O 2 ), without breaking the plutonium-silicate bounds, to

Fig 6 .

 6 Fig 6.Comparison of PuSiO4 and its surrogates according to their ways of synthesis.

  Ce 4.67 (SiO 4 ) 3 O and G-Ce 2 Si 2 O 7 have known plutonium analogs. Thus, this work was dedicated to the hydrothermal synthesis of PuSiO 4 considering the transposition of the conditions optimized for ThSiO 4 , USiO 4 and CeSiO 4 or starting from Pu(III) silicate precursors.

	23-25

Among the solid Ce(III) silicates precursors considered,

Hydrothermal synthesis of PuSiO 4 from Pu(III) silicate precursors solid compounds. In

		and Th, 62, 63 as Pu
	analogs failed (synthesis pathways A, B2, B3 and B4 in Table 1).
	The detailed protocols for these experiments are available in
	supporting information.
	order to develop hydrothermal synthesis
	from Pu(III), several solid Pu(III) based silicate precursors
	(Pu 4.67 (SiO 4 ) 3 O and Pu 2 Si 2 O 7 ) were prepared from mixtures of
	nanometric hydrated PuO 2 and SiO 2 taking into account the
	protocols described by De Alleluia et al.	23, 24 and Uchida et
	al.

SiO 4 ) 3 O and Pu 2 Si 2 O 7 which are identifiable by PXRD. Both Pu 2 Si 2 O 7 and Pu

	4.67 (SiO 4 ) 3 O were
	blue colored compounds. They were stable during more than a
	week under aerated atmosphere without exhibiting any traces
	of PuO 2 , which could result from the oxidation of plutonium
	(III).

Table 1 .

 1 Synthesis parameters for the reported PuSiO4 hydrothermal syntheses.

	Synthesis pathway	Label	Plutonium precursor	Reactive media	pH initial	T (°C)	Δt (days)	Final phase
		(12)			8.8		7
	A1	(13) (14)	Pu IV aq	HNO 3	6.2 2.6	150	7 12	PuO 2
		(15)			1.9		15

(8) 3.6 PuSiO 4 + PuO 2 (9) 2.0 PuO 2 (10) Pu III 4.67 (SiO 4 ) 3 O HCl 4.0 150 20 PuSiO 4 + PuO 2 (11)* Pu III 2 Si 2 O 7 HCl 3.4 150 14 PuSiO 4 + PuO 2 + Pu 2 Si 2 O 7

	*: CPu = 0.21 M for all of the syntheses except for (11), CPu = 0.42 M.
	Synthesis pathways: -A1: based on ThSiO4 synthesis in carbonate ions free reactive media. 62
	-B1, B2, B3: based on PuSiO4,

3, 14 ThSiO4 63 and USiO4 74 syntheses

	in carbonate ions rich reactive media, respectively.
	-C1 and C2: based on CeSiO4 syntheses from Ce(III) silicate precursors. 77
	The nature of the final phase obtained was strongly dependent
	on the molar ratio between PuO 2 and SiO 2 in the starting
	mixtures. The synthesis of Pu 2 Si 2 O 7 was performed with a
	molar ratio PuO 2 :SiO 2 = 1:1 (sample 1) whereas Pu 4.67 (SiO 4 ) 3 O
	(sample 2) was prepared starting with a molar ratio PuO 2 :SiO 2
	equal to 8:6 (corresponding to the stoichiometry of Pu 8 (SiO 4 )

6

described by De Alleluia et al.

23, 24

  Pu 2 Si 2 O 7 and Pu 4.67 (SiO 4 ) 3 O precursors to facilitate the discussion. The formation of Pu 2 Si 2 O 7 at 1350°C could be considered as quite surprising considering the results reported by de Alleluia et al. 23, 24 , who did not obtain Pu 2 Si 2 O 7 after a heat treatment up to 1500°C under reducing atmosphere (H 2 ). However, this might easily be explained by the high specific surface of the nanometric compounds used during our experiments (nanometric Pu(IV) oxo/hydroxo obtained by rapid precipitation and 5-20 nm commercial SiO 2 ), which enabled a high chemical reactivity. In order to prepare PuSiO 4 , some amounts of solid Pu(III) silicate precursors, i.e. Pu 4.67 (SiO 4 ) 3 O or Pu 2 Si 2 O 7 (synthesis presented thereafter), were dispersed in a 1 mol•L -1 acid solution (HNO 3 and HCl for synthesis pathway C1 and C2, respectively, see Table

  2 •xH 2 O, and nanometric or poorly crystallized PuO 2 which were both identified by PXRD (by analogy with Na 3 (NpO 2 )(CO 3 ) 2 •xH 2 O,

81 Figure S10) and Raman spectroscopy (from Na 3 (PuO 2 )(CO 3 ) 2 •xH 2 O reference spectrum, 82 Figure S11).

  4 from Ce(III) silicate precursors, 77 hydrothermal treatments were developed in air at 150°C for 7 to 20 days on Pu 2 Si 2 O 7 and Pu 4.67 (SiO 4 ) 3 O precursors for various initial pH values ranging from 1 to 7. None of these experiments succeeded in forming PuSiO 4 . Only mixtures containing the remaining precursor and/or PuO 2 crystalline phases were evidenced by PXRD.

However, a clear difference was observed between the hydrothermal treatment performed in acidic media, which led to PuO 2 and SiO 2 mixtures for pH  2 and those developed in much more alkaline reactive media for which the starting Pu(III) silicate was poorly altered (Figure

1

and Figure S12, for Pu 4.67 (SiO 4 ) 3 O and Pu 2 Si 2 O 7 respectively

  Si 2 O 7 and Pu 4.67 (SiO 4 )

	The oxidation of plutonium (III) may be caused by both the
	oxygen of the working atmosphere and the considered nitric
	media which could facilitate the oxidation of Pu(III) bearing
	species. It has to be noticed that extending the duration of the
	hydrothermal treatment from 7 to 20 days in weakly acid
	media (i.e. pH = 4) led to similar results. Moreover, additional
	experiments performed at lower temperature (60°C) and for
	pH = 2, starting from Pu 2 Syntheses in hydrochloric acid reactive media (synthesis pathway
	C2). In order to determine if less oxidative reactive media could be
	suitable to form PuSiO 4 , hydrothermal treatments of Pu 2 Si 2 O 7 and
	Pu 4.67 (SiO 4 ) 3 O precursors were performed at 150°C in air, for
	various initial pH values ranging from 2 to 8. These conditions
	corresponded to maximal concentrations of Pu of 0.21 mol•L	-1
	and 0.42 mol•L	-1 in solution (i.e. 0.84 mmol and 1.7 mmol of
	Pu, respectively in 4mL), when considering the full dissolution
	of plutonium precursors.

3

O, did not allow to form PuSiO 4 (Figure

S13

) but only led to the formation of a PuO 2 and Pu(III) silicate mixture, probably due to the slow dissolution kinetic of Pu(III) silicate at low temperature. Therefore, decreasing the temperature did not seem to prevent the oxidation reaction leading to the formation PuO 2 .

  4 , an additional experiment was performed starting with higher content of Pu 2 Si 2 O 7 (i.e. 1.7 mmol of Pu) at pH = 3.4. The hydrothermal treatment performed for 14 days at 150°C in air led to the formation of mixture composed by PuSiO 4 , PuO 2 and Pu 2 Si 2 O 7 .

Table 2 .

 2 Assignment of the bands associated to the silicate groups observed by Raman spectroscopy for PuSiO4 and comparison with other isostructural silicate based phases.

  2 Si 2 O 7 , the sample was covered by square-based bipyramid grains, which is characteristic of silicate phases of the zircon group resulting from hydrothermal treatment. This change in morphology was quite similar to what was observed during the progressive conversion of Ce(III) silicates into CeSiO 4 . PuSiO 4 crystals were precipitated onto the surface of a dense substrate which might correspond to the remaining Pu 2 Si 2 O 7 precursor or to a PuO 2 and SiO 2 mixture. Such kind of precipitation on a solid substrate might be compared to the results obtained for the coffinitisation process for USiO 4 formation from UO 2 and SiO 2 . All of these observations underline the kinetic limitation of the precursor chemical reactivity and suggest that the PuSiO 4 formation occurs by a

	PuSiO 4
	(a.u.)	10			pH = 4.0
	Intensity	1			Pu 4,67 (SiO 4 ) 3 O precursor
		PuO 2
	10	20	30	40	50	60
			2• (degree)
	Fig 3. PXRD patterns obtained after hydrothermal treatment (T = 150°C,
	t = 20 days, air atmosphere) of Pu4.67(SiO4)3O (1) starting with 0.84 mmol of
	Pu in hydrochloric medium (4mL) and with pH = 4.0 (10). Characteristic XRD lines of PuO2 and PuSiO4 were extracted from references 1 and 3 ,
	respectively.
		PuSiO 4
	(a.u.)	7			pH = 3.4
	Intensity	1			Pu 2 Si 2 O 7 precursor
		PuO 2
	10	20	30	40	50	60
			2• (degree)

77

Moreover,

  oxidation of Pu(III) silicate species (at the solid-liquid interface or in solution), keeping low Pu(IV) concentrations in the solution and thus preventing the formation of Pu(IV) bearing hydroxide precipitates which could evolve rapidly into PuO 2 . Consequently, less oxidative media, such as a hydrochloric media, should be privileged to form PuSiO 4 . It was also observed that the yield of formation of PuSiO 4 was improved by increasing the amount of the Pu(III) reactant in the reactive media. Such higher yields could be explained either by the increase of the concentration of the intermediate species which are involved in the formation of PuSiO 4 or by the decrease of the oxidative species (such as O 2 ) / Pu(III) precursor ratio in the reactive media (avoiding the quick oxidation of Pu(III) silicate precursors leading to PuO 2 ).

Table 3 .

 3 Constants of complexation of Ce(IV) and An(IV) with carbonate associated to the limit complexes at I = 0 (25°C).

Comparison with the different zircon-type silicate systems

  Among all the attempts to prepare PuSiO 4 adapted from the optimized conditions for the potential surrogates (Ce, Th and U), the chemical route leading to the formation of CeSiO 4 from Ce(III) silicate solid precursors in hydrochloric reactive media

								89, 91
					logβ°1		logβ°2	logβ°3	logβ°4
	Pu			0.96	14.6 ± 0.2		28.6 ± 0.3	39.7 ± 0.4	48.1 ± 0.9	-58.5 ± 0.7
	Ce			0.97	14.8		28.0	40.5	51.9	-59.3 ± 0.3
	U			1.00	13.6 ± 0.2		26.9 ± 1.0	37.3 ± 1.0	46.0 ± 1.4	-54.5 ± 1.0
	Th			1.05	11.8 ± 0.2		22.0 ± 0.6	31.0 ± 1.0	38.5 ± 1.0	-47.0 ± 0.8
	* β°n is associated to the reaction An 4+ + n HO -= An(OH)n (4-n)+ .
	** Ksp° is associated to the reaction AnO2•xH2O + (2-x) H2O = An 4+ + 4 HO -
							77
	has been identified as the only suitable way to prepare
	plutonium silicate. Similar experiments performed in nitric
	conditions did not lead to the formation of PuSiO 4 . This
	difference compared to CeSiO 4 was explained by the stronger
	oxidative character of nitrous acid from nitric media
	(establishing a redox potential around 1.0 V/ENH) for Pu(III)
	compared to Ce(III).	77
	Pu 4+ + e	-⇋ Pu 3+		E° = 0.982 V/ENH	92
	Ce 4+ + e	-⇋ Ce 3+		E° = 1.72 V/ENH	93-95

  According to the dissolution and precipitation process hypothesized, it seems likely that at least one of the intermediate species involved in the PuSiO 4 formation could be a plutonium silicate aqueous species. As we previously stated during the synthesis of CeSiO 4 from Ce(III) silicate precursors,77 the formation of PuSiO

). Moreover, complexation of Ce 4+ or Pu 4+ by carbonate ions are stronger than that obtained for Th 4+ or U 4+ (Table S2). Both aspects would affect the availability of Pu 4+ to interact with other ligands. The feasibility of PuSiO 4 synthesis in the conditions reported in this article from plutonium (III) silicate precursors, could result from the formation of intermediate plutonium silicate species (Figure

S17

).

Acknowledgements

The authors would like to thank V. Brethenoux and J. Hennuyer (from CEA) for supporting the syntheses experiments, J. R. Sevilla (from CEA) for supporting SEM experiments and P. Coste (from CEA) for supporting Pu(III) silicate syntheses experiments.

FOR TABLE OF CONTENTS ONLY

The study of PuSiO 4 constitutes a crucial issue to get a better representation of the behavior of plutonium in silicate ions rich environments. This study reports an innovative way of synthesis of PuSiO 4 by hydrothermal in situ oxidation of solid Pu(III) silicate precursors. It also identifies how representative thorium, uranium and cerium are of plutonium chemistry in silicate ions rich reactive media.