Jean Slawinski
email: jean.slawinski@insep.fr

Nicolas Houel
email: nicolas.houel@univ-reims.fr

Camille Moreau
email: moreau_camille@live.fr

Alexia Mahlig
email: alexia.mahlig@usherbrooke.ca

Daniel Dinu
email: daniel.dinu@insep.fr

Contribution of segmental kinetic energy to forward propulsion of the centre of mass: Analysis of sprint acceleration

Keywords: Inertial sensors, sprint performance, kinetic energy, power, performance

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Introduction

Sprint performance is determined by the ability of the sprinter to generate high velocity (V0), high power (Pmax) and forces (F0) at the centre of mass (CoM). Performance can thus be characterised by the force-velocity (F-v) and power-velocity (P-v) profiles of the sprinter's CoM [START_REF] Morin | Mechanical determinants of 100-m sprint running performance[END_REF][START_REF] Rabita | Sprint mechanics in world-class athletes: a new insight into the limits of human locomotion[END_REF]Slawinski et al., 2017b). The mean power (Pmean) produced during a 40m sprint has also been shown to be closely related to sprint performance time [START_REF] Rabita | Sprint mechanics in world-class athletes: a new insight into the limits of human locomotion[END_REF].

In physics, power is the rate, with respect to time, at which work is done; thus power is the time derivative of work. As such, several studies have evaluated external work (Wext), defined as the sum of the potential and kinetic energy variations measured at the centre of mass level, in order to identify the overall mechanical determinants of sprinting. A recent study of twelve young, male athletes performing a maximal 60m sprint [START_REF] Matsuo | External mechanical work done during the acceleration stage of maximal sprint running and its association with running performance[END_REF] showed that performance mainly depended on horizontal anterior-posterior Wext during the propulsion phase. In other words, the greater the mechanical work produced, the higher the sprint performance. Indeed, the use of the kinetic energy (KE) of the body segments, because it take into account mass and velocity of each segment, supplies useful information concerning the upper and lower limbs' contributions to the translation of the body in the forward direction during rapid movement [START_REF] Hubley | A work-energy approach to determine individual joint contributions to vertical jump performance[END_REF]J Slawinski et al., 2010).

Internal mechanical work (Wint) is an important component of total work production during sprinting [START_REF] Winter | Biomechanics and Motor Control of Human Movement[END_REF] and analysis of this variable provides important information to understand overall performance. In the literature, two methods have been used to calculate Wint during sprinting. The first method involves calculation of the power produced at each lower limb joint (ankle, knee and hip) and its contribution to the total power produced (sum of the power at each joint). This method has been used at discrete instants of the acceleration phase: during the starting block phase [START_REF] Bezodis | Relationships between lower-limb kinematics and block phase performance in a cross section of sprinters[END_REF][START_REF] Brazil | Lower limb joint kinetics in the starting blocks and first stance in athletic sprinting[END_REF][START_REF] Mero | Effects of muscletendon length on joint moment and power during sprint starts[END_REF][START_REF] Otsuka | Effect of a Wide Stance on Block Start Performance in Sprint Running[END_REF], during the first step [START_REF] Brazil | Lower limb joint kinetics in the starting blocks and first stance in athletic sprinting[END_REF][START_REF] Charalambous | Lower limb joint kinetics and ankle joint stiffness in the sprint start push-off[END_REF][START_REF] Debaere | Biomechanical determinants of sprint acceleration in adult and young athletes Biomechanical determinants of sprint acceleration in adult and young athletes[END_REF]2013;[START_REF] Mero | Effects of muscletendon length on joint moment and power during sprint starts[END_REF], during the second step [START_REF] Debaere | From block clearance to sprint running: Characteristics underlying an effective transition[END_REF]Debaere et al., 2013;[START_REF] Jacobs | Intermuscular coordination in a sprint push-off[END_REF], at 14-m [START_REF] Johnson | Muscle power patterns in the mid-acceleration phase of sprinting[END_REF] and at maximal velocity (Vmax) [START_REF] Belli | Moment and power of lower limb joints in running[END_REF][START_REF] Bezodis | Lower-limb mechanics during the support phase of maximum-velocity sprint running[END_REF][START_REF] Vardaxis | Power patterns of the leg during the recovery phase of the sprinting stride for advanced and intermediate sprinters[END_REF].

However, the data from these studies do not show how lower limbs joint powers or the contribution of each joint, evolve throughout the course of the acceleration (table 1). More recently, [START_REF] Schache | Lower-limb joint mechanics during maximum acceleration sprinting[END_REF] calculated lower limb joint powers during the entire acceleration phase using data from a combination of 8 force plates integrated within the track and 22 Vicon 3D motion analysis cameras. They demonstrated that positive power produced at the hip, knee and ankle joints decreased as running speed increased, and that the contribution of the ankle joint to the total power produced by the lower limb increased with increasing CoM velocity.

The authors concluded that the hip, and in particular the ankle joints, provided key sources of positive power that contributed to maximising the body's forward kinetic energy during a rapid acceleration. Despite some disparities, overall, these studies suggest that ankle joint power might play an important role in the production of mean power over the sprint (Pmean) since the contribution of ankle joint power to total power increases until Vmax. In contrast, the contribution of hip joint power is greater during the start and the acceleration phase while the contribution of the knee is stable throughout the acceleration phase. Although these studies provided important information to increase understanding of performance, they did not consider the trunk and upper limbs which play an important role in sprinting (Slawinski et al., 2010;[START_REF] Slawinski | Effect of postural changes on 3D joint angular velocity during starting block phase[END_REF]Slawinski et al., , 2017a)).

The second method involves calculation of Wint or internal power (Pint) using total body kinetic energy (KE). Recently, using this method, it was demonstrated that Pint accounts for 41% of the total power measured during a 20m sprint acceleration [START_REF] Pavei | Comprehensive mechanical power analysis in sprint running acceleration[END_REF]. In order to understand the role of each segment (and particularly the upper limbs) in the production of Pint, a series of papers measured the KE of each segments during the starting block phase and first step (Slawinski et al., 2010;[START_REF] Slawinski | Effect of postural changes on 3D joint angular velocity during starting block phase[END_REF]Slawinski et al., , 2017a)). These authors found that the trunk contributed 41% of the kinetic energy, the arms and forearms around 17% and the lower limbs about 42%. These data suggested that the upper limbs and particularly the trunk play a central role in the production of the total body KE, however, only the starting block phase and first step has been analysed the entire acceleration phase remains to be explore.

The aim of this study was therefore to calculate the KE of each limb segment and the contribution of each to total body kinetic energy throughout the entire acceleration phase of a 40m sprint. We hypothesised that the contribution of the lower limb segments and trunk would increase during the acceleration phase and in consequence, the contribution of the upper limbs would decrease.

Material and Methods

Subjects and experimental procedure Nine athletes who were not specifically sprinters (mean height 181.6 ± 6.8 cm; mean mass 70.3 ± 9.2 kg; mean age 19.5 ± 1.1 years) performed two maximal 40-m sprints: this distance was chosen to ensure that the entire acceleration phase could be recorded (Morin et al., 2015). Each sprint was performed from a stand-up start and a five-minute recovery period was imposed between the two sprints. All participants provided informed consent for their participation. The protocol was in accordance with the ethical standards of the Declaration of Helsinki and the study was approved by the Institutional National Science in Sport Review Board.

During the sprints, participants wore a MVN Biomech suit (Xsens Technologies BV, Enschede, The Netherlands) onto which 17 miniature inertial measurement units were strapped (nanotechnology inertial measurement units, nIMU). Each nIMU contains three gyroscopes, three accelerometers and three magnetometers in a 35-g box about the size of a matchbox. This system was validated in running [START_REF] Reenalda | Continuous three dimensional analysis of running mechanics during a marathon by means of inertial magnetic measurement units to objectify changes in running mechanics[END_REF] and for high speed movement [START_REF] Blair | Concurrent validation of an inertial measurement system to quantify kicking biomechanics in four football codes[END_REF]. Each nIMU captures the 6-degrees-of-freedom of the segment on which it is fixed, in real time at a sampling frequency of 240 Hz. Sensors were placed following the manufacturers recommendations; feet (dorsum), shanks (medial surface of the tibias), thighs (lateral surface, above the knees), pelvis (middle of both the posterior superior iliac spines), shoulders (middle of the scapula spine), upper arms (lateral, above the elbow), forearms (lateral side of wrist), hands (dorsum), sternum and the back of the head. A static (N-pose) and dynamic calibration was carried out for each participant. Sensor to segment orientations were then determined using regression equations [START_REF] Blair | Concurrent validation of an inertial measurement system to quantify kicking biomechanics in four football codes[END_REF][START_REF] Roetenberg | Xsens MVN: full 6DOF human motion tracking using miniature inertial sensors[END_REF]. Care was taken to ensure there were no materials that could provoke magnetic disturbances in the testing environment.

Data analysis

The raw data from the sensors (positions at each time-point) were exported to a customized MatLabTM program (7.10.0, R2010a, Natick, USA). This program was used to calculate the orientations of hands, fore-arms, upper-arms, head, upper-trunk, lower-trunk, pelvis, upper legs, lower legs and feet, wrist, elbow, shoulder, trunk, head, pelvis, hip, knee, ankle and metatarsus joint angles using a Newton-Euler method, and center of mass (CoM) positions and velocities of the body center of mass (CoM). The joints rotations sequences answer to the standard of the International Society of Biomechanics [START_REF] Wu | ISB recommendation on definitions of joint coordinate systems of various joints for the reporting of human joint motion-Part II: shoulder, elbow, wrist and hand[END_REF][START_REF] Wu | ISB recommendation on definitions of joint coordinate system of various joints for the reporting of human joint motion--part I: ankle, hip, and spine[END_REF]. Sixteen rigid segments (head-neck, thorax, abdomen, pelvis, upper arms, forearms, hands, thighs, legs and feet) were defined, and the kinetic energy (KE) during the entire acceleration phase was calculated for each segment according to the De Leva anthropometrical model [START_REF] De Leva | Adjustments to Zatsiorsky-Seluyanov's segment inertia parameters[END_REF].

The KE was calculated as the sum of translational and rotational KE of each segment. All the KE datas were then filtered using a zero phase butterworth filter of fourth order and 12Hz cutting frequency. This choice was made according to the residual method [START_REF] Winter | Biomechanics and Motor Control of Human Movement[END_REF].

In order to understand the evolution of the KE during the 40-m sprint, we chose to divide the sprint into three different phases based on the maximal power (Pmax) and maximal velocity of the CoM (Vmax). Acceleration occurred during the first two phases while during the third phase, CoM velocity was stable. Phase 1 corresponded to the first movement made by the sprinter (determined by an increase in CoM velocity of 0.1 m.s -1 ) to reach Pmax, Phase 2 was began at Pmax until Vmax and phase 3 began at Vmax until the end of the 40 m. We used the method proposed by Samozino to compute Pmax and the time at which it was reached [START_REF] Morin | A simple method for computing sprint acceleration kinetics from running velocity data: Replication study with improved design[END_REF][START_REF] Samozino | A simple method for measuring power , force , velocity properties , and mechanical effectiveness in sprint running[END_REF]. This simple method uses an mono-exponential model to describe the increase in velocity during the acceleration phase (di Prampero et al., 2005) (figure 1). Using this method, we also computed linear force -velocity (F-v) and second-order polynomial power -velocity (P-v) relationships for each participant in order to obtain maximal theoretical force, velocity and power values (respectively, F0, V0 and Pmax) (figure 2 A and B).

F0 and V0 are the respective intercepts of the force and velocity axes of the F-v curve. Pmax is the maximum of the P-v relationship. Once Vmax and Pmax had been determined, the start and end-points of each of the 3 phases were defined according to the timing of these two variables.

Using a method previously published by our team, we calculated mean KE (KEmean) for each phase and each segment (Slawinski et al., 2017a). The KEmean of each segment was then summed to obtain whole body KEmean, uppers limbs KEmean (sum of left and right upper arms, forearms and hands KEmean), lowers limbs KEmean (sum of left and right thigh, lowers legs and feet KEmean) and head-trunk limb KEmean (sum of head-neck, thorax, abdomen, pelvis KEmean)

for the 3 sprint phases. KEmean of each segment was expressed as a percentage of whole body KEmean to determine the contribution of each segment to the forward velocity of the CoM during the whole sprint.

Statistical analysis

The KEmean of the different segments and their contributions to total body KE were compared across the three phases using a repeated measures ANOVA, and a Bonferroni post-hoc test was used to compare the results for each phase. The level of significance was set at p ≤ 0.05.

Results

F-v and P-v relationships

Mean V0 for all participants was 8.47 ± 0.50 m.s -1 , mean F0 was 462 ± 86 N and mean

Pmax was 971 ± 129 W.

The durations of phase 1 (start to Pmax), phase 2 (Pmax to Vmax) and phase 3 (Vmax to 40m), were respectively 0.90 ± 0.17 s, 4.02 ± 0.89 s and 0.93 ± 0.77 s.

Mean kinetic energy

KEmean of the total body increased significantly (p ≤ 0.001) from phase 1 to phase 2 and from phase 2 to phase 3. Simirlarly, KEmean of the head, neck, trunk, right and left upper arms and thigh segments increased significantly (p ≤ 0.001) from phase 1 to phase 2 and from phase 2 to phase 3. The KEmean of the other distal segments (right and left forearms, hands, legs and feet) only increased from phase 1 to phase 2 (figure 3 A, B, C, D).

Segmental contribution

The post-hoc test demonstrated that the contribution of the head-trunk limb to the forward velocity of the CoM increased significantly between phases 1 and 2 and between phases 1 and 3 (p ≤ 0.05; table 2), but not between phases 2 and 3 (table 2). Inversely, the contribution of the upper and lower limbs to the forward velocity of the CoM decreased significantly from phase 1 to phase 2 and from phase 1 to phase 3 (p ≤ 0.05; table 2), but not from phase 2 to phase 3 (table 2). Details of the contributions of the different segments are presented in table 2.

Discussion and implication

In this study, we calculated the KE of each body segment and the contribution of the main limbs to the production of total body KE during the entire acceleration phase of a 40m sprint.

This was the first time that kinetic energy of all the segment has been measured during a 40 m run. This was possible though the use of an inertial measurement system allowing to continuously measure the body kinematics in ecological conditions. This approach allows to understand how maximal power, that has been identified as a strong sprint performance predictor, is produced by the different limbs [START_REF] Di Prampero | Running at altitude: the 100-m dash[END_REF]. The present results demonstrated that KEmean of the total body increased until the phase 3. This increase is explained by the increase in KEmean of head-trunk-limb and the proximal limb segments (arms and thigh).

KEmean of distal segments (fore arms, hands, legs and feet) did not contribute to the increase in KEmean of the total body when the sprinter has reached is Vmax (between phase 2 and 3). The results also confirmed that, during the entire sprint, the head-trunk-limb and upper limbs together contributed more than 50% of total body KE. This contribution of the head-trunk-limb increased between the phase 1 and 2 demonstrating that the head-trunk-limb plays a central role in the velocity production during the acceleration phase.

F-v and P-v relationships

The present results highlight that, thanks to the use Pmax and Vmax, a sprint running race can be carved in three main phases. This approach allow to analyze the sprint through measurable mechanical parameters and not only with the classical phases obtained from technical observation (block clearance, driving phase, acceleration phase, top speed and maintenance or decrease) [START_REF] Mann | The Mechanics of Sprinting and Hurdling[END_REF]. The present data obtained with Xsens match with those obtained for a fifth division soccer players who have a close Vmax (Jimenez-reyes and Cuadrado, 2019). Indeed for a Vmax of 8.7 m.s -1 these authors found a F0 of 6.7 N.kg -1 and a Pmax of 14.5 W.kg -1 and in the present work, for a Vmax of 8.47 m.s -1 , F0 was 6.6 N.kg -1 and a Pmax was 13.9 W.kg -1 .

Mean kinetic energy and Segmental contribution

The first important result was that total body KE increased from the start to the end of the 40-m sprint. This increase was mathematically associated with the increase in trunk KE and the KE of the upper arms and thighs, which attach directly to the trunk (figure 3). This is not surprising given that the trunk, upper arms and thighs collectively represent more than 60% of total body mass. Thus, the hip and shoulder joints played a key role throughout the entire sprint.

This supports the results of recent studies that have demonstrated the importance of hip muscle activity for sprint performance. For example, the muscles group of quadratus femoris, hamstring and gluteus are significantly larger in sprinters than in non-sprinters [START_REF] Handsfield | Adding muscle where you need it: non-uniform hypertrophy patterns in elite sprinters[END_REF]. Also, the mean power output developed during resistance exercise to develop hamstring strength, such as the hip thrust, is directly correlated to performance on 10, 20, 40 and 60-m sprints [START_REF] Loturco | Vertically and horizontally directed muscle power exercises: Relationships with top-level sprint performance[END_REF]. The hamstring muscles are strongly active in late swing, just before ground contact (Morin et al., 2015;[START_REF] Schache | Mechanics of the human hamstring muscles during sprinting[END_REF]. Their role at this time point is two-fold: 1) to brake the leg at the end of the swing phase and 2) to increase thigh velocity to prepare for foot contact [START_REF] Morin | Sprint Acceleration Mechanics: The Major Role of Hamstrings in Horizontal Force Production[END_REF]. We believe that the strong contribution of the trunk and thighs to forward motion of the CoM observed in this study throughout the sprint acceleration reflects the specific role of the hip muscles in sprint performance. To go deeper into the analysis of the role of the hip, we plotted typical continuous KE of upper, lower limb and head-trunk limb expressed as a percentage of the total energy (figure 4). This figure

showed that lower limb and the head-trunk limb are in opposite phase, and suggested that trunk and legs make an opening and a closing movement during the sprint. This closing-opening movement is particularly important (great amplitude) during the phase 1 of the sprint. This suggested that this movement is a mechanism allowing to produce a great KE energy variation and to reach a higher Pmax at the end of the phase 1. The efficiency of this closing-opening movement may particularly depend on the action of the hip muscles. The KE of the upper limb being also in opposite phase with the one of the head-trunk limb, we can hypothesise that upper and head-trunk limbs also make an opening-closing movement. This movement, as for lower limb, may help to create a greatest Pmax. Thus, the sprinter would realise a double closingopening movement between lower and head-trunk limb and between head-trunk and upper limb that increase Pmax at the end of the phase 1.

The second important result was that, as hypothesised, the contribution of the upper limbs to total body KE decreased as sprint velocity increased. In a previous study, it has been showed that the upper limbs contribute 15 to 22% of the total body KE during the starting block phase and first step; the exact contribution depends on the training level of the sprinters and on their position during the start phase (Slawinski et al., 2010;Slawinski et al., 2017a). The present results confirmed this level of contribution (16%) of the upper limbs during the start of the acceleration (phase 1) and revealed that their contribution decreased significantly (to 11%) at maximal velocity (phase 3) (table 2). Compare to previous study the lower contribution of upper limb observed in the present study may be linked to the lower training status in sprint running of the participants. This decrease of the upper limb contribution during the 40m sprint is not surprising since the upper limb mass is much smaller than that of the trunk or lower limbs, thus, as sprint velocity increases, the relative contribution of the upper limbs must decrease. This was corroborated by the increase in the contribution of trunk and the head-neck segment (57.8% of the total body mass [START_REF] Winter | Biomechanics and Motor Control of Human Movement[END_REF]) with increasing sprint velocity.

The third important result was that, in contrast with our hypothesis, the contribution of the lower limbs reduced over the course of the acceleration. This could be explained by the fact the lower limbs represent a smaller proportion of total body mass (32.3%) than the trunk and head-neck segments [START_REF] Winter | Biomechanics and Motor Control of Human Movement[END_REF], and that small variations in legs velocity occurred between phases 1 and 3. In contrast to CoM velocity, step rate does not increase gradually throughout the acceleration phase: the rate of the first step after leaving the starting block is already at 95% of the rate reached at Vmax (Debaere et al., 2013). Thus lower limb velocity remains relatively constant from the sprint start until Vmax. The reduction in the relative contribution of the lower limbs to forward motion of the body CoM seemed to compensate for the increase in trunk contribution from phase 1 to phase 2 (in phase 2, the contribution of the trunk was greater than that of the lower limb, in contrast with phase 1; table 2). However, the decrease in the lower limb contribution at Vmax is surprising, particularly in the light of the recent results of Schache et al. ( 2019) that showed that ankle power increased at Vmax. This difference could be explained firstly by the fact that we evaluated lower limb segmental energy, which is mechanically different to joint power which was evaluated by [START_REF] Schache | Lower-limb joint mechanics during maximum acceleration sprinting[END_REF]; and secondly, the role of the trunk, thigh and thus hip joint could be to create sagital movement, while the role of the ankle joint appears to be to create large vertical reaction forces at Vmax [START_REF] Weyand | Faster top running speeds are achieved with greater ground forces not more rapid leg movements Faster top running speeds are achieved with greater ground forces not more rapid leg movements[END_REF].

Conclusion

The results of this study revealed that, as well as the lower limbs, the upper limbs, and particularly the trunk, contribute to total body KE during sprint acceleration. Trunk KE contributed largely to total body KE throughout the entire acceleration, while the contribution of the upper and lower limbs reduced over the course of the sprint. Others studies must be conducted in sprinters with a better maximal velocity and maximal power in order to confirm if movement of closing-opening between legs, trunk and arms are determinant in the production of a hight kinetic energy and a maximal power. If this was confirmed, the start phase and first step would no longer be considered as a simple pushing phase but as a succession of a closing and opening movement, where the trunk, hip and shoulders would play a central role.

Declaration of interest statement

Figures

  Figures

Figure 1 :

 1 Figure 1: Exponential model applied to the increase of the typical non-fitted velocity of the CoM during the 40-m sprint.

Figure 2 :

 2 Figure 2: Average force-velocity (A) and power-velocity (B) relationships derived from the exponential model of the increase in CoM velocity during the acceleration phases.

Figure 3 :

 3 Figure 3: Total body kinetic energy (A) and kinetic energy of the 16 body segments. Mean kinetic energy was calculated for the 3 phases of the 40-m sprint (Start to Pmax; Pmax to Vmax and Vmax to the end of the sprint). The KEmean of each segment was summed for each limb. Headtrunk-limb (B) is composed of the head-neck, thorax, abdomen and pelvis; lower limb (C) is composed of the right and left thighs, legs and feet; and upper limb (D) is composed of the right and left upper arms, forearms and hands. * Significantly different from phase 1 (p ≤ 0.05); £ Significantly different from phase 2 (p ≤ 0.05).

Figure 4 :

 4 Figure 4: Typical example of continuous plotting of the lower limb, Head-trunck and lower limb expressed as a percentage of total kinetic energy.

All authors disclose any financial and personal relationships with other people or organisations 293 that could inappropriately influence (bias) their work. 294 Tables Table 1: Maximal hip, knee and ankle joint power (mean ± SD) according to the distance of the sprint acceleration (rear and front block refer to the starting block phase). All the values were established from data found in the literature (see text for references).


Marta N Sanz-Ortiz

Fernando Rodríguez

Gérard

Gérard Demazeau

Spin transition in Co 3+ by optical absorption and time-resolved spectroscopy under pressure: an appraisal of the different spin states

Keywords: Co 3+, CoF 3, spin crossover, intermediate spin, Jahn-Teller

Spin transition in Co3+ by optical absorption and time-resolved spectroscopy under pressure: An appraisal of the different spin states

Introduction

There is some controversy about the existence of intermediate spin (IS; S=1) in Co 3+ ions (3d 6 ) around the S = 0 ( 1 A 1 ) to S = 2 ( 5 T 2 ) spin crossover transition (SCO). The observation of magnetic moments with values of μ eff > 0 for T>0 K that increase with temperature in Co 3+ oxides [START_REF] Imada | Metal-insulator transitions[END_REF] has been explained in terms of either a mix of IS and low spin (LS) states around SCO or high spin (HS) and LS states, which actually corresponds to S = 1 or S = 2 and S = 0 spin states in thermal equilibrium, respectively [START_REF] Imada | Metal-insulator transitions[END_REF][START_REF] Maris | Evidence for orbital ordering in LaCoO 3[END_REF][START_REF] Haverkort | Spin state transition in LaCoO 3 studied using soft X-ray absorption spectroscopy and magnetic circular dichroism[END_REF]. Insulating oxides and fluorides are attractive systems for elucidating such a controversy since they provide a crystal-field strength, CF , for Co 3+ at, or nearly below, the SCO ( 5 T 2 ↔ 1 A 1 ): CF = 2.1 [START_REF] Burns | Mineralogical Applications of Crystal Field Theory[END_REF] and 1.7 eV [START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF], respectively. According to the Tanabe-Sugano diagram for Co 3+ (3d 6 ), the SCO crystal field is 20 times the Racah parameter B, which is SCO = 1.9 eV for B = 0.095 eV derived from the optical spectra [START_REF] Burns | Mineralogical Applications of Crystal Field Theory[END_REF][START_REF] Allen | The electronic spectra of the hexafluoro complexes of the first transition series[END_REF]. Magnetic measurements and X-ray emission spectroscopy (XES) performed on CoF 3 and LaCoO 3 clearly indicate that the low temperature ground state corresponds to HS (S = 2) [START_REF] Grannec | Propriétés magnétiques des hexafluorures Na 3 MeF 6 (Me = Co, Ni, Cu)[END_REF][START_REF] Gütlich | Spin Crossover in Transition Metal Compounds II[END_REF][START_REF] Vankó | Temperature-and pressure-induced spin-state transitions in LaCoO 3[END_REF] and LS (S = 0) [START_REF] Vankó | Temperature-and pressure-induced spin-state transitions in LaCoO 3[END_REF][START_REF] Kozlenko | Temperature-and pressure-driven spin-state transitions in LaCoO 3[END_REF], respectively. XES was employed to explore the increase of spin with *Corresponding author. Email: rodriguf@unican.es temperature in LaCoO 3 as a probe for IS [START_REF] Vankó | Temperature-and pressure-induced spin-state transitions in LaCoO 3[END_REF]. However, the proof of IS on the basis of an S = 1 state in Boltzmann equilibrium with a S = 0 state, as deduced from magnetic measurements [START_REF] Kozlenko | Temperature-and pressure-driven spin-state transitions in LaCoO 3[END_REF], is doubtful since the magnetic data can also be interpreted in terms of the 5 T 2 HS state split by the spin-orbit and trigonal crystal field in LaCoO 3 [START_REF] Ropka | 5 D term origin of the excited triplet in LaCoO 3[END_REF]. High-resolution X-ray diffraction experiments were performed in order to prove the involvement of the IS state in the magnetic properties of LaCoO 3 by measuring the distortion of the CoO 6 octahedron due to the E ⊗ e Jahn-Teller (JT) effect associated with IS ( 3 T) [START_REF] Maris | Evidence for orbital ordering in LaCoO 3[END_REF]. It is shown that CoO 6 exhibits a slight distortion with three different Co-O bond distances: 1.87, 1.93 and 1.99 Å at T = 295 K. The distortion parameter of the octahedron due to the JT effect, defined as [START_REF] Rodríguez | Correlations between structure and optical properties in Jahn-Teller Mn 3+ fluorides: A study of TlMnF 4 and NaMnF 4 under pressure[END_REF], is ρ = 0.12 Å. But such a distortion seems to be very small for a E ⊗ e JT effect if we take into account well known distortions in E ⊗ e systems such as LaMnO 3 with an averaged four short Mn-O distances of 1.94 Å and two long Mn-O of 2.18 Å (ρ ≈ Q θ = 0.24 Å) [START_REF] Rodríguez-Carvajal | Neutron-diffraction study of the Jahn-Teller transition in stoichiometric LaMnO 3[END_REF] and the big JT effect (E ⊗ e) required to stabilize the IS state (E JT ≈ e /4 ≈ 0.6 eV/Co 3+ ) [14].

ρ = [Q 2 θ + Q 2 ε ] 1/2
In this work we investigate the electronic structure of Co 3+ -doped LaAlO 3 and CoF 3 by optical spectroscopy under pressure. The aim is to obtain the crystal-field spectra and particularly the orbital splitting related to the JT distortion for different electronic ground states: weakly T ⊗ e for HS ( 5 T 2 ) and strongly E ⊗ e (plus T ⊗ e) for the supposedly low-lying first-excited state IS ( 3 T 1,2 ).

Experimental

CoF 3 was obtained from Sigma-Aldrich and Co-doped LaAlO 3 crystals were synthesized following the procedures indicated elsewhere [START_REF] Sanz-Ortiz | Synthesis under pressure and characterizations through optical spectroscopy of Jahn-Teller cations (LS Ni 3+ , IS Co 3+ ) as probes diluted in a perovskite matrix[END_REF]. Due to the high reactivity of CoF 3 in air, silicone oil and paraffin, special care was paid for handling and loading the diamond anvil cell (DAC) in Argon atmosphere using a glove box.

The optical spectra of Co-doped LaAlO 3 were obtained in a Cary 6000i spectrophotometer operating directly in reflectance mode and in absorption mode, using KBr-diluted sample pellets (12.5 mol%) in the latter case. The absorption spectra of CoF 3 were directly obtained by compacting CoF 3 powders with no hydrostatic medium in the DAC together with several ruby chips homogeneously distributed in the gasket hole. This procedure guarantees a full absorption measurement from the sample. The experimental setup is a homemade single beam microspectrometer (Nikon microscope with an output optical fibre attached to a HR-320 Jobin-Yvon monochromator), using a SR440 lock-in amplifier and suitable Hamamatsu InGaAs and R928 photomultiplier detectors for synchronous detection.

Results and discussion

Figure 1 shows the three possible spin states for Co 3+ and the corresponding one-electron level diagram taking into account the strength of the JT effect associated with each state. For illustration purposes, Figure 1 also shows the absorption spectra corresponding to trivalent transition-metal ions exhibiting JT distortions of T ⊗ e (Cs 2 NaCoF 6 ) and E ⊗ e (Na 2 MnF 5 ), as well as others having regular octahedron coordination (Rb 2 KCrF 6 ). The observed absorption bands correspond to electronic transitions between the octahedral e g and t 2g orbitals split in a different way depending on the strength of the JT distortion. For a strong E ⊗ e JT effect, the spectrum of Na 2 MnF 5 consists of three bands located in the visible range at 1.55, 2.23 and 2.62 eV, consistently with a JT distortion of Q θ = 0.29 Å [START_REF] Rodríguez | Correlations between structure and optical properties in Jahn-Teller Mn 3+ fluorides: A study of TlMnF 4 and NaMnF 4 under pressure[END_REF]. They correspond to spin-allowed electronic transitions between one-electron orbitals e g and t 2g , split both by the octahedral crystal field and the low symmetry distortion due to the JT effect [START_REF] Rodríguez | Correlations between structure and optical properties in Jahn-Teller Mn 3+ fluorides: A study of TlMnF 4 and NaMnF 4 under pressure[END_REF]. The JT stabilization energy is given by 1/4 of the e g splitting: E JT = 1/4 e = 0.39 eV/Mn 3+ for e = 1.55 eV in Na 2 MnF 5 . The corresponding t 2g splitting, t = 0.39 eV, can be obtained from the two bands at higher energy and corresponds to approximately 1/4 e . The situation for Co 3+ (T ⊗ e) is different. The one-electron diagram is basically the same for Co 3+ and Mn 3+ but the distortion is smaller for Co 3+ due to the weaker T ⊗ e coupling with respect to E ⊗ e. The first transition energy for d 6 corresponds to t and the difference between the other two transitions, which are those usually observed in VIS-NIR, gives e . The two bands observed in HS Co 3+ fluorides, actually correspond to the t 2g → e g transition split by the JT effect (T ⊗ e) and eventually by the crystal anisotropy [START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF]. It is worth noting that in an eventual IS ground state the optical spectrum of Co 3+ (band structure and transition energies) would be similar to the spectrum of Mn 3+ in HS for a similar low-symmetry distortion associated with E ⊗ e. In fact, the splitting of the absorption band for Cs 2 NaCoF 6 and CsCoF 4 directly gives e = 0.3 and 1.0 eV, respectively (Figure 2). The latter splitting is smaller than e for Mn 3+ in Na 2 MnF 5 and NaMnF 4 [START_REF] Rodríguez | Correlations between structure and optical properties in Jahn-Teller Mn 3+ fluorides: A study of TlMnF 4 and NaMnF 4 under pressure[END_REF] thus indicating that distortions are significantly shorter in Co 3+ , as corresponds to the weakly T ⊗ e JT effect [START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF][START_REF] Rodríguez | Correlations between structure and optical properties in Jahn-Teller Mn 3+ fluorides: A study of TlMnF 4 and NaMnF 4 under pressure[END_REF]. Q θ (and ρ) values in Cs 2 NaCoF 6 , CsCoF 4 [START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF] and Na 2 MnF 5 , NaMnF 4 [START_REF] Rodríguez | Correlations between structure and optical properties in Jahn-Teller Mn 3+ fluorides: A study of TlMnF 4 and NaMnF 4 under pressure[END_REF] are 0.08, 0.18, 0.29 and 0.38 Å, respectively. These distortions are in agreement with their corresponding e values as they are obtained by the relationship that links the JT distortion,

ρ = [Q 2 θ + Q 2 ε ] 1/2
, and the tetragonal splitting e , as:

e = 5.1ρ, ( 1
)
where e is given in eV and ρ in Å; ρ being positive or negative for elongated or compressed distortions, respectively [START_REF] Rodríguez | Correlations between structure and optical properties in Jahn-Teller Mn 3+ fluorides: A study of TlMnF 4 and NaMnF 4 under pressure[END_REF]. Such a correlation applies also for Mn 3+ in epidote (ρ = 0.28 Å) [START_REF] Burns | Mineralogical Applications of Crystal Field Theory[END_REF], and is obeyed for Co 3+ fluorides as well (Figure 1) [START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF]. It must be noted that in CrF 3- 6 the first absorption band does not show any structure related to the JT effect as it corresponds to an orbital singlet ground state ( 4 A 2 ) with ρ = 0 Å, and CF = 1.9 eV for R Cr-F = 1.89 Å (Figure 1) [START_REF] Hernández | Opto-structural study of A 2 BMF 6 : Cr 3+ elpasolite crystals. Variation of the (CrF 6 ) 3complex optical properties with Cr-F bond distance, hydrostatic pressure and excited-state crossover[END_REF]. [START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF][START_REF] Allen | The electronic spectra of the hexafluoro complexes of the first transition series[END_REF]. The spectra show one absorption band with two components located at 1.53 and 1.83 eV in Cs 2 NaCoF 6 [START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF], 1.39 and 1.81 in Li 3 CoF 6 [START_REF] Allen | The electronic spectra of the hexafluoro complexes of the first transition series[END_REF], 1.17 and 2.18 eV in CsCoF 4 [START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF], and 1.45 and 1.89 eV in the reported CoF 3 . The absorption bands are related to crystal-field transitions of Co 3+ from the HS ground state ( 5 T 2 ) to the excited state 5 E split by the JT distortion (T ⊗ e). Note that the splitting in CoF 3 ( e = 0.45 eV) is similar to that measured in Li 3 CoF 6 (0.42 eV) [START_REF] Allen | The electronic spectra of the hexafluoro complexes of the first transition series[END_REF] or Cs 2 NaCoF 6 (0.30 eV) but lower than in CsCoF 4 (1.01 eV). As indicated in [START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF], besides the JT effect, CsCoF 4 provides an additional contribution to e due to crystal anisotropy [START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF]. The CoF 3 spectrum is consistent with a HS ( 5 T 2 ) ground state, which experiences a significant JT effect yielding a e g -splitting e similar to that attained in other Co 3+ fluorides. From Equation 1, we estimate a JT distortion: ρ ≈ -0.09 Å.

A similar band structure is observed in Co 3+ -doped LaAlO 3 at ambient conditions (Figure 2). The bands at 1.74 and 2.52 eV are consistent with e = 0.8 eV and CF = 2.1 eV [START_REF] Sanz-Ortiz | Synthesis under pressure and characterizations through optical spectroscopy of Jahn-Teller cations (LS Ni 3+ , IS Co 3+ ) as probes diluted in a perovskite matrix[END_REF]. The crystal-field splitting CF indicates that the system is beyond the SCO (1.8 eV) [START_REF] Aguado | Pressure-induced Jahn-Teller suppression and simultaneous high-spin to low-spin transition in the layered perovskite CsMnF 4[END_REF], and thus the electronic ground state must correspond to the LS 1 A 1 state as was confirmed by magnetic measurements. Co 3+ is non-magnetic (LS: S = 0) at low temperatures and its magnetic moment increases with temperature due to thermal population of either IS (S = 1) or HS (S = 2) states. A similar behaviour was observed in the neat compound LaCoO 3 [START_REF] Kozlenko | Temperature-and pressure-driven spin-state transitions in LaCoO 3[END_REF]. Nevertheless, the spectroscopic data reveal that the thermally populated state corresponds to the HS state ( 5 T 2 ) rather than the IS state ( 3 T 1,2 ). The latter state would exhibit a strong E ⊗ e JT effect yielding e splitting similar to, or larger than, the one attained in Mn 3+ in fluorides [START_REF] Rodríguez | Correlations between structure and optical properties in Jahn-Teller Mn 3+ fluorides: A study of TlMnF 4 and NaMnF 4 under pressure[END_REF][START_REF] Aguado | Pressure-induced Jahn-Teller suppression and simultaneous high-spin to low-spin transition in the layered perovskite CsMnF 4[END_REF] or in Ni 3+ in oxides [START_REF] Sanz-Ortiz | Synthesis under pressure and characterizations through optical spectroscopy of Jahn-Teller cations (LS Ni 3+ , IS Co 3+ ) as probes diluted in a perovskite matrix[END_REF]. Therefore, the present results indicate that the HS state lies above the LS state, in agreement with conclusions given elsewhere [START_REF] Ropka | 5 D term origin of the excited triplet in LaCoO 3[END_REF]. The variation of the CoF 3 spectra with pressure through the SCO clarifies this behavior. Figure 3 shows the pressure dependence of the absorption spectra of CoF 3 . The JT-related splitting ( e ) is observed in the explored pressure range. The band at 1.89 eV experiences a significant blue shift of 0.2 eV as the pressure increases from 0 to 20 GPa according to an increase of the crystal-field strength, while the band at 1.45 eV sustains only a slight shift with pressure, showing that both CF and e increase with pressure. This result is not surprising since e can increase with pressure even if, as shown elsewhere [START_REF] Aguado | Pressure effects on NaMnF 4 : Structural correlations and Jahn-Teller effect from crystal-field spectroscopy[END_REF], the JT distortion, ρ, reduces with pressure. The increase of CF and e as 0.009 and 0.005 eV/GPa, respectively, and the band enhancement at 1.9 eV above 10 GPa suggest that HS-LS crossover already occurs around this pressure. These results confirm the stability of the JT distortion with pressure for Mn 3+ (E ⊗ e) and Co 3+ (T ⊗ e) in fluorides as well as the pressure-induced HS to LS SCO.

Figure 1 .

 1 Figure 1. (Left) Schematic representation of the energy for the three possible spin states of six-coordinated Co 3+ . The one-electron level diagram includes the JT effect for each state. Arrows indicate the transitions corresponding to the spectra on the right side. (Right) Absorption spectra of Co 3+ taken from [5], Mn 3+ [12] and Cr 3+ [16] exhibiting JT distortions of T ⊗ e (Cs 2 NaCoF 6 ) and E ⊗ e (Na 2 MnF 5 ), and regular octahedron coordination (Rb 2 KCrF 6 ).

Figure 2 .

 2 Figure 2. Absorption spectrum of Co 3+ -doped LaAlO 3 at ambient conditions (left) and Cs 2 NaCoF 6 [5], CsCoF 4 [5] and CoF 3 [present work]. The arrows in the one-electron level diagram indicate the observed transitions.

Figure 2

 2 Figure2shows the absorption spectra of Co 3+ -doped LaAlO 3 and several Co 3+ fluorides. The absorption bands of Co 3+ -doped LaAlO 3 and CoF 3 are mounted on an intense background due to the light scattering in compressed powders. Background subtraction allows us to separate the two contributions and obtain the absorption bands due to Co 3+ . Their position and structure are similar to those observed in other Co 3+ fluorides[START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF][START_REF] Allen | The electronic spectra of the hexafluoro complexes of the first transition series[END_REF]. The spectra show one absorption band with two components located at 1.53 and 1.83 eV in Cs 2 NaCoF 6[START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF], 1.39 and 1.81 in Li 3 CoF 6[START_REF] Allen | The electronic spectra of the hexafluoro complexes of the first transition series[END_REF], 1.17 and 2.18 eV in CsCoF 4[START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF], and 1.45 and 1.89 eV in the reported CoF 3 . The absorption bands are related to crystal-field transitions of Co 3+ from the HS ground state ( 5 T 2 ) to the excited state 5 E split by the JT distortion (T ⊗ e). Note that the splitting in CoF 3 ( e = 0.45 eV) is similar to that measured in Li 3 CoF 6 (0.42 eV)[START_REF] Allen | The electronic spectra of the hexafluoro complexes of the first transition series[END_REF] or Cs 2 NaCoF 6 (0.30 eV) but lower than in CsCoF 4 (1.01 eV). As indicated in[START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF], besides the JT effect, CsCoF 4 provides an additional contribution to e due to crystal anisotropy[START_REF] Reinen | The Jahn-Teller effect in octahedral T Ground states -an experimental and DFT study on MIIIX6 polyhedra [MIII: Ti, V, Co; X: Cl, F] in various solids[END_REF]. The CoF 3 spectrum is consistent with a HS ( 5 T 2 ) ground state, which experiences a significant JT effect yielding a e g -splitting e similar to that attained in other Co 3+ fluorides. From Equation1, we estimate a JT distortion: ρ ≈ -0.09 Å.A similar band structure is observed in Co 3+ -doped LaAlO 3 at ambient conditions (Figure2). The bands at 1.74 and 2.52 eV are consistent with e = 0.8 eV and CF = 2.1 eV[START_REF] Sanz-Ortiz | Synthesis under pressure and characterizations through optical spectroscopy of Jahn-Teller cations (LS Ni 3+ , IS Co 3+ ) as probes diluted in a perovskite matrix[END_REF]. The crystal-field splitting CF indicates that the system is beyond the SCO (1.8 eV)[START_REF] Aguado | Pressure-induced Jahn-Teller suppression and simultaneous high-spin to low-spin transition in the layered perovskite CsMnF 4[END_REF], and thus the electronic ground state must correspond to the LS 1 A 1 state as was confirmed by magnetic measurements. Co 3+ is non-magnetic (LS: S = 0) at low temperatures and its magnetic moment increases with temperature due to thermal population of either IS (S = 1) or HS (S = 2) states. A similar behaviour was observed in the neat compound LaCoO 3[START_REF] Kozlenko | Temperature-and pressure-driven spin-state transitions in LaCoO 3[END_REF]. Nevertheless, the spectroscopic data reveal that the thermally populated state corresponds to the HS state ( 5 T 2 ) rather than the IS state ( 3 T 1,2 ). The latter state would exhibit a strong E ⊗ e JT effect yielding e splitting similar to, or larger than, the one attained in Mn 3+ in fluorides[START_REF] Rodríguez | Correlations between structure and optical properties in Jahn-Teller Mn 3+ fluorides: A study of TlMnF 4 and NaMnF 4 under pressure[END_REF][START_REF] Aguado | Pressure-induced Jahn-Teller suppression and simultaneous high-spin to low-spin transition in the layered perovskite CsMnF 4[END_REF] or in Ni 3+ in oxides[START_REF] Sanz-Ortiz | Synthesis under pressure and characterizations through optical spectroscopy of Jahn-Teller cations (LS Ni 3+ , IS Co 3+ ) as probes diluted in a perovskite matrix[END_REF]. Therefore, the present results indicate that the HS state lies above the LS state, in agreement with conclusions given elsewhere[START_REF] Ropka | 5 D term origin of the excited triplet in LaCoO 3[END_REF].

Figure 3 .

 3 Figure 3. Variation of the absorption spectra of CoF 3 with pressure at room temperature. The bands are obtained after background subtraction.

Acknowledgements

Financial support from the Spanish MEC (Project No MAT2005-00099) and MALTA INGENIO-CONSOLIDER 2010 (Ref. CDS2007-0045) are acknowledged. MNSO thanks the Spanish MEC for a FPU grant (Ref. AP-2004-5954).


Gérard Demazeau
email: demazeau@icmcb-bordeaux.cnrs.fr

High pressures and chemical reactivity: an approach to the synthesis of novel materials

Keywords: high pressures, reaction mechanisms, chemical nature of precursors, experimental conditions

In material synthesis, high pressures are characterized by different factors: the weak conveyed energy compared to the temperature; the negative V value (if V is the volume difference between the final unit-cell volume of the resulting material and the sum of the unit-cell volumes of the precursors); the 'in situ' improvement of the chemical reactivity inducing rapid reaction kinetics (such an improvement being observed in particular in the presence of a liquid phase as solvothermal processes). In addition, the reaction mechanisms and the chemical reactivity of the precursors can play an important role in highpressure synthesis. Consequently, in order to develop new high-pressure research domains involving novel materials, careful selection of either the reaction route or the chemical nature of precursors appears as a fruitful approach for decreasing the pressure and temperature conditions required for the synthesis.

In particular, such an approach is important for the synthesis of novel materials characterized by specific physical properties (hardness, magnetic or electrical properties, etc.) able to lead to industrial developments.

Introduction

Material synthesis, through processes involving different precursors as solid phases, very often requires high temperatures in order to activate the chemical diffusion of reactive species. Such a requirement can be considered as a negative factor, in particular, in two cases:

• the preparation of low-temperature structural forms;

• the stabilization of metastable materials. Different strategies have been set-up in order to bypass the use of high temperatures: in particular, the development of high pressures and the selection of either appropriate reaction mechanisms or the chemical nature of precursors.

This paper presents an analysis of each strategy and some illustrative examples.

Synthesis of novel materials under high pressures

The application of high pressures is characterized by three main factors:

• the small energy developed by pressure in liquid or solid phases (such energy being directly correlated to the compressibility of the medium submitted to pressure); • the negative V value, if V is defined as

V = V j Z j final materials - V i Z i precursors ,
where V /Z corresponds to the volume per unit formula with (j ) for the products and (i) for the reagents; • the improvement of the chemical reactivity.

The energy conveyed by high pressures in solid-state synthesis or in reaction processes involving a liquid phase (as solvothermal processes) is small, and such a parameter can be negligible compared to temperature effects.

The chemical reactivity seems to be improved due to the observed kinetic effects during solidstate synthesis (e.g., using Fe 2 O 3 and La 2 O 3 as precursors at a constant temperature (1000 • C), 70 h is required for obtaining a pure perovskite-phase LaFeO 3 at normal pressure (10 5 Pa) and only 5 min using high pressures close to 5 GPa [START_REF] Goodenough | Preparative methods in solid state chemistry[END_REF]). High pressures -in a first analysis -could appear as an expansive technique, but due to the low energy conveyed by the pressure parameter, the running cost is small compared to the conventional ceramic processes. The investment is important but the certified number of runs is now improved.

During these last 50 years, high pressures have been developed for preparing different new materials [START_REF] Demazeau | Materials chemistry under high pressures -some recent aspects[END_REF][START_REF] Mcmillan | High pressure synthesis of solids[END_REF][START_REF] Brazhkin | High pressure synthesized materials: treasures and hints[END_REF]. The relevant question could be, in the near future: how to combine pressure as a thermodynamic variable with the chemical reactivity for improving the domains of materials synthesis?

Two main strategies have been developed for associating high pressures and chemical reactivity:

• the development of specific media or chemical processes under high pressures;

• the selection of the chemical nature of precursors through low-temperature preparation processes or specific pretreatments (as mechanical milling).

Material synthesis using specific reactive media or chemical processes under highpressure conditions

During the last 30 years, solvothermal reactions were developed, the use of specific solvents (aqueous or non-aqueous) improving the chemical reactivity either in a homogeneous system or a heterogeneous one in sub-or supercritical conditions [START_REF] Demazeau | Solvothermal reactions: an original route for the synthesis of novel materials[END_REF]. Solvothermal reactions are characterized by a chemical reaction between precursors in a close system in the presence of a specific solvent and at a temperature higher than the boiling temperature of that solvent. Consequently, pressure is involved as autogenous pressure (if the pressure value is regulated both by the percentage of filling for the high-pressure vessel and the selected temperature) or as imposed pressure using at the beginning of the solvent under high pressure.

Solvothermal reactions can help the synthesis of novel materials with a specific structure (as the phyllosiloxides isostructural of the natural phyllosilicates), or can orientate the formation of metastable materials if the synthesis is kinetically controlled (as the stabilization of the metastable structural forms of MnS versus the physico-chemical properties of the solvent).

Solvothermal synthesis of the phyllosiloxides

The origin of this research was to find layered materials able to be developed as interphases between fiber and matrix in order to deflect cracks from mode 1 (perpendicular to the fiber) to mode 2 (parallel to the fiber) and able to be used at high temperature (T > 1000 • C) in an oxygen atmosphere [START_REF] Reig | Les phyllosiloxydes: de nouveaux matériaux d'interphase lamellaires tout-oxyde pour matériaux composites[END_REF]. Graphite and hexagonal BN, being oxygen sensitive in such a temperature range, and the thermal stability of phyllosilicates (as mica) being reduced due to the presence of hydroxyl groups [2OH -→ H 2 O (vapor)+ +O 2-] leading to the formation of anionic vacancies, a new family of layered oxides derived from natural phyllosilicates was prepared.

Using mica with the phlogopite structure as a structural model, a new composition was set-up through different substitutions: anionic (OH -→ O 2-) and cationic (Al 3+ → Si 4+ in T d sites and Mg 2+ → Al 3+ in O h sites) (Figure 1).

In order to synthesize such a proposed complex composition, a homogeneous precursor has been prepared through a sol-gel process. In a second step, this precursor (aerogel) has been treated using a conventional solid-state chemistry process (500 • C < T < 1000 • C). X-ray diffraction patterns of the resulting materials confirm that only a mixture of 3D silicates is formed.

Through a solvothermal process, using as solvent the same as that for preparing the sol (2-methoxy-ethanol) and the following experimental conditions (50 < P < 150 MPa, T = 600 • C, t = 54 h), a single-phase isostructural of mica with the phlogopite structure is observed through X-ray diffraction analysis. The physical characterizations (IR spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) confirm both the absence of hydroxyl group into the lattice and the lamellar structure of this material (Figure 2).

Consequently, using a solvothermal process, a new family of 2D oxides, isostructural of natural phyllosilicates, has been prepared for the first time [START_REF] Reig | A new class of bidimensional oxides: the phyllosiloxides[END_REF][START_REF] Reig | KMg 2 AlSi 4 O 12 phyllosiloxyde as potential interphase material for ceramic matrix composites. Part 1: chemical compatibility[END_REF].

Stabilization of metastable materials

The stabilization of metastable materials is important for enlarging the knowledge in material chemistry [START_REF] Brazhkin | Metastable phases and "metastable" phase diagrams[END_REF]. When the synthesis is kinetically controlled, metastable materials can be stabilized. Using MnCl 2 , 4H 2 O and SC(NH 2 ) 2 as precursors, the solvothermal preparation of MnS with the stable α-rocksalt form or the metastable β and γ forms (zinc-blende and wurtzite structures) is closely dependent on the chemical nature of the solvent.

When the solvent (water or ethylenediamine) is able to form a stable Mn 2+ complex, the stable α-rocksalt structure is observed. When the solvent is THF or benzene, the β zinc-blende structure and the γ -wurtzite structure were, respectively, stabilized [START_REF] Lu | Metastable MnS crystallites through solvothermal synthesis[END_REF].

Such an example underlines the role of the chemical mechanisms governing the solvothermal preparation process on the stabilization of metastable or stable structural forms for the same chemical composition.

The decomposition temperature of SC(NH 2 ) 2 (180 • C) as S 2-source being relatively high, a solvothermal reaction in lower temperature conditions (60-130 • C) using different precursors (thioacetamide and Mn(CH 3 COO) 2 ) and water as solvent has led to the formation of the γ -MnS form [START_REF] Zhang | Low-temperature hydrothermal synthesis of pure metastable γ -manganese sulfide (MnS) crystallites[END_REF]. Such a result would underline that the temperature would be a predominant parameter, compared to the nature of the solvent (ability to form a complex with Mn 2+ ), on the stabilization of a metastable structural form for MnS.

Material synthesis through chemical reactive precursors

High pressures due to the small energy involved in material chemistry are, in different cases, not sufficient for inducing the synthesis. Consequently, the improvement of the chemical reactivity for the precursors appears to be a possible strategy for improving material synthesis under high-pressure conditions. For illustrating such a strategy, several methods have been developed involving, in particular, either the preparation method of the precursor(s) or different characteristics of the reagent(s) (texturation, structure, etc.).

Synthesis of different compounds using precursors obtained by wet chemistry techniques

The perovskites with A cationic ordering (A A 3 )B 4 O 12 exhibit interesting physical properties (magnetism, low-field magnetoresistance, etc.) [START_REF] Shiraki | Ferromagnetic cuprates CaCu 3 Ge 4 O 12 and CaCu 3 Sn 4 O 12 with A-site ordered perovskite structure[END_REF]. CaCu 3 Mn 4 O 12 and other compounds of the (A A 3 )B 4 O 12 family require, using oxides as precursors, in the main cases, high pressures (close to 5 GPa) for obtaining a pure phase [START_REF] Chenavas | The synthesis and crystal structure of CaCu 3 Mn 4 O 12 : a new ferromagnetic-perovskite-like compound[END_REF]. Recently, new derivatives of CaCu 3 Mn 4 O 12 were prepared at a lower pressure (2 GPa) in the same temperature domain (≈1000 • C) using very reactive precursors obtained by wet chemistry procedures (Ca 2+ being replaced by rare earth) [START_REF] Sanchez-Benitez | Preparation under high pressures and neutron diffraction study of new ferromagnetic RCu 3 Mn 4 O 12 (R = Pr[END_REF].

Synthesis of diamond versus the structure of the graphite source

The reactivity of transition metals with carbon plays an important role on the flux-assisted conversion graphite → diamond. The ability for a transition metal to react with carbon increases with its number of electron vacancies in the d-orbitals [START_REF] Sung | Reactivities of transition metals with carbon: implications to the mechanism of diamond synthesis under high pressure[END_REF]. The mechanisms of diamond formation proceed in two steps: nucleation and growth. Graphite structures may determine the rate of diamond nucleation [START_REF] Sung | Kinetics of the graphite to diamond transition under high pressures[END_REF]. In addition, the structural relationship between diamond and graphite may also include the inheritance of texture [START_REF] Sung | Reactivities of transition metals with carbon: implications to the mechanism of diamond synthesis under high pressure[END_REF]. Consequently, the high-pressure synthesis of diamond requires controlling the preparation of the graphite precursor.

Synthesis of LaCuO 3 using a precursor characterized by a specific structure

The stabilization of Cu 3+ in oxygen lattices was obtained at the beginning of the 1970s [START_REF] Demazeau | Sur deux nouvelles phases oxygénées du cuivre trivalent: LaCuO 3 et La 2 Li 0.50 Cu 0.50 O 4[END_REF][START_REF] Goodenough | Sur une nouvelle phase oxygénée du cuivre +III: SrLaCuO 4[END_REF]. For preparing under very high oxygen pressures, LaCuO 3 with the perovskite structure and metallic behavior, La 2 CuO 4 and CuO were used as precursors [START_REF] Demazeau | Sur deux nouvelles phases oxygénées du cuivre trivalent: LaCuO 3 et La 2 Li 0.50 Cu 0.50 O 4[END_REF]. Recently, in order to investigate electronic properties of Cu 3+ in such a LaCuO 3 lattice through Mössbauer spectroscopy, it was necessary to dope LaCuO 3 lattice with 1% 57 Fe. With La 2 CuO 4 and CuO [doped with 57 Fe (1%)] as precursors, and the same experimental conditions that were used for the undoped material (6 GPa, 950 • C in the presence of KClO 3 as an oxygen source) no pure Cu 3+ perovskite was observed.

The preparation of the 57 Fe-doped LaCuO 3 required two steps: (i) the reaction between the precursors and (ii) the copper oxidation (Cu 2+ → Cu 3+ ). A new precursor, La 2 Cu 2 O 5 , was selected in order to reduce the preparation process to the second step. La 2 Cu 2 O 5 can adopt two different structural forms: orthorhombic [START_REF] Poeppelmeier | Structure determination of CaMnO 3 and CaMnO 2.5 by X-ray and neutrons methods[END_REF] and monoclinic [START_REF] Karppinen | Control on the copper valence and properties by oxygen content adjustment in the LaCuO 3-y system (0 = y = 0.5)[END_REF]. LaCu 2 O 5 (doped with 57 Fe (2%)) was prepared with the two structural forms. Using oxygen pressures in a Walker-type module apparatus (using KClO 3 as oxygen source), only the orthorhombic form is transformed to pure LaCuO 3 : 1% 57 Fe (11.5 GPa, 900 • C) [START_REF] Demazeau | A novel preparation process for LaCuO 3 involving a high pressure oxidation of precursor derived from the perovskite structure: an appropriate route for doping with 57 Fe Mössbauer probe[END_REF]. Such a result can be explained through the specific structure of the orthorhombic form La 2 Cu 2 O 5 isomorphous with Ca 2 Mn 2 O 5 . Cu 2+ occupying a (CuO 5 ) square pyramid in such a structure. The transformation (La 2 Cu 2 O 5 : 57 Fe→ 2 LaCuO 3 : 57 Fe) is associated with a high-pressure diffusion of oxygen into the bulk of the orthorhombic matrix [(CuO 5 ) + 1 2 O 2 → (CuO 6 )]. Using La 2 Cu 2 O 5 : 57 Fe with a monoclinic structure, characterized by two different Cu 2+ environments (square planar and octahedral) and with the same experimental P , T conditions, no pure perovskite phase was observed.

Consequently, the structure of the precursor, and in particular the repartition of oxygen vacancies, can play an important role for preparing the target material under high pressures.

Conclusions

High pressure is an important tool for preparing new materials. Two factors play a crucial role: (1) the negative V value, and (2) the increase of the chemical reactivity. Nevertheless, in order to enlarge the application domain of high pressures, chemical reactivity must be improved through (i) the development of specific media or chemical processes under high pressures and/or (ii) the selection of the chemical nature (in particular, the preparation method) or the structural form of the precursors.

Figure 1 .

 1 Figure 1. Proposal for a new layered oxide, isostructural of mica with the phlogopite structure (KMg 3 (AlSi 3 )O 10 OH 2 ), through the anionic substitution (OH -→ O 2-) compensated by cationic substitutions either on the T d sites (Al 3+ → Si 4+ ) or O h sites (Mg 2+ → Al 3+ ).

Figure 2 .

 2 Figure 2. Physico-chemical characterizations of the single phase issued from solvothermal synthesis: (a) comparison of the X-ray diffraction patterns; (b) comparison of the IR spectra; (c) SEM of the single phase and (d) TES.