Sam Ng

Pablo A Valdes

Sylvie Moritz-Gasser

Anne-Laure Lemaitre

Hugues Duffau

Guillaume Herbet
email: guillaume.herbet@umontpellier.fr

Guillaume Herbet Bâtiment

Marc Bloch

Intraoperative functional remapping unveils evolving patterns of cortical plasticity

Keywords: plasticity, recurrent glioma, direct electrostimulation mapping, awake surgery, cognitive mapping, cortical rearrangement

The efficiency with which the brain reorganizes following injury not only depends on the extent and the severity of the lesion, but also on its temporal features. It is established that diffuse lowgrade gliomas (DLGG), brain tumours with a slow-growth rate, induce a compensatory modulation of the anatomo-functional architecture, making this kind of tumours an ideal lesion model to study the dynamics of neuroplasticity. Direct electrostimulation (DES) mapping is a well-tried procedure used during awake resection surgeries to identify and spare cortical epicenters which are critical for a range of functions. Because DLGG is a chronic disease, it inevitably relapses years after the initial surgery, and thus requires a second surgery to reduce tumour volume again. In this context, contrasting the cortical mappings obtained during two sequential neurosurgeries offers a unique opportunity to both identify and characterize the dynamic (i.e. re-evolving) patterns of cortical re-arrangements.

Here, we capitalized on an unprecedented series of 101 DLGG patients who benefited from two DES-guided neurosurgeries usually spaced several years apart, resulting in a large DES dataset of 2082 cortical sites. All sites (either non-functional or associated with language, speech, motor, somatosensory and semantic processing) were recorded in the Montreal Neurological Institute (MNI) space. Next, we used a multi-step approach to generate probabilistic neuroplasticity maps that reflected the dynamic rearrangements of cortical mappings from one surgery to another, both at the population and individual-level.

Voxel-wise neuroplasticity maps revealed regions with a relatively high potential of evolving reorganizations at the population level, including the supplementary motor area (SMA, pmax = 0.63), the dorsolateral prefrontal cortex (dlPFC, pmax = 0.61), the anterior ventral premotor cortex (vPMC, pmax=0.43) and the middle superior temporal gyrus (STG pmax= 0.36). Parcelwise neuroplasticity maps confirmed this potential for the dlPFC (Fisher's exact test, pFDRcorrected = 6.6 e-5 ), the anterior (pFDR-corrected = 0.0039) and the ventral precentral gyrus (pFDR-corrected

Introduction

Neuroplasticity refers to the unique capacity of the brain to adapt its networks in response to experience or cognitive demands to maintain an optimal level of interactions with the everevolving internal or external environment. [START_REF] Mesulam | From sensation to cognition[END_REF][START_REF] Buonomano | Cortical plasticity: From Synapses to Maps[END_REF] Neuroplasticity can occur at multiple spatiotemporal scales, from fast remodelling of neurosynaptic maps to long-term rewiring of long-range white matter connections. [START_REF] Sampaio-Baptista | White Matter Plasticity in the Adult Brain[END_REF] From a behavioural standpoint, this neural malleability is mirrored in the human's capacity to learn an impressive array of new skills and to produce creative behaviours. For example, it is established that acquiring a new language [START_REF] Li | Neuroplasticity as a function of second language learning: Anatomical changes in the human brain[END_REF] or learning a new musical instrument [START_REF] Herholz | Musical Training as a Framework for Brain Plasticity: Behavior, Function, and Structure[END_REF] causes measurable structural and functional brain variations. [START_REF] Zatorre | Plasticity in gray and white: neuroimaging changes in brain structure during learning[END_REF] In the pathophysiological domain, neuroplasticity refers to the brain's ability to reorganize following structural damage (e.g. stroke, tumour, traumatic brain injury) in an attempt to maintain or re-establish normal function. Postlesional neuroplasticity has been the subject of numerous studies, especially in the context of acute lesions such as stroke or traumatic brain injury, with the objective of identifying the different strategies that the brain deploys to compensate for neuronal losses and the factors predictive of functional recovery. [START_REF] Rocha | Recovery of neural dynamics criticality in personalized whole-brain models of stroke[END_REF][START_REF] Salvalaggio | Post-stroke deficit prediction from lesion and indirect structural and functional disconnection[END_REF][START_REF] Stockert | The 2021 WHO Classification of Tumors of the Central Nervous System: a summary[END_REF] Over different lesion models, diffuse low-grade gliomas (WHO grade II diffuse glioma 10 , DLGG), a subgroup of primary brain tumours with a slow-growth rate, have been highlighted as an exemplary model for studying neuroplasticity. [START_REF] Duffau | Lessons from brain mapping in surgery for low-grade glioma: insights into associations between tumour and brain plasticity[END_REF][START_REF] Duffau | The huge plastic potential of adult brain and the role of connectomics: New insights provided by serial mappings in glioma surgery[END_REF] At the cellular level, the dynamic interplay between glioma cells and their local microenvironment results in complex electrical and synaptic integration into neural circuits [START_REF] Venkataramani | Glutamatergic synaptic input to glioma cells drives brain tumour progression[END_REF][START_REF] Venkatesh | Electrical and synaptic integration of glioma into neural circuits[END_REF] that promotes long-range modulations of neural signalling and activities. [START_REF] Jung | Emerging intersections between neuroscience and glioma biology[END_REF] At the macroscale level, there is increasing evidence that the slowgrowth kinetics of DLGG favour a dynamic network re-organization, [START_REF] Desmurget | Contrasting acute and slow-growing lesions: a new door to brain plasticity[END_REF][START_REF] Almairac | Homotopic redistribution of functional connectivity in insula-centered diffuse low-grade glioma[END_REF][START_REF] Ng | Precuneal gliomas promote behaviorally relevant remodeling of the functional connectome[END_REF] whose mechanistic principles remain to be characterised. These whole brain neuroplastic modulations may account for the common but remarkable observation that cognitive impairments in DLGG patients are relatively low both before and after neurosurgery, [START_REF] Lemaitre | Cognitive preservation following awake mapping-based neurosurgery for low-grade gliomas: A longitudinal[END_REF] despite sometimes large resections of cortical areas ordinarily considered as "eloquent". 20 As DLGG is a chronic disease, it will inevitably relapse years or even decades after the initial resection, requiring a second surgery to be performed in an attempt to again reduce tumour volume and prevent malignant transformation. [START_REF] Ramakrishna | Outcomes in Reoperated Low-Grade Gliomas[END_REF][START_REF] Shofty | Impact of repeated operations for progressive low-grade gliomas[END_REF] Currently, wide-awake surgery assisted with direct electrostimulation (DES) mapping is the gold standard treatment at diagnosis, as it has been shown to enhance both the extent of resection and life expectancy while drastically decreasing the likelihood of postoperative permanent, debilitating A C C E P T E D M A N U S C R I P T neurological and neuropsychological deficits. [START_REF] Lemaitre | Cognitive preservation following awake mapping-based neurosurgery for low-grade gliomas: A longitudinal[END_REF][START_REF] Witt Hamer | Impact of Intraoperative Stimulation Brain Mapping on Glioma Surgery Outcome: A Meta-Analysis[END_REF] DES mapping is increasingly used at the time of reoperation in the event of DLGG recurrence, as it allows surgical resection to be extended beyond the functional limits identified with DES mapping during the first surgery. [START_REF] Picart | Iterative Surgical Resections of Diffuse Glioma With Awake Mapping: How to Deal With Cortical Plasticity and Connectomal Constraints[END_REF][START_REF] Southwell | Intraoperative mapping during repeat awake craniotomy reveals the functional plasticity of adult cortex[END_REF][START_REF] Ng | Recurrent Low-Grade Gliomas: Does Reoperation Affect Neurocognitive Functioning?[END_REF] The observation that reoperations are possible supports the hypothesis whereby tumour-induced plasticity may continue in a constant-evolving manner, in a co-dependent manner with tumour expansion, resulting in evolving patterns of cortical rearrangements. [START_REF] Duffau | The huge plastic potential of adult brain and the role of connectomics: New insights provided by serial mappings in glioma surgery[END_REF] In this context, contrasting the cortical mappings obtained between two sequential surgeries offers a unique opportunity to characterise the dynamics of cortical reorganisation and to identify the anatomic factors that constrain them.

Here, we took advantage of a unique DES cortical mapping dataset (2082 exploitable stimulation sites) gained from a cohort of 101 patients having undergone two distinct awake surgical procedures usually spaced several years apart. In contrast to alternate methods that can be used to study the dynamics of neuroplasticity, DES offers the clear advantage of causally linking human behaviors and neuroanatomy with a high reproducibility and spatiotemporal resolution. [START_REF] Lu | Functional maps of direct electrical stimulation-induced speech arrest and anomia: a multicentre retrospective study[END_REF][START_REF] Sarubbo | Mapping critical cortical hubs and white matter pathways by direct electrical stimulation: an original functional atlas of the human brain[END_REF][START_REF] Tate | Probabilistic map of critical functional regions of the human cerebral cortex: Broca's area revisited[END_REF] The main objectives of this study were threefold: (i) to investigate the potential of cortical structures to functionally evolve and remodel from one surgery to another under the form of gradients (from lowest to highest potential); (ii) to categorize the topological features of cortical rearrangements as a result of functional responses; (iii) to provide clinically relevant maps that account for the evolving capability of cortical structures to compensate over time.

Materials and methods

Study population

Data processed in this study were obtained in a clinical context and the procedures detailed below follow our standard clinical approach. All patients gave informed consent. Approval for the study was granted by the Institution Review Board of Montpellier University Medical Center (No.202000557).

The population screened for inclusion included all patients operated in our institution by a single surgeon (H.D.) between 2007 and 2021. The inclusion criteria were: (1) a diagnosis of DLGG, confirmed by histopathological and/or molecular analysis, (2) two surgical procedures spaced at least 6 months apart, (3) the use of cortical DES mapping under awake

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conditions in both procedures. Intraoperative reports, photographs, camera recordings and neuroimaging data were consecutively collected and subjected to a retrospective analysis. Of note, only patients who remained free of radiation therapy (a treatment that may diminish the propensity for neuroplastic reorganization) [START_REF] Makale | Mechanisms of radiotherapyassociated cognitive disability in patients with brain tumours[END_REF] were included in the study.

In vivo electrostimulation procedure and behavioural paradigms

The surgical procedure was extensively described elsewhere [START_REF] Duffau | New Philosophy, Clinical Pearls, and Methods for Intraoperative Cognition Mapping and Monitoring "à la carte" in Brain Tumor Patients[END_REF][START_REF] Sanai | Functional Outcome after Language Mapping for Glioma Resection[END_REF] and the technique remained unchanged from Surg.1 to Surg.2. Briefly, patients systematically underwent an asleep-awakeasleep procedure. DES cortical mapping was performed during the awake phase, after craniotomy and dura (re-)opening. Electrical mapping was performed with a bipolar probe (inter-tip spacing: 5mm, NIMBUS stimulator, Newmedic, France) delivering a biphasic current (1ms pulse width, 60Hz, amplitude from 1.50mA to 3.50mA). DES never exceeded 4s to limit electrical spreading and to maintain anatomo-functional specificity. The amplitude was gradually increased up until functional responses (i.e., transient speech disorders) were elicited within the ventral premotor cortex (vPMc). Then, the same amplitude was maintained constant during the whole procedure. Cortical sites were considered as positive ones if they elicited an inability to perform at least one of the intraoperative tasks within the 4s time delay, in a reproducible way, during at least three non-consecutive stimulations (see Figure 1A). Cortical sites that did not fulfil the criteria for a positive response were considered as negative ones.

After completion of DES cortical mapping, the tumour removal was performed using subpial dissection. The resection was achieved until DES subcortical responses were obtained (following the principle of functionally guided tumour removal). [START_REF] Tate | Probabilistic map of critical functional regions of the human cerebral cortex: Broca's area revisited[END_REF] Intraoperative tasks included upper limb movements, number counting (1-10), a picture naming task, a non-verbal semantic association task when tumours involved the right hemisphere (Pyramids and Palm Trees test). [START_REF] Herbet | Electrical stimulation of the dorsolateral prefrontal cortex impairs semantic cognition[END_REF][START_REF] Moritz-Gasser | Mapping the connectivity underlying multimodal (verbal and non-verbal) semantic processing: A brain electrostimulation study[END_REF] Other cognitive tasks (e.g., line bisection task) were occasionally used [START_REF] Duffau | Constant Multi-Tasking With Time Constraint to Preserve Across-Network Dynamics Throughout Awake Surgery for Low-Grade Glioma: A Necessary Step to Enable Patients Resuming an Active Life[END_REF][START_REF] Ng | White matter disconnectivity fingerprints causally linked to dissociated forms of alexia[END_REF][START_REF] Yordanova | Combining resting state functional MRI with intraoperative cortical stimulation to map the mentalizing network[END_REF] depending on the location of the tumour and the date of the surgery but were not considered in this study. Intraoperative cognitive monitoring was performed by a senior speech therapist (S.M.G.) or neuropsychologist (A.L.L., G.H.) who remained blind to the application of DES during the procedure. Note that each task was administered before surgery in order to decrease the likelihood of false positive responses during intraoperative mapping. The neuropsychologist and/or speech therapist checked whether movement disorders

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were due to contraction/inhibition during each task (or dual tasking). This assessment was performed systematically for the face and the upper limbs.

The following positive (i.e. functional) responses were considered: (1) language impairments (including anomia and paraphasia), (2) speech disorders (including speech arrest and dysarthria), (3) negative motor responses of the upper limb, (4) positive motor responses of the face and upper limb, (5) dysesthesia and ( 6) semantic association disorders.

The above-mentioned intraoperative manifestations were determined as follows: a negative motor response was defined as a complete inhibition of movement without the loss of muscle tone or consciousness; [START_REF] Lüders | Cortical electrical stimulation in humans. The negative motor areas[END_REF][START_REF] Rech | A probabilistic map of negative motor areas of the upper limb and face: a brain stimulation study[END_REF] a positive motor response was defined as the contraction of a muscle or muscle group causing an involuntary movement; [START_REF] Penfield | Somatic Motor and Sensory representation in the Cerebral Cortex of man as studied by Electrical Stimulation[END_REF] speech arrest was defined as a complete discontinuation of the ongoing number counting or continuous speaking, without oral, facial, jaw, or tongue positive movements [START_REF] Sanai | Functional Outcome after Language Mapping for Glioma Resection[END_REF][START_REF] Rech | A probabilistic map of negative motor areas of the upper limb and face: a brain stimulation study[END_REF] (of note, it was not possible to perform a concomitant movement task of the face and thus to strictly distinguish a facial negative motor response from a speech arrest); anomia was defined as an inability to name an object, while still being able to pronounce the words "this is a [picture]" (naming task) [START_REF] Metz-Lutz | Standardisation d'un test de dénomination orale: contrôle des effets de l'âge, du sexe et du niveau de scolarité chez les sujets adultes normaux[END_REF] , thus allowing to rule out a speech arrest manifestation; paraphasia was specifically defined by a misnaming of an object, using the same paradigm; dysarthria was defined as a disorder of articulatory planning and/or speech sound production (word groping, sound distortion, prosodic abnormalities), without oral, facial, jaw, or tongue positive movements; a semantic association disorder was defined as in incorrect response or an inability to respond during the Pyramid and Palm Tree task.

Functional responses in one of these categories at Surg.1 were only considered if the same task(s) were applied at Surg.2. Note that some stimulations could produce concomitant disorders and were further considered in both disorder categories in further analyses (e.g., speech arrest plus negative motor response of the upper limb). Two photographs were done: one at the end of the cortical mapping, and one at the end of the resection. In addition, procedures were recorded with a built-in camera attached to the operating lights.

Neuroimaging processing

T1-weighted and fluid-attenuated inversion recovery (FLAIR) preoperative and postoperative MRI were systematically acquired the day before and at 3-months after both surgeries. MRIs were co-register to the Montreal Neurological Institute (MNI) space using enantiomorphic

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Spatial positioning of DES sites

The MNI coordinates for each stimulation site were determined after inspection of operative reports, intraoperative photographs (positive stimulations) and intraoperative videos (negative sites), as illustrated in Figure 1B and 1C. This method was previously demonstrated to provide an high inter-observer reliability. [START_REF] Lu | Functional maps of direct electrical stimulation-induced speech arrest and anomia: a multicentre retrospective study[END_REF][START_REF] Roux | Combining Electrostimulation With Fiber Tracking to Stratify the Inferior Fronto-Occipital Fasciculus[END_REF] The MRI obtained 3 months after Surg.1 was systematically used as the reference image for spatial positioning, since it allowed a comparison between the post-resection photograph obtained during Surg.1 and the pre-resection photograph obtained during Surg.2. In addition, 3-dimensional pial-mesh reconstructions of the MRI obtained 3 months after Surg.1 were generated with BrainVISA/Anatomist software (Version 5.0, CEA I2BM, CATI Neuroimaging, Inserm IFR49, and CNRS, France) to allow a 3D visualization of individual cortical structures and a semi-automated MNI coordinate-surface matching of the model (Figure 1D). To demonstrate the reproducibility of the procedure used to position cortical sites despite a more challenging identification of gyro-sulcal anatomy at reoperation, [START_REF] Morshed | Reducing complication rates for repeat craniotomies in glioma patients: a single-surgeon experience and comparison with the literature[END_REF] intra-and inter-observer correlation coefficients were computed. More specifically, the first observer positioned two times consecutively 100 stimulation sites randomly selected within the left hemisphere. Pearson correlations were performed between each set of coordinates to derive measures of intra-observer agreement (one for each axis).

Likewise, a second observer also positioned the same 100 sites to derive this time measures of inter-observer agreement. We also used simple two-tailed t-tests to check for differences between each coordinate dataset. Note that the procedure was performed by two experts in neuroanatomy. Overall, we found a high degree of intra and inter-observer agreement (see Supplementary Figure 1).

Probabilistic distributions of functional sites

Voxel-wise density maps

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For each DES site, we assumed that the associated functional response fi (where i corresponds to a given stimulation with i=1,…, n for n stimulations) is maximal at the centre of the stimulated area. Furthermore, to account for the spread of the electrical current nearby this central position, we approximated fi in all surrounding voxels (from 1 at the centre to 0 in all voxels sufficiently distant from this centre) with a standard kernel regression technique. Using the function "fslmaths" in FMRIB Software Library program (FSL, version 6.0, https://fsl.fmrib.ox.ac.uk/fsl), we applied a full width at half maximum (FWHM) of 8mm (bandwidth 3.4mm) for each DES site, which is more conservative than previous studies pointing out that bandwidth values were acceptable until 4-5mm in the same experimental settings. [START_REF] Sarubbo | Mapping critical cortical hubs and white matter pathways by direct electrical stimulation: an original functional atlas of the human brain[END_REF][START_REF] Sarubbo | Towards a functional atlas of human white matter: Functional Atlas of White Matter[END_REF] In order to represent the spatial distribution of all stimulation sites, we then computed a voxel-wise overlap map by summing individual kernel-transformed maps.

Consequently, for each voxel x of the brain, we obtained a density overlap map with Densityx=

∑ 𝑓 𝑖 𝑛 𝑖=1

Voxel-wise probability maps

To map the potential of cortical areas to be associated with positive or negative responses, we computed voxel-wise probability maps by calculating the ratio of positive responses and negative responses in each voxel. In particular, we defined the probability that a given voxel x of the brain is implicated in a functional response as a positive probability, and the probability that a given voxel x of the brain is implicated in a negative response as a negative probability.

Thus, we calculated positive and negative probability maps at Surg.1 and Surg.2 with the following formula:

Positive_Probabilityx= ∑ 𝑓_𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒 𝑖 𝑣 𝑖=1 (∑ 𝑓_𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒 𝑖 𝑣 𝑖=1 + ∑ 𝑓_𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒 𝑘 𝑤 𝑘=1 ) ⁄ Negative_Probabilityx= ∑ 𝑓_𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒 𝑘 𝑤 𝑘=1 (∑ 𝑓_𝑝𝑜𝑠𝑖𝑡𝑖𝑣𝑒 𝑖 𝑣 𝑖=1 + ∑ 𝑓_𝑛𝑒𝑔𝑎𝑡𝑖𝑣𝑒 𝑘 𝑤 𝑘=1
) ⁄

where f_positivei and f_negativej are the functional responses of v positive stimulation(s) (with i=1,…,v) and w negative stimulation(s) (with k=1,…,w). To capture the propensity of cortical areas to be functional at Surg.1, while becoming nonfunctional at Surg.2, which can be interpreted as a marker of cortical remodelling, we further computed a probability map representing the intersection between positive probability maps at Surg.1 and negative probability maps at Surg.2 as follows (i.e. a plasticity probability map):

Voxel-wise plastic probability maps

Plasticity_Probabilityx=Surg.1_positive_probabilityx* Surg.2_negative_probabilityx

Parcel-wise probability map

Next, we used predefined cortical parcels to contextualize the ratio between positive and negative responses in terms of anatomical location. Our goal was to allow an explicit assignment of DES effects in specific anatomical structures. This approach also permits a substantial reduction of the dimensionality of the stimulation dataset in comparison to voxelbased analyses. To estimate the probability of obtaining a positive response within a priori anatomical parcellations, we used the Brainnetome atlas (https://atlas.brainnetome.org/bntatlas, composed of 123 labelled parcels, see Supplementary materials for a list of all cortical parcels).

More specifically, we divided the number of positive DES by the total number of DES (positive and negative) in a given parcel. For instance, the probability of observing a positive response during Surg.1 within the region 6_1 of the left Precentral gyrus (left PrG_6_1) was 0.973 (i.e., 215 positive DES/ 221 positive and negative DES). Each stimulation site was generated as an MNI-registered sphere using Marsbar Toolbox (radius=5mm), implemented in the MATLAB environment (https://marsbar-toolbox.github.io). To determine the location of each sphere in terms of anatomical parcels, we used the Lesion Quantification Toolkit (https://wustl.app.box.com/v/LesionQuantificationToolkit) [START_REF] Griffis | Lesion Quantification Toolkit: A MATLAB software tool for estimating grey matter damage and white matter disconnections in patients with focal brain lesions[END_REF] .

The function "util_get_parcel_damage" was applied for each stimulation point (see Supplementary Figure 2 and Supplementary materials for more details and links to custom MATLAB codes). The ratio of positive and negative DES was then compared between Surg.1 and Surg.2, within each anatomical parcel with >3 DES, using Fisher's exact tests. Benjamini-Hochberg's false discovery rate (FDR) correction for multiple comparisons was used in each hemisphere separately, with a threshold set at P < 0.05 (Rstudio, 2021.09.1 Build 372, www.r-project.org).

Clustering analyses K-mean clustering

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Cluster comparisons

Based on a qualitative analysis, we next identified: (i) Clusters presenting with spatial similarities but potential shifts of their coordinates from Surg.1 to Surg.2. To provide a quantitative overview of cluster shifts, the MNI coordinates of stimulations sites forming similar clusters were analysed with two-tailed t-tests. FDR correction for multiple comparisons was used for each hemisphere separately, with a threshold set at P < 0.05; (ii) Clusters that disappeared from Surg.1 to Surg.2 and their related centroids; (iii) Clusters that appeared from Surg.1 to Surg.2 and their related centroids.

Dynamic remodelling of individual functional cortical maps Individual functional cortical maps

To better highlight the neuroplasticity potential outlined in previous analyses, we directly contrasted the changes in the distribution of functional responses from one surgery to another at the individual level. First, we identified cortical areas that were uniformly covered by DES from one surgery to another in each patient by generating a reference binarized map

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corresponding to the intersection map between the sum of all stimulations at Surg.1 and Surg.2.

By doing so, areas eliciting positive (or negative) response during only one of the two electrostimulation sessions were not considered in further statistical analyses.

Second, we computed an individual functional cortical map accounting for positive responses at Surg.1 and Surg.2 in each patient. Briefly, we computed functional responses in each patient using a standard kernel regression technique to approximate a cortical functional map (with each voxel having a value between 0 and 1) exclusively within the previously generated intersection map (i.e. within voxels covered by DES during both Surg.1 and Surg.2).

Individual functional cortical maps were computed to investigate plasticity in general (i.e. all positive functional responses were selected), or in the different functional responses taken separately. It should be mentioned that non-responsive DES sites for a given function of interest were considered as non-functional DES (e.g. in language mapping, DES eliciting positive motor responses without associated language disorders were considered negative for language). More details and links to custom MATLAB codes are provided in Supplementary figure 3 and Supplementary materials).

Surg.1 and Surg.2 statistical comparisons

Each patient's functional cortical maps (FCMs) at Surg.1 and Surg.2 were then used to study the cortical neuroplastic changes from one surgery to another. To do so, non-parametric statistics were performed, using the "randomise" function from FSL with 5000 permutations and threshold free cluster enhancement. To achieve a paired comparison analysis, we calculated the difference between Surg.1 and Surg.2 FCMs within-subjects (FCMSurg.1 -FCMSurg.2, using "fslmaths" function) and then performed a one sample t-test across subjects, which is equivalent to a paired t-test (https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/Randomise/UserGuide). We corrected for multiple comparisons with a family-wise error (FWE) threshold set at P < 0.05. Since we assumed a prior hypothesis that individual FCMs were more restricted at Surg.2 compared to Surg.1 (FCMSurg.1-FCMSurg.2>0) due to higher neuroplastic compensations, we only provide the results of one sample t-tests. Of note, we also performed the same analysis with the opposite contrast (difference between Surg.2 and Surg1.) but found no significant results.

To mitigate the potential effect of clinical or biological factors that may modulate cortical plasticity, we next repeated the same analysis using 'stimulation intensity changes from Surg.1 to Surg.2', 'histomolecular classification', 'the use of chemotherapy between Surg.1 and probabilistic neuroplasticity maps are displayed in Figure 2B. The maximal probabilities pmax (from 0, lowest neuroplasticity, to 1, highest neuroplasticity) were observed within the right SMA (pmax = 0.63), the right dlPFC (pmax = 0.61), the left supramarginalis gyrus (SMG, pmax= 0.49), the left anterior part of the ventral PMC (pmax = 0.43), the left dlPFC (pmax = 0.39) and the middle portion of the left STG (STG, pmax = 0.36).

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Parcel-wise probability maps

Parcel-wise probabilistic maps and related significant statistical comparisons are illustrated in Figure 2C-F. The number of stimulations (positive and negative) in each parcel is detailed in Supplementary materials.

The analyses indicated significant changes in probability distributions between positive and negative DES in the following parcels: left PrG6_1 parcel (Fisher's exact test, pFDR-corrected = 2.6 e-5 ) the left PrG6_2 (pFDR-corrected = 0.046), the left PrG6_6 (pFDR-corrected = 0.046), the right MFG7_2 (pFDR-corrected = 6.6 e-5 ), the right PrG6_1 (pFDR-corrected = 0.0058) and the right PrG6_6

(pFDR-corrected = 0.0039). In all above-mentioned parcels, the ratio of positive/negative stimulations consistently decreased at Surg.2, with the exception of the left supramarginal gyrus (left IPL6_4, pFDR-corrected = 0.046).

Clustering analyses

Clustering analysis results, locations of cluster centroids and significant shifts of cluster MNI coordinates between Surg.1 and Surg.2 are provided in Figure 3.

Within the left hemisphere, significant cluster migrations were found for language responses (nSurg. see Figure 3H).

Dynamic reorganizations of individual functional cortical maps

The overlap of cortical areas being stimulated both at Surg.1 and Surg.2 at the individual level is displayed in Figure 4A. Statistically significant differences between both functional cortical maps are provided as 1-p maps (FWE-corrected, threshold free cluster enhancement, 5000 permutations). The results of FCM comparisons when adjusted for each covariate independently (i.e. stimulation intensity changes, histomolecular classification, the use of chemotherapy and the time interval between Surg.1 and Surg.2) still remained significant, with only modulations in terms of cluster size. In addition, no linear significant relationships were observed between FCM measures and each covariate taken separately (see Supplementary Significant voxels were found both within the residual functional cortical map, the regrowth functional cortical map and the extra-lesional functional cortical map, highlighting cortical plasticity potential both within and directly around the cortical tumoral infiltration (see Supplementary Figure 7).

Discussion

The unique ability of the cerebral cortex to rearrange the topological organization of its cortical networks has been highlighted in response to various pathological settings, including peripheral lesions (e.g. in case of depravation of normal sensory inputs), [START_REF] Buonomano | Cortical plasticity: From Synapses to Maps[END_REF][START_REF] Feldman | Map Plasticity in Somatosensory Cortex[END_REF] congenital or brain injuries such as brain tumours and stroke. [START_REF] Hartwigsen | Neuroimaging of stroke recovery from aphasia -Insights into plasticity of the human language network[END_REF][START_REF] Saur | Dynamics of language reorganization after stroke[END_REF] It is widely acknowledged that lesion-induced plasticity is a time-dependent process, though it has been rarely investigated longitudinally, especially in the context of slow-growing tumours where neuroplastic changes are widespread. In other words, the longitudinal study of DLGG patients may help address fundamental questions about the properties of tumour-induced plasticity: Is postlesional plasticity a time-limited process? Is the potential for cortical remodelling equivalent across brain areas?

In the present study, we capitalized on a large and longitudinal electrostimulation dataset acquired from patients with DLGG to test the hypothesis that lesion-induced plasticity follows an evolving and spatially constrained scheme across the human cortex. Overall, our results

show that (i) cortical areas have a potential for evolving rearrangements which is however graded across the cortex; (ii) this dynamic potential appears to be constrained by the underlying functional neuroanatomy (e.g., primary sensorimotor cortices) and (iii) the efficiency of cortical rearrangements might be domain-specific, with higher plastic potentials in cortical circuits underpinning language and speech production.

The temporal pattern of tumour expansion is believed to be a key mechanistic principle governing the efficiency with which neuroplasticity deploys in DLGG patients. [START_REF] Desmurget | Contrasting acute and slow-growing lesions: a new door to brain plasticity[END_REF] However, there is only a handful of studies that have investigated the longitudinal features of gliomainduced neuroplasticity in general 50,51those using DES mapping being based on small datasets. [START_REF] Picart | Iterative Surgical Resections of Diffuse Glioma With Awake Mapping: How to Deal With Cortical Plasticity and Connectomal Constraints[END_REF][START_REF] Southwell | Intraoperative mapping during repeat awake craniotomy reveals the functional plasticity of adult cortex[END_REF] Here we showed that the potential of cortical areas to undergo glioma-induced rearrangement evolves from one surgery to another, albeit with an important variability across the cerebral cortex. Furthermore, we demonstrated that cortical plasticity evolves not only within the lesion, but also around the cortical infiltration. This confirms that important spatial cortical rearrangements occur as a consequence of a dynamic interplay between the brain and the tumour, as reported in previous case series. [START_REF] Southwell | Intraoperative mapping during repeat awake craniotomy reveals the functional plasticity of adult cortex[END_REF]52 In addition, the present study suggests a marked heterogeneity in the plasticity potential across cortical structures. Such differing potential can be conceived as a gradient, the value of which varies as a function of cortical areas. Low gradients of plasticity were mainly found within unimodal cortical structures, those which belong to what have been previously called the "minimal common brain" (i.e. a universal trunk formed by cortico-subcortical structures that is essential for basic cerebral functions, with low interindividual variability), 53 including the primary somatosensory and motor cortices.

This result aligns with that of a recent MEG study showing only subtle ipsilesional longitudinal changes of motor activations in patients with recurrent gliomas. 50 Admittedly however, the restricted plasticity potential observed in the primary motor areas at reoperation has to be interpreted in the light of a limited tumoral regrowth within cortical areas of the precentral gyrus (in particular, the primary hand area), while adjacent connective tracts were generally more infiltrated compared to the initial surgery. Increasing sample size in future studies may help capture a more in-depth picture of the potential of all cortical motor areas to be rewired following glioma relapse, an important point considering that plasticity of the primary sensorimotor cortex at first operation has been sometimes described. 52,[54][55][56] Remarkably, cluster analyses confirmed that positive language sites within the middle portion of the STG tended to turn negative during the second surgery, thus supporting the results of a recent MEG study indicating that the neural network underpinning language may be especially plastic in recurrent glioma patients, the mechanism of which would be a laterality shift in hemispheric specialization. 51 Interestingly, we observed higher gradients of neuroplasticity within the vPMC, a result that was not particularly expected given the role of this region in speech production and its topologically-constrained neuroplasticity potential at initial surgery. 57 Indeed, the speechrelated lateral part of the precentral gyrus was substantially rearranged between two surgeries, with a migration of sites from the ventral to the dorsal premotor cortex. This intragyral reorganisation may be explained by the fact that the third branch of the superior longitudinal fasciculus, which is known to convey speech articulatory related information from the supramarginal gyrus to the vPMC, 58 also projects more dorsally in the precentral gyrus. In other words, one can speculate that the dorsal trajectory of cortical rearrangements might depend on

A C C E P T E D M A N U S C R I P T
the connective properties of the superior longitudinal fasciculus III. Likewise, the high potential for evolving neuroplastic compensation within the posterior dlPFC appears to be challenging given its topological positioning into the anatomo-functional architecture. Indeed, the dlPFC acts as a high-centrality cortical hub, and is thought to support transmodal integration. [START_REF] Margulies | Situating the default-mode network along a principal gradient of macroscale cortical organization[END_REF] Yet, our findings might be sustained by further functional compensation within the contralateral dlPFC, given the expected contribution of both dlPFC especially in semantic processing. [START_REF] Herbet | Electrical stimulation of the dorsolateral prefrontal cortex impairs semantic cognition[END_REF] Our results may have important clinical implications in at least two directions. First, DLGG are diffuse neoplasms showing a recurrent infiltration within the brain parenchyma. In the context of multistage surgical management, [START_REF] Duffau | New concepts in the management of diffuse low-grade glioma: Proposal of a multistage and individualized therapeutic approach[END_REF] a better understanding of the mechanistic principles of cortical reallocation over time is of major importance to assess re-operability in patients for whom the glioma recurs (e.g. earlier reoperation might be suggested in the event of dlPFC, vPMc, or STG infiltration). Second, our results pave the way for future interventional therapies aimed at fostering individual dynamics of plasticity or "meta-plasticity" (a term recently reappraised to account for the susceptibility of brain plasticity to adapt its learning rules in an ever changing context). [START_REF] Duffau | Introducing the concept of brain metaplasticity in glioma: how to reorient the pattern of neural reconfiguration to optimize the therapeutic strategy[END_REF] For example, cortical areas associated with a high potential of continuous plasticity may be potential targets for non-invasive neuromodulation therapy.

Such a proactive approach coupled with cognitive stimulation may help accelerate cortical redistributions and thus allows earlier reoperation and greater extent of resection while minimizing the likelihood of postoperative deficits. 62

Limitations

Our main findings must be interpreted in light of several limitations. First, the cerebral cortex was not uniformly covered by cortical electrostimulations given that the topological distribution of DES sites was conditional to that of tumour (re)infiltration. Second, due to the clinical context of the study, some biological and therapeutical variables with a potential influence on neuroplasticity could not be directly controlled between the two surgeries (e.g. time interval between Surg.1 and Surg.2, switch of antiepileptic drugs, use of adjuvant chemotherapy), although adjusted statistical analyses did not suggest critical a determinant effect of these variables on plasticity changes. Third, thirteen patients (12.9%) presented with a shift of their histomolecular grade toward higher-grade gliomas, which may impact our results. In these patients, it is possible that the dynamic potential for cortical reorganization between both surgeries might have been lowered. [START_REF] Desmurget | Contrasting acute and slow-growing lesions: a new door to brain plasticity[END_REF] Fourth, we considered negative stimulation sites as

A C C E P T E D M A N U S C R I P T
cortical areas free of function, which is a necessary reductionist (but clinically relevant) approach given the fact that these areas are not stimulated for the large array of existing cognitive and fine-grained motor functions. For example, all aspects of motor control ere not assessed, [START_REF] Viganò | Negative motor responses to direct electrical stimulation: Behavioral assessment hides different effects on muscles[END_REF][START_REF] Fornia | Direct Electrical Stimulation of Premotor Areas: Different Effects on Hand Muscle Activity during Object Manipulation[END_REF] meaning that the neuroplasticity potential of motor regions might have been underestimated. This potential shortcoming is however mitigated by the clinical observation that surgical removal of negative sites is not associated with a postoperative decline of intraoperatively unassessed functions. [START_REF] Lemaitre | Cognitive preservation following awake mapping-based neurosurgery for low-grade gliomas: A longitudinal[END_REF][START_REF] Ng | Recurrent Low-Grade Gliomas: Does Reoperation Affect Neurocognitive Functioning?[END_REF] Last, the probability of observing a loss of functional sites at Surg.2 (i.e. a functional area at Surg.1 turning negative at Surg.2) was higher than the probability of observing a displacement of positive sites (i.e. the migration of one positive stimulation at Surg.1 to another area at Surg.2). This suggests that remote, nonlocal compensatory cortical sites are potentially recruited (either in the ipsilateral or contralateral hemisphere), beyond the cortical surface exposed during surgery which is necessarily limited.

Conclusions

Overall, our findings indicate that the slow re-growth of DLGGs favours a gradual and evolving remodelling of the cerebral cortex from one surgery to another. This dynamic potential can be expressed as a gradient to the extent that some cortical areas (the dorsolateral prefrontal cortex, the ventral premotor cortex, the superior temporal gyrus) display a higher propensity to reorganize than others (primary sensorimotor cortices). Furthermore, the efficiency of cortical reorganization may be domain-specific, with a higher potential for cortical circuits underpinning language and speech production.

  Downloaded from https://academic.oup.com/brain/advance-article/doi/10.1093/brain/awad116/7111299 by CNRS user on 14 April 2023

  1 = 45 stimulations [4 clusters], nSurg.2 = 46 stimulations [3 clusters], see Figure 3A), with a significant antero-posterior shift of paired centroids within the dlPFC (mean MNIycoordinates change = -10.4  3.0 t(32) = 3.46, p FDR-corrected = 0.007). In addition, a language cluster located within the left STG during Surg.1 (centroid MNI coordinates [-65.1; -22.3; 7.7]) disappeared at Surg.2. Cluster shifts were also found in speech arrest/dysarthria (nSurg.1 = 167 stimulations [2 clusters], nSurg.2 = 115 stimulations [2 clusters], see Figure 3C) with a ventrodorsal shift of a cluster located within the medio-dorsal part of the precentral gyrus (mean MNIx-coordinates change = 8.3  2.0, t(87) = 4.23 pFDR-corrected < 0.0001; mean MNIz-coordinates change = 9.1  2.0, t(87) = 4.49, pFDR-corrected < 0.0001). An additional cluster was identified at Surg.2 regarding stimulations eliciting negative motor responses (left precentral gyrus, centroid MNI coordinates [-35.9; -5.6; 61.6], see Figure 3E). No cluster change was observed for stimulations provoking positive motor responses (Figure 3G). Within the right hemisphere, cluster locations were also comparable regarding DESinduced non-verbal semantic disorders (nSurg.1 = 9 stimulations [1 cluster], nSurg.2 = 12 stimulations [1 cluster], see Figure 3B). Additional clusters were identified at Surg.2, especially regarding stimulations eliciting speech arrest/dysarthria (right SMG, centroid MNI coordinates [64.2; -24; 32], see Figure 3D) and negative motor responses (right precentral gyrus, centroid MNI coordinates [49.6; 0.88; 50.7], see Figure 3F). Interestingly an antero-posterior shift of a cluster located within the right upper precentral gyrus/right SMA toward the right upper precentral gyrus was observed (mean MNIy-coordinates change = -13.8  3.0, t(28) = 4.55, pFDRcorrected <0.0001, Figure 3F). No significant changes in cluster coordinate positioning were observed between the two surgeries for stimulations provoking positive motor responses within the right hemisphere (nSurg.1 = 64 stimulations [2 clusters], nSurg.2 = 54 stimulations [2 clusters],

  Materials). Results by behavioural response (Figure4B) indicate significant reorganizations (i) for language responses within both the left dlPFC and the left anterior STG, (ii) for speech arrest/dysarthria within the left vPMC and to some extent at the junction between the right dorsal PMC and the precentral gyrus (iii) for semantic response within the right dlPFC and (iv) for negative motor responses within the right vPMC. Cortical areas eliciting positive motor responses (left and right hemispheres) and negative motor responses (left hemisphere) remained unchanged from Surg.1 to Surg.2. Overall, high neuroplastic reorganizations occurred in the left dlPFC, the left vPMC, the posterior part of the inferior frontal gyrus (IFG), the anterior part of the left STG and the right dlPFC (Figure4C).

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Figure 1 Figure 2 Figure 3 Figure 4 1 Figure 1 2

 123411 Figure 1 Methodological framework for intraoperative stimulation and acquisition of normalized stimulation sites. (A) Intraoperative protocol during wide-awake surgery with cognitive monitoring. (B and C) Illustration of intraoperative cortical mapping at Surg.1 and Surg.2, based on intraoperative photographs and camera recordings. (D) Positioning of stimulation sites (Surg.2, left hemisphere) onto a 3-dimensional pial mesh reconstruction normalized in the Montreal Neurological Institute space (BrainVISA/Anatomist software [Version 5.0, CEA I2BM, CATI Neuroimaging, Inserm IFR49, and CNRS, France]). (E) Eligible cortical stimulation sites during Surg.1 (485 positive stimulations, 806 negative stimulations) (F) Eligible cortical stimulation sites during Surg.2 (351 positive stimulations, 440 negative stimulations). The same colour legends apply for the whole figure. DES: Direct electrostimulation, LH: left hemisphere, RH: right hemisphere, Surg.: Surgery

  Figure 2 180x124 mm ( x DPI)

  Figure 3 180x233 mm ( x DPI)

  R I P T Downloaded from https://academic.oup.com/brain/advance-article/doi/10.1093/brain/awad116/7111299 by CNRS user on 14 April 2023Clustering analyses were performed to assess the extent to which the distribution pattern of functional sites was changing from Surg.1 to Surg.2. In this analysis, only positive stimulations were considered. Clustering analyses were performed separately at Surg.1 and Surg.2. For a given behaviour response of interest (i.e. language disorders, speech arrest and dysarthria, positive motor responses and negative motor responses), the MNI coordinates of all stimulation sites were analysed with R software using the Flexible Procedures for Clustering package (fpc, version 2.2-9). Note that somatosensorial responses were not considered here, because of the limited amount of available data per hemisphere during Surg.2. A Duda-Hart statistic was used to determine whether a given dataset should be clustered. Datasets with a Duda-Hart statistic >1.645 were further subjected to a k-mean clustering; the remaining ones were represented as

single clusters. The optimal number of clusters (range: 2-10) was determined using the most conservative result (i.e. the smallest number of suggested clusters) obtained from the average silhouette width method and the Calinski Harabasz index method. Stimulation sites and their related centroids were further plotted in a z/y MNI graph representation to allow an easier visualization of their locations.

functional cortical map, regrowth functional cortical map and extra-lesional functional cortical map

  R I P T Downloaded from https://academic.oup.com/brain/advance-article/doi/10.1093/brain/awad116/7111299 by CNRS user on 14 April 2023Surg.2' and the 'time interval between Surg.1 and Surg.2' as nuisance covariates. Briefly, we independently tested the effect of these covariates with both a 1/0 and a 0/1 contrasts within the corresponding design matrix (FEAT interface, https://fsl.fmrib.ox.ac.uk/fsl/fslwiki/GLM) before running "randomise" function from FSL.To assess whether the spatial patterns of tumour regrowth modulated cortical plasticity, we next confronted the individual FCMs to each patient's tumoral infiltration pattern, based on FLAIR images obtained 3 months after Surg.1 and immediately before Surg.2. As a result, in each

	Residual Data availability Data will be made available upon reasonable request to the corresponding author. Exhaustive datasets are provided in the Supplementary materials, including custom MATLAB codes. Visualization of the results were made with MRIcroGL (https://www.nitrc.org/projects/mricrogl) and SurfIce (https://www.nitrc.org/projects/surfice). A C C E P T E D M A N U S C R I P T	Downloaded from https://academic.oup.com/brain/advance-article/doi/10.1093/brain/awad116/7111299 by CNRS user on 14 April 2023
	Results
	Participants

patient we were able to discriminate (1) a residual functional cortical map (changes of FCM only within post-Surg.1 tumour residual), (2) a regrowth functional cortical map (changes of FCM only within changes of FLAIR signal observed between post-Surg.1 and pre-Surg.2 MRIs) and (3) an extra-lesional functional cortical map (changes of FCM only outside pre-Surg.2 tumour signal). Non-parametric statistics were then performed to compare each patient's FCM, using the "randomise" function from FSL with 5000 permutations and threshold free cluster enhancement (see above). Methodological details are illustrated in Supplementary Figure

4

.

  Taken together, these novel findings provide a new insight into the neuroplastic properties of cortical networks and are of immense interest for the clinical management of patients, especially in case of tumour recurrence. Downloaded from https://academic.oup.com/brain/advance-article/doi/10.1093/brain/awad116/7111299 by CNRS user on 14 April 2023 50. Bulubas L, Sardesh N, Traut T, et al. Motor Cortical Network Plasticity in Patients With

	Recurrent	Brain	Tumors.	Front	Hum	Neurosci.	2020;14:118.
	doi:10.3389/fnhum.2020.00118
	51. Traut T, Sardesh N, Bulubas L, et al. MEG imaging of recurrent gliomas reveals functional
	plasticity of hemispheric language specialization. Hum Brain Mapp. 2019;40(4):1082-
	A C C E P T E D M A N U S C R I P T A C C E P T E D M A N U S C R I P T A C C E P T E D M A N U S C R I P T A C C E P T E D M A N U S C R I P T A C C E P T E D M A N U S C R I P T 1092. doi:10.1002/hbm.24430 52. Duffau H, Denvil D, Capelle L. Long term reshaping of language, sensory, and motor maps after glioma resection: a new parameter to integrate in the surgical strategy. J Neurol Neurosurg Psychiatry. 2002;72(4):511-516. doi:10.1136/jnnp.72.4.511 53. Ius T, Angelini E, Thiebaut de Schotten M, Mandonnet E, Duffau H. Evidence for potentials and limitations of brain plasticity using an atlas of functional resectability of WHO grade II gliomas: Towards a "minimal common brain." NeuroImage. 2011;56(3):992-1000. doi:10.3171/2013.5.JNS122235 55. Rossi M, Viganò L, Puglisi G, et al. Targeting Primary Motor Cortex (M1) Functional Components in M1 Gliomas Enhances Safe Resection and Reveals M1 Plasticity Potentials. Cancers. 2021;13(15):3808. doi:10.3390/cancers13153808 56. Nakajima R, Kinoshita M, Nakada M. Motor Functional Reorganization Is Triggered by Tumor Infiltration Into the Primary Motor Area and Repeated Surgery. Front Hum Neurosci. 2020;14:327. doi:10.3389/fnhum.2020.00327 57. van Geemen K, Herbet G, Moritz-Gasser S, Duffau H. Limited plastic potential of the left ventral premotor cortex in speech articulation: Evidence From intraoperative awake mapping in glioma patients: Ventral Premotor Cortex and Speech. Hum Brain Mapp. 2014;35(4):1587-1596. doi:10.1002/hbm.22275 A C C E P T E D M A N U S C R I P T A C C E P T E D M A N U S C R I P T	Downloaded from https://academic.oup.com/brain/advance-article/doi/10.1093/brain/awad116/7111299 by CNRS user on 14 April 2023
	58. Giampiccolo D, Duffau H. Controversy over the temporal cortical terminations of the left
	within-patient arcuate fasciculus: design	a	study. reappraisal. Neuro-Oncology. Brain.	2022;24(5):781-793. 2022;145(4):1242-1256.
	left doi:10.1093/neuonc/noab275 temporo-parietal and doi:10.1093/brain/awac057		frontal	stroke.	Brain.	2020;143(3):844-861.
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doi:10.1016/j.neuroimage.2011.03.022 54. Schucht P, Ghareeb F, Duffau H. Surgery for low-grade glioma infiltrating the central cerebral region: location as a predictive factor for neurological deficit, epileptological outcome, and quality of life: Clinical article. JNS. 2013;119(2):318-323.

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Funding

No funding was received towards this work.

Overall, 101 patients underwent repeat awake-guided surgical resections for a DLGG (49 female [48.5%], mean age at first surgery: 34.5  9.1). Details about mean tumour volumes before/after surgeries and the extent of resections are provided in the Supplementary materials.

Pre-surgical, post-surgical and tumour regrowth infiltration maps are displayed in Supplementary Figure 5. The mean time interval between Surg.1 and Surg.

Competing interests

The authors report no competing interests.

Supplementary material

Supplementary material is available at Brain online.


Panyi Wang

Yingying Cui

Senlong Zhao

Renguang Ye

Muzhi Cai
email: caimuzhi@cjlu.edu.cn

Laurent Calvez

Jean Rocherulle

Feifei Huang

Hongli Ma

Gongxun Bai

Junjie Zhang

Shiqing Xu

Xianghua Zhang

Effects of melting temperature on Color-changing in Germano-tellurite Niobate Glass

Keywords: Germano-tellurite niobate glass, Color-changing mechanism, X-ray photoelectron spectroscopy, Raman spectra

Germano-tellurite niobate glasses have gained significant interest because of their applications in lasers and optical devices. However, germano-tellurite niobate glasses tend to turn brown or dark, which is a major disadvantage for visible optical systems. This work is devoted to exploring the mechanism of this colorchanging. Glasses with a molar composition of 70TeO2-10GeO2-10K2O-10Nb2O5 (TGKN) were synthesized by conventional melt-quenching at various melting temperatures. Different melting temperature leads the glasses to present different colors. The precise glass composition was revealed by ICP-OES, and the thermal properties, optical properties, and microstructure of the TGKN glasses were investigated. The XRD patterns and DSC curves suggest that the formation of Te metal is not the principal reason for the color-changing. A novel interpretation that the generation of T-Nb2O5 causes the color-changing was proposed based on the transmittance, XPS and Raman spectra. This work will provide a new perspective on clarifying the effects of melting temperature on color-changing in the germano-tellurite niobate glass.

Introduction

Tellurium oxide based glasses have attracted a strong interest over recent years due to their low phonon energy, good transmission at visible and infrared wavelength from ~0.4 to ~6 µm, high resistance to humidity and enhanced non-linear optical properties [START_REF] Jackson | Towards high-power mid-infrared emission from a fibre laser[END_REF][START_REF] Kato | Influence of Al2O3 incorporation on the third-order nonlinear optical properties of Ag2O-TeO2 glasses[END_REF][START_REF] Zaki | Structural investigation of new tellurite glasses belonging to the TeO2-NbO2.5-WO3 system, and a study of their linear and nonlinear optical properties[END_REF][START_REF] Désévédavy | Review of tellurite glasses purification issues for mid-IR optical fiber applications[END_REF][START_REF] Sayyed | Impact of Ag2O on linear, nonlinear optical and gamma-ray shielding features of ternary silver vanadio-tellurite glasses: TeO2-V2O5-Ag2O[END_REF][START_REF] Gowda | Structural, thermal and spectroscopic studies of Europium trioxide doped lead boro-tellurite glasses[END_REF]. However, compared to other oxide glass systems, for example, silicate or germanate glass systems, tellurite glasses generally present low glass transition temperature (Tg) and onset of crystallisation temperature (Tx) causing their poor thermal stability, which can be a drawback. Introducing GeO2 into tellurite glass can improve their thermal performances, and thus germano-tellurite glasses are widely investigated [START_REF] Zhao | Spectroscopic properties of GeO2-and Nb2O5-modified tellurite glasses doped with Er 3+[END_REF][START_REF] Pepe | Enhanced gain bandwidth of Tm 3+ and Er 3+ doped tellurite glasses for broadband optical amplifier[END_REF][START_REF] Tang | Effect of introduction of TiO2 and GeO2 oxides on thermal stability and 2 μm luminescence properties of tellurite glasses[END_REF][START_REF] Hou | Broadband mid-infrared 2.0 μm and 4.1 μm emission in Ho 3+ /Yb 3+ co-doped tellurite-germanate glasses[END_REF]. Among various kinds of germano-tellurite glasses, germano-tellurite niobate glasses have gained significant interest because of their applications in lasers and optical devices [START_REF] Zhao | Spectroscopic properties of GeO2-and Nb2O5-modified tellurite glasses doped with Er 3+[END_REF][START_REF] Monteiro | Optical and spectroscopic properties of rare earth-doped (80-x) TeO2-xGeO2-10Nb2O5-10K2O glasses[END_REF][START_REF] Lu | Er 3+ doped germanate-tellurite glass for mid-infrared 2.7 μm fiber laser material[END_REF][START_REF] Lu | Enhanced effect of Er 3+ ions on 2.0 and 2[END_REF][START_REF] Truong | Second harmonic generation in germanotellurite bulk glass-ceramics[END_REF][START_REF] Łukaszewicz | Neodymium-doped germanotellurite glasses for laser materials and temperature sensing[END_REF].

A number of studies have found that germano-tellurite niobate glasses have suitable visible and midinfrared optical property [START_REF] Lu | Er 3+ doped germanate-tellurite glass for mid-infrared 2.7 μm fiber laser material[END_REF][START_REF] Lu | Enhanced effect of Er 3+ ions on 2.0 and 2[END_REF]. However, our experiments also found that the germano-tellurite niobate glasses easily become brown even dark, which is one of the main obstacles that hinder the usage of germanotellurite niobate glasses in visible optical devices. The reasons for this color-changing are difficult to predict because of the number of variables involved, especially for multicomponent glass systems. Up to now, little attention has been paid to the color-changing mechanism in these glasses. Several studies have documented that color-changing of tellurite glass is due to the reduction of tellurium under synthesis conditions [START_REF] Machado | The reduction of tellurium in binary glasses in the system TeO2 -Sb2O3[END_REF][START_REF] Bachvarova-Nedelcheva | Synthesis, structure and properties of silver tellurite glasses[END_REF]. Silva et al. demonstrates that in the TeO2-Sb2O3 system glass system, the oxidation of Sb 3+ to Sb 5+ leads the reduction of Te 4+ to Te 0 during the melting process, which is accompanied by a gradual devitrification and darkening process in the glass [START_REF] Machado | The reduction of tellurium in binary glasses in the system TeO2 -Sb2O3[END_REF]. On the other hand, some studies suggest that the color-changing is because of the reduction of niobium valence states [START_REF] Ghussn | Color bleaching and oxygen diffusion in a niobium phosphate glass[END_REF][START_REF] Holubová | The influence of niobium on the structure of Nb2O5-ZnO-P2O5 glasses[END_REF]. Both reasons do not explain very well the phenomenon observed in our experiment, as will be detailed later.

In this work, the glasses with molar composition of 70TeO2-10GeO2-10K2O-10Nb2O5 presenting high thermal stability, high refractive index and relative low phonon energy were synthesized at different melting temperature by melt-quenching method [START_REF] Monteiro | Optical and spectroscopic properties of rare earth-doped (80-x) TeO2-xGeO2-10Nb2O5-10K2O glasses[END_REF][START_REF] Monteiro | Optical and spectroscopic properties of germanotellurite glasses[END_REF]. It was found that glasses synthesized at different melting temperatures present different colors. The accurate glass composition was investigated by inductively coupled plasma optical emission spectroscopy (ICP-OES) to exclude the effect of the evaporation process. To find the reason for color-changing, the optical properties and thermal properties of the prepared glasses were investigated by transmission spectra and differential scanning calorimetry (DSC), respectively. The valence state of Te and Nb were verified by X-ray photoelectron spectroscopy (XPS). The microstructure and submicrostructure of the glasses were studied by X-ray diffraction analysis (XRD), XPS and Raman spectra, respectively. Based on the results, a different interpretation was proposed to interpret the color-changing of the prepared glasses, which could be extended to explain the color-changing of other tellurite niobate glasses.

Experimental

Synthesis

Glasses with molar composition of 70TeO2-10GeO2-10K2O-10Nb2O5 were synthesized by conventional melt-quenching method, named as TGKN. All the glass samples were synthesized by using high-purity raw materials from Aladdin Co., Ltd, including TeO2 (99.99%), GeO2 (99.99%), K2CO3 (99.99%), Nb2O5 (99.99%), Na2CO3 (99.99%) and ZnO (99.99%). The stoichiometric chemicals were well-mixed and melted at 900 ℃, 1050 ℃ and 1150 ℃ for 40 min in a covered alumina crucible in ambient air, respectively named as TGKN900, TGKN1050 and TGKN1150. In addition, a glass sample melted with two-step process (1150 ℃ for 30 min, then 900 ℃ for 10 min), donated as TWO-STEP, was also synthesized as shown in Figure 1. In order to study the cause of color-changing, the tellurite glasses with molar composition of 70TeO2-20ZnO-10Na2O, 70TeO2-30K2O, 70TeO2-10GeO2-20K2O, and (100-x) TeO2-xNb2O5 (x=10 and 20) were synthesized like the TGKN glasses. The melts were poured onto a preheated steel plate and pressed by another plate for shaping. After annealing at around glass transition temperature (355 ℃), all samples were cut and polished into 10×10×1.5 mm 3 for further measurement. The whole synthesis process is shown in Figure 1. The elements analysis was conducted using a simultaneous axial view inductively coupled plasma optical emission spectroscopy (ICP-OES) (Agilent 720, USA). The DSC scans (NETZSCH DSC 200 PC) were carried out on samples of 10 mg in an Al pan. The overall accuracy of this measurement is expected to be within 1 K. The Tg was determined at a heating rate of 10 K/min. The amorphous nature of the prepared glasses was characterized by X-ray diffraction measurements (XRD) (D2 PHASE Bruker, German). All measurements were carried out at room temperature using Cu Kα radiation (λ=1.54056 Å). A step size of 0.02 ᵒ (2θ) was used with a scan speed of 0.1  /s. The transmittance spectrum of glass samples ware measured by UV-Vis spectrophotometer (UV-3600 Shimadzu, Japanese) ranging from 300 to 800 nm. High-resolution photoelectron spectra were collected on a Thermo Scientific K-Alpha spectrometer equipped with dual aluminium-magnesium anode using Al Kα (radiation (h=1486.6 eV). The electron analyzer was set at a pass energy of 50 eV. For self-consistency, the C 1s line of 284.8 eV binding energy was used as a reference for all charge shift corrections as this peak arises from hydrocarbon contamination and its binding energy is generally accepted as remaining constant, irrespective of the chemical state of the sample. This hydrocarbon contamination was further enhanced as the XPS measurements had to be performed on uncleaved surfaces of the glasses as the glass rod samples shattered under vacuum cleavage. Collecting the XPS spectrum of a sample for at least 5 times of cyclic signal accumulation and the base pressure in the analysis chamber during these measurements was less than 5.010 -7 mBar. Polarised Raman scattering spectra measurements were carried out on a confocal Raman spectrometer (Renishaw, UK). Raman scattering spectra were obtained in the range 200-1000 cm -1 by exciting the samples with the 532 nm line of a DPSS laser (RL532C50, Renishaw) with an output power of 22.5 W. The signal was detected by a thermoelectric cooled (-70 ℃) standard chargecoupled array detector. Each Raman spectrum is an accumulation of ten scans with an exposure time of 10 s.

Raman spectra deconvolution was done with the Origin 2021 software, using Gaussian component curves.

Result and Discussion

The transmittance spectra in the wavelength range of 300-800 nm of the TGKN series glasses are shown in Figure 2a. The digital photos of the glass samples are shown in the inset of Figure 2a. It displays that the glass sample melted at 900 ℃ (TGKN900) is slightly yellow, whereas the glass sample melted at 1050 ℃ (TGKN1050) and 1150 ℃ (TGKN1150) presents brown and black color, respectively. In addition, the TGKN900 and TGKN1050 samples are transparent, whereas the TGKN1150 sample becomes partly devitrified. Although TGKN1050 shows lower transmittance than that of TGKN900, the absorption edge of both samples is located at around 370 nm, which is consistent with other reports [START_REF] El-Deen | IR and UV spectral studies for rare earths-doped tellurite glasses[END_REF][START_REF] Jlassi | Thermal and optical properties of tellurite glasses doped erbium[END_REF][START_REF] Çelikbilek | Thermochromic behavior of tellurite glasses[END_REF][START_REF] Kaky | Physical, structural, optical and gamma radiation attenuation properties of germanate-tellurite glasses for shielding applications[END_REF]. However, the absorption edge of the TGKN1150 is situated at around 600 nm. Since elements evaporate during the melting process, it was necessary to check if this evaporation process had anything to do with the color-changing in glasses. The elemental profiling of the TGKN series glasses was performed by ICP-OES, as shown in Figure 1b-f. These figures show that all the elements (Te, Ge, K, and Nb) firstly decrease and then increase with the increasing melting temperature. In TGKN900, TGKN1050, and TGKN1150, the mass percent concentration of Te element is 56 %, 48 %, and 50 % respectively.

Considering the experiment accuracy, the changes of Ge, K, and Nb are hardly negligible compared to the changes of the Te element. At the same time, the mass percent concentration of the Al element keeps increasing with the increasing melting temperature because the reaction between the alumina crucible and the TGKN glass melt benefits from a higher melting temperature. However, Te evaporation is not able to explain the color change on the unique basis of the mass Te concentration change between samples TGKN1050 and TGKN1150. In order to investigate the association between color-changing and evaporation processes, some experiments have been done. Firstly, the TGKN1150 glass was re-melted at 900 ℃, and the glass becomes slightly yellow. Besides, the glass synthesized by a two-step process (1150 ℃ for 30 min, then 900 ℃ for 10 min) presenting slightly yellow, as shown in Figure 1. These results suggest that 1) the evaporation process is not the direct reason for the color-changing in these glasses; 2) the color-changing in these glasses seems to be associated with kinetic processes.

It has been noted that the formation of Te metal particles may decrease the transmittance of glass and may change the color of the glass [START_REF] Machado | The reduction of tellurium in binary glasses in the system TeO2 -Sb2O3[END_REF]. In contrast, this could not explain the phenomena well in our work.

The XRD patterns of all three samples are shown in Figure 3a. Unlike the previous work [START_REF] Machado | The reduction of tellurium in binary glasses in the system TeO2 -Sb2O3[END_REF][START_REF] Bachvarova-Nedelcheva | Synthesis, structure and properties of silver tellurite glasses[END_REF], there is no prominent Te metal crystal signal peak, and only a broad halo characteristic of amorphous samples was observed [START_REF] Machado | The reduction of tellurium in binary glasses in the system TeO2 -Sb2O3[END_REF]. In order to further confirm if the glass sample contains Te metal particles, the DSC of the TGKN glass samples was performed, as shown in Figure 3b. It provides that the TGKN900 and TGKN1150 samples show an endothermic deep at around 450 ℃, which corresponds to the melting temperature of Te, giving sense to the presence of small quantities of metallic Te in these two glass samples. As a consequence, it clearly demonstrates that the color-changing is not entirely due to the presence of metallic Te. Although it is not exactly rigorous, some other tellurite glass systems, including TeO2-ZnO-Na2O, TeO2-K2O, and TeO2-GeO2-K2O were synthesized at different temperatures like the TGKN series of glasses to verify the above conclusion.

Comapared to TGKN series of glasses, the color of all these glass samples does not change with the different melting temperature. Therefore, the principal reason for the color-changing of TGKN series glasses has to be further discussed. In addition, the Tg of TGKN1150 is the highest among the TGNK series glasses, which suggests that TGNK1150 has the highest polymerized glass network. Niobium having various stable valence states, it can also be considered as responsible for this colorchanging. Thus, the binary TeO2-Nb2O5 glass systems were synthesized to investigate the influence of Nb on transmittance changes. It was found that the color of the binary TeO2-Nb2O5 glass melted at different temperatures is different, which is similar to that of TGKN series glasses. Besides, the color of the glass becomes darker with increasing Nb2O5 content. Typically, the low valence states of niobium (Nb 3+ or Nb 4+ ) presents deep color, whereas Nb 5+ appears slightly yellow. However, in this work, all the glass samples were synthesized in an ambient environment at high temperatures, suggesting the glass during melting is under oxidative state, and niobium consequently tends to form Nb 5+ . This was proved by the experiment of synthesizing TGKN glass samples utilizing NbO2 as raw material. As shown in Figure 4a, the glass synthesized utilizing Nb2O5 as raw materials presents almost the same transmittance spectrum as the glass synthesized with NbO2. X-ray photoelectron spectroscopy (XPS) has been found to be a helpful technique in estimating the ratio of the different valence states in a glass. In order to further investigate the valence state of Nb, the Nb 3d spinorbit doublet spectra for the TGKN series glasses were done and are shown in Figure 3b. The position and the width of the Nb 3d5/2 and Nb 3d3/2 peaks hardly changed with the increasing melting temperature, which indicates that the valence state of niobium ions probably does not change in all TGKN series glass samples.

Besides, it was found the Nb 3d core-level spectra of TGKN series glass samples only can be fitted with two spin-orbit doublets related to Nb 5+ and Nb 4+ oxidation states of niobium ions [START_REF] Delheusy | X-ray investigation of subsurface interstitial oxygen at Nb/oxide interfaces[END_REF][START_REF] Ma | Thermal effect on the oxides on Nb (100) studied by synchrotron-radiation x-ray photoelectron spectroscopy[END_REF] respectively, as shown in Figure 4 c-e. It suggests that no Nb 3+ or even lower oxidation states of niobium are present in these glass samples. Therefore, considering Nb 5+ and Nb 4+ presents slight yellow and blue color, respectively, it is assumed that the valence states of Nb are not responsible for the color-changing (darken) of TGKN series glasses either. Nb 3d core-level fitted with Nb 5+ and Nb 4+ peaks for TGKN900 (c), TGKN1050 (d), and TGKN1150 (e) Thus, to find out the principal reason for the color-changing, the structure of the TGKN series glass was studied. XPS is also a useful technique in assessing the local glass structure [START_REF] Bachvarova-Nedelcheva | Synthesis, structure and properties of silver tellurite glasses[END_REF]. The full spectra of the TGKN series glasses are shown in Figure 5a, and the atomic orbitals of each element are visible. No other elements except Al which comes from the Al2O3 crucible were observed in the spectrum of TGKN glass series, which is consistent with the ICP-OES result, as shown in the inset in Figure 5a.

The 3d spin-orbit doublet spectra for the TGKN series glasses are shown in Figure 5b. Figure 5b displays that the peak positions of Te 3d shift to higher energy with increasing melting temperature. Normally, the conversion from [TeO3] to [TeO4] configuration may be responsible for this chemical shift. However, this is not consistent with the ICP-OES result that the content of Te in TGKN900 is highest among all the TGKN series glasses, which means TGKN900 should have the most [TeO4] configurations. Considering the change of the binding energy of Ge 3d in TGKN series glasses is neglectable, as shown in Figure 5c, these results suggest that more Te-O-configurations may convert into Te-O-X configurations (X= Al or Nb). Besides, the O1s core-level spectra can always provide information to confirm this interpretation, as shown in Figure 5d.

A slight asymmetry is observed on the higher binding energy side of the prominent peak, having an average value of ~530.5 eV. This peak is also fitted with two contributions, as shown in Figure 5e-5f, one due to oxygen atoms in a bridging environment (BO) and the other due to oxygen atoms in a non-bridging environment (NBO) [START_REF] Bachvarova-Nedelcheva | Synthesis, structure and properties of silver tellurite glasses[END_REF][START_REF] Mekki | XPS and magnetic studies of vanadium tellurite glassses[END_REF][START_REF] Chen | Optical linear & nonlinearity and Faraday rotation study on V2O5 nanorod doped glass and glass-ceramic: Impact of optical basicity[END_REF]. Obviously, TGKN1050 and TGKN1150 contain more BOs, and this is consistent with the conversion from Te-O-to Te-O-X configurations that can increase BO. Raman spectroscopy is another effective method to investigate the structure of the glass. Figure 6a displays the Raman spectra of TGKN series glasses, and Figure 6b illustrates the deconvoluted Raman spectra of the TGKN900 glass sample. The spectrum is normalized and superimposed. The broad peaks are deconvoluted into five peaks using Gaussian distribution to distinguish the exact mode of vibrations. The bands are peaked at ~460, ~590, ~680, ~780, and ~891 cm -1 . The observed first intense at ~460 cm -1 is due to the symmetric vibration of Te-O-Te, Te-O-Ge and/or Ge-O-Ge linkages.

According to the report [START_REF] Monteiro | Optical and spectroscopic properties of germanotellurite glasses[END_REF], the ~590 cm -1 weak band could be assigned to the presence of GeO6 octahedra or Nb-O-Ge linkages or NbO6 octahedra. Notably, previous studies also point that the low-frequency side should of the ~590 cm -1 band is associated with NbO6 octahedra [START_REF] Takahashi | Precursive stage of nanocrystallization in niobium oxidecontaining glass[END_REF][START_REF] Marcondes | High niobium oxide content in germanate glasses: Thermal, structural, and optical properties[END_REF]. From Figure 6 b-d, it can be found that the band shifts from 601 cm -1 in TGKN900, to 596 cm -1 in TGKN1050, to 587 cm -1 in TGKN1150, respectively. Combining the XPS analysis, this indicates the vibration intensity of NbO6 octahedra which commonly acts as glass modifiers becomes more intense in TGKN1050 and TGKN1150.

The ~680 and ~780 cm -1 intense band is assigned to the vibration of [TeO4] and [TeO3], respectively [START_REF] Kalampounias | A temperature dependence Raman study of the 0.1Nb2O5-0.9TeO2 glass-forming system[END_REF][START_REF] Pan | Engineering tunable broadband near-infrared emission in transparent rare-earth doped nanocrystals-in-glass composites via a bottom-up strategy[END_REF]. It is worth mentioning that both ~680 cm -1 and ~780 cm -1 bands shift to higher frequency regions. The ~680 cm -1 band shifts from 668 cm -1 in TGKN900, to 680 cm -1 in TGKN1050, to 695 cm -1 in TGKN1150, respectively. The ~780 cm -1 band shifts from 762 cm -1 in TGKN900, to 781cm -1 in TGKN1050, to 797 cm -1 in TGKN1150, respectively. Meanwhile, the FWHM of ~680 cm -1 band of TGKN900, TGKN1050, and TGKN1150 are 84, 101, 110 cm -1 , respectively, whereas the FWHM at ~780 cm -1 band of the corresponding glass are 139, 117, 98 cm -1 , respectively. This indicates that the [TeO3] configuration tends to convert to [TeO4] configuration in TGKN1050 and TGKN1150, which is consistent with the result of XPS.

Normally, the ~891 cm -1 band can be attributed to the presence of NBOs and Al-O vibrations. The FWHM at ~891 cm -1 band of TGKN900, TGKN1050 and TGKN1150 are 65, 81, 89 cm -1 . Since the XPS results indicate that NBOs decrease with increasing melting temperature, it is reasonable to assign the change of FWHM to the presence of Al-O vibrations. According to the literature, compared to Nb2O5, Al2O3 may be more likely to incorporate into the TeO2-rich glass network. Thus, combining the XPS and Raman spectrum, it can be speculated that Al may incorporate into the tellurium glass network through connecting [TeO3] configuration or replacing Nb in the glassy network, and the former increases the BO contents, while the latter induce the formation of Nb-O contents. Previous studies demonstrate a more ordered/crystalline Nb-rich region (~1-2 nm) could evolve in the elastic softening period [START_REF] Lipovskii | The origin of electrooptical sensitivity of glassy materials: crystal motifs in glasses[END_REF][START_REF] Lipovskii | Mid-range structure of niobium-sodium-phosphate electro-optic glasses[END_REF]. Notably, the T-Nb2O5 (orthorhombic structure) consists of 4×4 blocks of corner-shared NbO6 octahedra, with connected blocks sharing the octahedron edges that could induce the absorption of visible light [START_REF] Jia | Theoretical and Experimental Study on Exciton Properties of TT-, T-, and H-Nb2O5[END_REF]. The report [START_REF] Jia | Theoretical and Experimental Study on Exciton Properties of TT-, T-, and H-Nb2O5[END_REF] illustrates the bandgap of T-Nb2O5 is around 2.08 eV, which is close to the cut-off absorption edge of TGKN1150, as shown in Figure 2a. Nb2O5 is classified as an intermediate oxide and can act as a network former in the form of [NbO4] tetrahedron or a network modifier in the form of [NbO6] octahedra [START_REF] Lin | Structure and non-linear optical performance of TeO2-Nb2O5-ZnO glasses[END_REF]. Therefore, Nb2O5 are possibly be linked through faces or edges when they act as [NbO6] octahedra, as shown in Figure 6e. Consequently, the T-Nb2O5 type Nb-O clusters can be formed. Considering that the formation of T-Nb2O5 depends on the temperature (600-800 °C) [START_REF] Jia | Theoretical and Experimental Study on Exciton Properties of TT-, T-, and H-Nb2O5[END_REF], the longer the glass melt is in the corresponding time, the more T-Nb2O5 clusters may be formed in the glass. It just well explains the phenomenon observed in this work. TGNK1150 has the highest pouring temperature among the TGNK series glasses and consequently has the longest time during 600-800 °C. Thus, TGNK1150 may contain the most T-Nb2O5 clusters causing the most drastic color-changing phenomenon among TGNK series glasses. This speculation also explains why the color-changing in TGKN series glasses is associated with kinetic processes. However, more specific and direct evidence is needed to confirm the formation of T-Nb2O5, and further studies of these Nb-O clusters are in progress. However, more specific and direct evidence is required to confirm the formation of T-Nb2O5, and further studies of these Nb-O clusters are in progress.

Conclusion

The glasses with the molar composition of 70TeO2-10GeO2-10K2O-10Nb2O5 were synthesized by conventional melt-quenching at different melting temperatures. It is found that these glasses present different colors, and the glasses melted at higher temperature show darker colors than those melted at low temperature.

Figure 1

 1 Figure 1 Schematic illustration for the synthesis of TGKN glasses

Figure 2

 2 Figure 2 Transmittance spectra (a), ICP-OES data of TGKN series glasses (b-f)

Figure 3

 3 Figure 3 XRD pattern (a) and DSC curves (b) of TGKN series glasses

Figure 4 (

 4 Figure 4 (a) Transmittance spectra of TGKN series glasses utilizing different niobium oxides as raw

Figure 5

 5 Figure 5 XPS diagrams of TGKN series glasses and different element orbitals: (a) full spectrum; (b) Te 3d

Figure 6

 6 Figure 6 Raman spectra of TGKN of glass series (a); Gaussian peak deconvolution of the Raman

  The glass composition was revealed by ICP-OES. The XRD pattern and DSC curves suggest that this colorchanging is not due to the formation of Te metal particles. The Nb 3d core-level spectra indicate no Nb 3+ or even low valence states of Nb in the TGKN series glasses. The structure of the TGKN glasses was analyzed by XPS and Raman spectra. ICP-OES and XPS full spectrum suggest that the Al was incorporated into the glass matrix. Both the Te 3d core-level XPS spectra and Raman spectra illustrate that the [TeO3] configuration tends to convert to [TeO4] configuration with increasing melting temperature. It can be speculated that Al incorporate into the tellurium glass network through connecting [TeO3] configuration or replacing Nb in the glassy network, and the former increases the BO contents, while the latter induces the formation of Nb-O clusters. The transmittance spectra indicate that these Nb-O clusters can form T-Nb2O5 by being linked by faces or edges, inducing the color-changing. Further study for these Nb-O clusters in glasses is on-going.

Funding

National Key Research and Development Project of China (2018YFE0207700); Natural Science Foundation of Zhejiang Province (Q21F050026); National Natural Science Foundation of China Joint Fund Project (U1909211); The Fundamental Research Funds for the Provincial Universities of Zhejiang (2022YW32).

Notes

The authors declare no competing financial interest.

Author contribution statement:

Panyi Wang: Investigation, Writing -original draft.

Yingying