# Columbite single-crystal CoV2O6 under high pressure: An XRD and Raman spectroscopy study

Josu Sánchez-Martín, Pierre Bouvier, Gaston Garbarino, Samuel

Gallego-Parra, Olivier Isnard, Plácida Rodríguez-Hernández, Alfonso Muñoz,

Daniel Errandonea, Julio Pellicer-Porres

► To cite this version:

Josu Sánchez-Martín, Pierre Bouvier, Gaston Garbarino, Samuel Gallego-Parra, Olivier Isnard, et al. Columbite single-crystal CoV2O6 under high pressure: An XRD and Raman spectroscopy study. Journal of Physical Chemistry C, 2025, 129 (22), pp.10364 - 10374. 10.1021/acs.jpcc.5c02341. hal-05173088

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Columbite single-crystal CoV<sub>2</sub>O<sub>6</sub> under high pressure:
An XRD and Raman spectroscopy study

J. Sánchez-Martín*,<fnref n="1" />, P. Bouvier<fnref n="2" />, G. Garbarino<fnref n="3" />, S. Gallego-Parra<fnref n="3" />, O. Isnard<fnref n="2" />, P. Rodríguez-Hernández<fnref n="4" />, A. Muñoz<fnref n="4" />, D. Errandonea<fnref n="1" /> and J. Pellicer-Porres<fnref n="1" />

<footnote n="1">¹Departamento de Física Aplicada-ICMUV, Universidad de Valencia, Dr. Moliner 50,
Burjassot, 46100 Valencia, Spain. E-mail: josu.sanchez@uv.es</footnote>

<footnote n="2">²Université Grenoble Alpes, CNRS, Institut Néel, 25 rue des Martyrs, 38042, BP166X,
Grenoble, France</footnote>

<footnote n="3">³European Synchrotron Radiation Facility, 38043 Grenoble, France</footnote>

<footnote n="4">⁴Departamento de Física, MALTA-Consolider Team, Universidad de La Laguna, San Cristóbal de La Laguna, E-38200 Tenerife, Spain</footnote>
ABSTRACT

The application of high pressure to columbite CoV2O6 resulted in two phase transitions, characterised by single-crystal X-ray diffraction and Raman spectroscopy up to 54.9(1) GPa. The first phase transition occurs at 19.9(1) GPa, preserving the space group (Pbcn) but undergoing a sudden symmetrisation of the polyhedra forming the structure, as well as a group shift of the vibrational modes towards higher wavenumbers. The named columbite-II structure remains stable up to 45.6(1) GPa. Beyond this pressure a different monoclinic polymorph (space group P21/c) was deduced from the high-quality diffraction data. The two observed phase transitions are reversible, but below 4.9(3) GPa the sample mainly transforms into the previously known brannerite polymorph. The anisotropic compressibility of the lattice axis and the Birch-Murnaghan equation of state were obtained from the XRD analysis. Our experimental results are supported by DFT calculations.
# I. INTRODUCTION

Metastable crystal structures of compounds can be achieved either in nature or in the laboratory by means of appropriate thermodynamic conditions. The pressure-temperature conditions provide access to a relative minimum of free energy where the sample can be trapped when returned to ambient conditions. A compound with a given stoichiometry can therefore exhibit completely different physical properties depending on its crystal structure, offering future scientific opportunities.

The family of metavanadates ($MV_2O_6$, where M is a divalent metal) presents a rich variety of crystal structures depending on the metal considered, such as the most common brannerite type [1–5] or the $NiV_2O_6$-type [2,6,7], among others [1,2,6].

The monoclinic brannerite structure (space group $C2/m$) is the natural stable form of cobalt metavanadate. The brannerite structure has been described as having a distorted cubic close-packed oxygen network [8,9]. although this is not strictly true. In fact, the monoclinic angle forces the distorted hexagonal oxygen planes, parallel to (001) planes, to systematically shift in the stacking sequence. Co and V cations are located in the octahedral sites between the oxygen layers. While the Co coordination is nearly octahedral, the V coordination deviates strongly from the ideal one, to the point that the oxygen coordination has often been described as 5+1. The arrangement of $VO_6$ octahedra sheets is similar to that of $TiO_2$ anatase, including edge-sharing two-wide zigzag chains of $VO_6$ octahedrons parallel to the b-axis. The so-called $\gamma$-$CoV_2O_6$ polymorph has been prepared in the laboratory [10,11]. This triclinic phase ($NiV_2O_6$-type) presents linear, edge-sharing, $CoO_6$ octahedra. However, the non-equivalent V atoms in the asymmetric unit have a different coordination. While two of them maintain the distorted $VO_6$ coordination, the third is tetrahedrally coordinated. $\gamma$-$CoV_2O_6$ is slightly denser than brannerite.

To obtain a substantially denser polymorph high-pressure synthesis must be employed. In fact, it has been 50 years since a metastable subset of this family was first reported in compounds containing Ni, Mg, Co, Zn, Mn and Cd as divalent metals. The metastable phase is obtained after applying pressures of 50-80 kbar and temperatures of 800-1200 °C and has the orthorhombic columbite-type structure (space group $Pbcn$) [8,9]. In this case the oxygen atoms are situated in a hexagonal close packed arrangement, with cation atoms in the octahedral interstices. Along the crystallographic c-axis, Fig. 1, there
are α-PbO₂-like columns defined by edge-sharing octahedra. The octahedra in each column are centered on either Co or V atoms. Along the a-axis, however, Co and V vertex-sharing octahedra alternate in a Co-V-V-Co sequence to form corrugated columns. The volume of columbite per unit formula is about 10% less than that of brannerite. Three non-equivalent positions are needed to describe the oxygens. They are color-coded in Fig. 1. In a single VO₆ octahedron, there are three O3 oxygen atoms, two O1 and one O2, while the CoO₆ octahedra includes four O2 and two O1 oxygen atoms. Along c, the Co octahedra are linked by oxygen atoms labelled O2, whereas O1 and O3 link V octahedra. Along a, the connections between Co and V octahedra are assured by O1 and O2 types of oxygen atoms. V-V octahedra are linked by O3 type oxygens [8,9].

With respect to high pressure (HP) characterization, only MgV₂O₆ has been studied in its columbite structure up to 22.5 GPa, with no phase transition detected by Raman measurements [12]. Considering other starting structures in metavanadates, brannerite ZnV₂O₆ and MgV₂O₆ both undergo a phase transition to a monoclinic structure (space group C2) at 16.6 [13] and 19 GPa [14] respectively. SrV₂O₆ and BaV₂O₆ transform from an orthorhombic structure (space group C222) to monoclinic structure (space group C2) at around 4 GPa, then both suffer amorphization between 9 and 10 GPa [15,16]. PbV₂O₆ remains stable in its orthorhombic structure (space group *Pnma*) up to 20 GPa [17]. On the other hand, many more non-vanadate compounds with a columbite-type packing have been studied under HP, such as the promising photocatalysts TiO₂ [18,19], various niobates [20–22], even mixed with vanadium [23], natural minerals [24,25], etc.

In this work, single crystals of columbite CoV₂O₆ have been measured up to 55 GPa by X-ray diffraction (XRD) and Raman experiments, both supported by ab initio density functional theory (DFT) calculations. Two phase transitions were found at 20 and 46 GPa, the first leading to an isostructural polymorph and the second implying a monoclinic distortion (space group *P2₁/c*). When the samples were returned to ambient conditions, a mixed phase dominated by the brannerite polymorph was found. From the XRD data, the bulk modulus (B₀) and linear compressibility were calculated for the two orthorhombic phases. These results contribute to a better understanding of the metavanadate family and the columbite-type materials.
## II. METHODS

### A. Synthesis

Pure phase precursor powders with the same chemical formula of the product were mixed in order to prepare CoV₂O₆. The blend was subsequently heat treated at 700 °C for 60 min under high pressure of 5.5 GPa, using a Conac type [26] press. This synthesis process has been followed by quenching the sample to room temperature [9]. Single crystalline pieces of typical size 20×20×5 µm and dark colour have been selected from the obtained samples.

### B. Details of high-pressure experiments

High-pressure experiments on single-crystal CoV₂O₆ were performed using a membrane-driven diamond-anvil cell (DAC) with low fluorescence 250/300 µm beveled diamond cullets. A pressure chamber of 140 µm in diameter and 35 µm in thickness was drilled in a rhenium gasket. Helium, loaded at 1.2 kbar, was used as the pressure-transmitting medium (PTM) to ensure high hydrostatic pressure conditions up to 42 GPa [27], just below the highest pressure reached in this study. The pressure was measured by means of the R1-line emission of a ruby ball placed close to the sample, using IPPS-Ruby2020 equation of state [28]. The ruby signal was measured before and after each measurement in order to control the pressure drift during long acquisitions. The recorded pressure was set at the average of these two pressure values and the uncertainty estimated as half of the difference between them. The homogeneity of the pressure in the DAC was followed from both the width and the splitting between the R1 and R2 ruby lines.

XRD measurements were performed at the ID15B beamline of the ESRF [29] using a monochromatic beam of wavelength 0.41 Å. The beam was focused down to a 1 × 1 µm² full width at half maximum (FWHM). An Eiger2 X CdTe 9 M PCD device detector was used to collect the XRD patterns, with a sample-to-detector distance of 180.98 mm, which was calibrated using the XRD images obtained from a single-crystal Vanadinite standard [30]. The collected XRD images were processed with CrysAlisPro (available from Rigaku Americas Corporation) and the structural analysis was performed with OLEX2 [31–33] and Jana2020 [34].
The Raman measurements were performed at room temperature using a 514.54 nm laser (Cobolt Fandango) and a 750 mm spectrometer (SP2750, Acton Research) with a 1800 grooves per millimetre grating (blazed at 500 nm), equipped with a cooled charge-coupled camera (PyLoN, Princeton), and a 50 µm entrance slit size that provides a resolution of 1 cm⁻¹. A set of Bragg filters (BNF-Opti grate) was used in order to reject the excitation line. The spectra were recorded in backscattering geometry with a 50× objective (Nikon) to focus the incident laser beam and collect the scattered light from inside the DAC through the diamond anvil. The wavenumber scale was calibrated using the plasma lines of a Ne-Ar lamp. The incident laser power was fixed at 8 mW (measured before the DAC) in order to avoid any laser heating of the sample. The Raman spectra covering a 15–1120 cm⁻¹ spectral range were recorded using two monochromator positions, with a maximum of 240 s acquisition time averaged over two to four acquisitions. In the 25–150 cm⁻¹ range, we have subtracted the contribution of N₂/O₂ rotations lines. Spectral parameters (position and full width at half maximum) were obtained with MATLAB [35] assuming a Pseudo-Voigt peak profile.

### C. Ab initio density-functional theory calculations

Ab initio calculations were performed within the framework of density functional theory (DFT) [36] using the Vienna ab initio Simulation Package (VASP) [37]. The projector augmented-wave (PAW) method [38,39] was employed. To ensure accurate convergence results, the plane-wave kinetic cut-off was extended up to 540 eV. The exchange-correlation energy was described via the generalized gradient approximation (GGA) with the Armiento and Mattsson (AM05) prescription [40,41]. The integrations over the Brillouin zone (BZ) were carried out with a k-special point sampling with a Monkhorst-Pack [42] grid of 2x6x6. To properly treat the strongly correlated states, the DFT +U method of Duradev et al. [43] was employed. This method utilizes a single parameter, $U_{eff} = U-J$, where U and J are the effective onsite Coulomb and exchange parameters, respectively. The values used for $U_{eff}$ [44] were 3.25 eV for V and 3.32 eV for Co. Between the two structural configurations considered for the columbite-type, the antiferromagnetic one was found to be lower in energy.

The unit cell parameters and atomic positions were fully optimized to obtain, at selected volumes, the relaxed structure. For the optimization, the criteria used were that the forces on the atoms were less than 2 meV/Å and that the deviations of the stress
tensors from a diagonal hydrostatic form were lower than 0.1 GPa. Our ab initio
calculations provide a data set of volumes, energies, and pressures (from the stress tensor)
that are fitted with a Birch-Murnaghan equation of state [45] to obtain the theoretical
equilibrium volume, the bulk modulus, and the pressure derivatives.

Lattice-dynamic calculations of the phonon modes were carried out at the zone
centre (Γ point) of the BZ with the direct force-constant approach provided by the
Phonopy package [46]. These calculations provide the frequency of the normal modes,
their symmetry, and their polarization vectors. This allows the identification of the
irreducible representations and the character of the phonon modes at the Γ-point.
### III. RESULTS AND DISCUSSION

#### A. High pressure single-crystal X-ray diffraction

High pressure single crystal XRD experiments show that columbite CoV₂O₆ remains stable at ambient temperature up to a pressure of 19.0(1) GPa. Refinement data and atomic positions at selected pressures are available in the Supplementary Material (SM) (Tables S1 and S4, respectively). Beyond this pressure, the unit-cell volume diminishes by 4.7%. Therefore, a first order isostructural phase transition is observed. In addition, the position of all atoms is slightly shifted, preserving the overall columbite arrangement but enhancing the regularity of the octahedra. The largest shift corresponds to Co. Its only atomic coordinate not determined by symmetry changes by almost 20%. The structural information has been deposited in the Cambridge Crystallographic Data Centre (CCDC) under reference number 2387761. More refinement data and atomic positions at selected pressures are provided in the SM (Tables S2 and S5, respectively). The isostructural phase, which we will henceforth designate as “columbite-II”, is illustrated in Fig. 1. As illustrated in the *a*-projection, the columns delineated by oxygen atoms exhibit a straighter configuration in the high-pressure polymorph, accompanied by a reduction in polyhedral distortion. It is worth to point out the relationship between the columbite-II phase and the TiO₂-II (α-PbO₂-type) structure. This phase can be considered as a (slightly distorted) TiO₂-II superstructure originated when the stacking cation planes are occupied by Co and V atoms in a Co-V-V sequence. In Fig 1 the TiO₂-II unit cell is marked with dashed lines. The pressure behaviour of the cation environment is further analysed in Fig. 2. Panels (a) and (b) display the evolution of Co-O and V-O bonds, respectively, as well as the average bond values. In the Co-O case, all bonds undergo a significant reduction at the phase transition. Respect to V polyhedra, the V-O bond lengths in the high-pressure phase exhibit a reduced dispersion around the average value, which, in addition, maintains a smooth decrease across the transition. Fig. 2 (c) represents the polyhedral volume, demonstrating that the larger CoO₆ octahedra undergo a more rapid reduction in volume under pressure compared to the VO₆ ones. After the phase transition, both volumes exhibit a notable similarity and maintain a comparable rate of decrease. Finally, Fig. 2 (d) addresses the issue of octahedra distortion. In ambient conditions, the VO₆ distortion index is observed to be larger than that of the CoO₆ octahedra. High pressure affects mostly the VO₆ octahedra, not only as volume is
continuously decreasing, but also at the phase transition. In spite of that, CoO₆ octahedra continue to be far more regular after the phase transition than the V centred octahedra. The change in VO₆ distortion index [47] is accompanied by an increase in effective coordination [47] of almost two units up to 46.0(1) GPa (inset). Taking all into account, we think that this symmetrisation of the columbite structure could be due to the loss of its antiferromagnetic nature [48,49]. This statement is backed by the DFT calculations reported in Fig. 2, which were performed using a non-magnetic version of the structure and remain in a fairly good agreement with the experimental results. Nevertheless, further experiments on the magnetic properties of the columbite-II structure after the phase transition would be necessary to properly support this conclusion.

The columbite-II structure remains stable up to 43.5(1) – 46.0(1) GPa. In this pressure range the diffraction quality drops significantly due to crystal cracking, progressively showing the increasing coexistence with a different monoclinic polymorph (space group *P*2₁/*c*) referenced as δ-CoV₂O₆. The structure of this phase was solved with the greatest degree of certainty within the 47.4(1) – 50.0(1) GPa pressure range. Refinement data and atomic positions extracted from the three largest pressures can be found at the SM (Tables S3 and S6, respectively). The structural information has been registered at de CCDC under deposition number 2387766. This second phase transition is associated with a 6.4% drop in unit-cell volume. The δ-CoV₂O₆ polymorph is represented in Fig. 3. Again, the atomic arrangement is clearly related to a high pressure TiO₂ polymorph, in particular the baddeleyite structured one described in Ref. [50]. Fig. 3 includes the TiO₂ monoclinic unit cell drawn with dashed lines. Both structures share spatial group. Along the a-axis the TiO₂ unit cell includes two cation planes. CoV₂O₆ contains three planes, in a V-Co-V sequence. Consequently, the *a* lattice parameter of both compounds is related by factor of 3/2. In both polymorphs, the oxygen atoms defining the hexagonal planes are slightly displaced from the ideal positions, with shifts occurring both along and perpendicular to the plane. As a result, the Ti and V coordination increases from 6 to 7. The associated polyhedrons define zigzag chains along the c-axis where the polyhedrons share edges. However, there are differences between the two structures due to the Co position, which, unlike the position of the V and Ti atoms, is in a Wyckoff site with inversion symmetry. The resulting Co coordination is sixfold. The connectivity between the above mentioned zigzag chains extending along the c-axis is
different in the monoclinic high pressure polymorphs of CoV₂O₆ and TiO₂. The pressure evolution of the Co and V environment is also included in Fig. 2.

The unit-cell lattice parameters and volume of all CoV₂O₆ polymorphs were obtained from the structure solving of the XRD data up to 50.0(1) GPa. The results are presented in Fig. 4 together with the DFT calculations of the columbite and columbite-II structures, showing excellent agreement. Second order Birch-Murnaghan equations of state (EOS) [45] were fitted using the EosFit7 software [51] for both columbite structures (dashed lines in Fig. 4). The second order nature of the EOS was confirmed from the Eulerian strain-normalized pressure dependence of the data [52]. The EOS fit parameters are gathered in Table 1. For experimental and calculated columbite-II data, 20.0(1) GPa was taken as the starting point for the fittings. The bulk modulus exhibited by columbite CoV₂O₆ (162(3) GPa) falls within the range of other metavanadates, such as the brannerite-type MgV₂O₆ (147(2) GPa) [14] and ZnV₂O₆ (188(5) GPa) [13].

The isothermal compressibility for all crystallographic axes of the columbite structures was obtained using the online PASCAL tool [53]. The obtained linear compressibilities are shown in Table 2 for both columbite structures, as well as experimental and DFT results. All axes exhibit comparable compression parameters, with the c-axis displaying the least compressibility, as expected based on the edge-sharing octahedra contact along this direction (Fig. 1). After the first phase transition, all axes become less affected by pressure, in particular the b-axis, which more than halves its compressibility parameter and leaves the a-axis as the most susceptible to change under pressure (almost doubling the other two axes).

## B. High pressure single-crystal Raman spectroscopy

As already discussed, columbite-type CoV₂O₆ is described by the orthorhombic space group Pbcn. The primitive unit cell contains 4 molecules, resulting in a total of 108 vibrational modes. At the centre of the Brillouin zone, the point group symmetry (mmm) classifies the Raman active modes as follows [54]:

$$ \Gamma_{\text{Raman}} = 13A_g + 14B_{1g} + 13B_{2g} + 14B_{3g}. $$

Before starting the HP Raman experiments we tested columbite-type CoV₂O₆ both in powder and in single crystal form. The Raman spectrum obtained from the powder
sample exhibited weaker Raman intensity and displayed only two additional modes compared to the spectrum obtained from the single crystal sample. Consequently, a small single crystal was selected for loading into the DAC.

High pressure Raman spectra of a columbite-type $CoV_2O_6$ single crystal are shown in Fig. 5. Once the sample was situated in the DAC pressure chamber, a total of 25 vibrational modes were identified in ambient conditions prior to the loading of the PTM. Three of the modes (asterisks in Fig. 5) were observed again only in the 3.4(1) GPa measurement, and another one in the 4.9(1) GPa downstroke pattern. Based on comparison with the spectrum of the recovered sample, see Section III.C, we identify these modes as belonging to a residual brannerite phase.

The wavenumbers of the observed Raman modes are shown in Table 3, along with the DFT calculated ones and their pressure coefficients. The symmetry assignment of the Raman modes is made by comparison with calculations, considering both the wavenumber and the pressure coefficient of the mode. In the absence of polarized Raman experiments, our symmetry assignment is therefore tentative. For comparison, we also present in Table 3 the mode assignment [8]. In that work, the assignment was also not based on polarized studies, but on the characterization of $ZnV_2O_6$ and the subsequent analysis of a series of compounds derived from cationic substitution. The agreement between the data presented in Table 3 is, in general, excellent.

When pressure was applied, 21 modes were followed up to 17.6(2) GPa. Selected Raman spectra from this range are shown in black in Fig. 5, while the evolution of each mode under the pressure effects is plotted in Fig. 6. All the observed modes evolve to higher wavenumbers with increasing pressure, except for the highest mode $A_g^{13}$ (see Fig. 6), which slightly softens from 10.7(2) GPa until the phase transition. This behaviour is paralleled in the DFT calculations from 17.3 to 20.3 GPa. Overall, the DFT simulated modes match most of the experimental modes in both starting wavenumber at ambient conditions and pressure coefficient (see Table 3). Additionally, DFT calculated infrared modes and its pressure coefficients can be found on Table S7.

Continuing with the experiment, the first phase transition to the isostructural columbite-II polymorph was observed at 19.9(1) GPa (blue spectra in Fig. 5). The phase transition induces the appearance of two additional modes (black dots in Fig. 6) and an overall shift of the spectrum to higher wavenumbers. The shifted modes are shown in Fig. 7.
6 using the same color scheme as the columbite phase. All modes of this isostructural phase go to higher wavenumbers with increasing pressure and can be followed up to 44.0(2) GPa.

It is noticeable that the modes above 730 cm⁻¹ vanish in the phase transition. We could follow the B₃g¹⁴ and B₂g¹² modes [810(4) and 830(5) cm⁻¹ in ambient conditions, respectively] only up to 9 GPa. Above this pressure, only the A_g¹³ mode [851(2) cm⁻¹ at ambient conditions] is detected. Even this very intense mode disappears in the phase transition. The transition from columbite to the columbite-II phase does not involve changes in either the symmetry or the Wyckoff position of the atoms, so the disappearance of the modes cannot be attributed to modifications of Raman selection rules induced by the phase transition. Even the symmetrized version of the columbite-II structure derived from the TiO₂-II polymorph (see Section III.A) shares the same spatial group, so arguments based on similarity to a more symmetric phase cannot be considered either. On the other side, a HP Raman experiment on anatase TiO₂ has been performed [55,56]. The transition to TiO₂-II is indeed associated with a large intensity reduction of the more energetic modes, although they remain easily measurable.

Fig. 7 shows the phonon vibration pattern corresponding to the A<sub>g</sub><sup>13</sup> mode in the columbite [0 GPa, panel a)] and columbite-II phases [around 22 GPa, panel b)]. Co atoms remain nearly static (shifts along *b* and *c* are forbidden by symmetry in this mode). The oxygen atoms with the largest amplitude vibrate quite closely in the direction of V-O bonds, which approximately matches Co-O octahedra edges. As a result, V-O bonds stretch. Co-O bonds bend, also contributing to the restoring forces. The most significant difference between the phonon patterns of columbite and columbite-II is the motion of the V atoms, which in columbite-II has a smaller amplitude and rotates to become closer to the perpendicular to the (100) stacking planes. According to calculations, the wavenumber of the A<sub>g</sub><sup>13</sup> mode (and also those of the closer modes) drops abruptly at the phase transition (Fig. 6). We interpret the drop as a consequence of the reduction of the restoring forces induced by the changes in the V motion just described.

When further pressure is applied, at 45.6(1) GPa, the sample undergoes a second phase transition to monoclinic δ-CoV₂O₆, Fig. 5. The spectrum of this phase is similar to the baddeleyite-structured TiO₂ reported in Ref. [50]. Since there are only two molecules in the primitive unit cell, the number of vibrational modes is halved compared to the
previous phase. Of the 54 vibrational modes, 24 are Raman active ($12A_g + 12B_g$). We were able to identify 22 modes in the measured spectra (empty black circles of Fig. 6). Data dispersion is significantly increased with respect to the lower pressure polymorphs due to spectra degradation caused by the loss of crystalline quality of the sample after the phase transition.

Subsequently, measurements were conducted during the downstroke. The phonon wavenumbers derived from these spectra are represented in Fig. 6 as void triangles. At a pressure of 33.4(9) GPa, the spectrum is consistent with the columbite-II structure. The polycrystalline nature of the sample in the downstroke enabled the characterization of some modes that were not observed in the upstroke (Fig. 6). At a pressure of 17.9(9) GPa in the downstroke, the measured spectrum corresponds to the columbite phase. Additional modes were observed, specifically the $B_{2g}^{11}$ and $A_g^{12}$ modes at 640 and 687 cm$^{-1}$, respectively. Following the high-pressure experiment and DAC opening, one further Raman measurement was conducted. A detailed discussion of this measurement can be found in Section III.C.

### C. Sample recovered to ambient conditions

Prior to concluding the Raman experiment, the last run of measurements were obtained opening the DAC and thereby releasing all the remaining pressure. Fig. 8 illustrates the Raman signal of the sample in two distinct sections (Area A and B). The Raman patterns exhibit a notable disparity due to the non-homogeneous nature of the sample subsequent to the HP experiment. The fitted modes in Area A (see Fig. 8) are consistent with the brannerite polymorph, as the Raman profile is analogous to those reported for brannerite-type $ZnV_2O_6$ [13] and $MgV_2O_6$ [14]. The most intense modes in Fig. 8 also correspond with the extra peaks (asterisks) observed in Fig. 5, indicating the presence of some brannerite $CoV_2O_6$ impurities at the beginning of the experiment. Area B demonstrates that this phase is not the sole remaining constituent, as it exhibits different vibrational modes in comparison to Area A.

Additional XRD measurements were conducted on the recovered sample to identify the polymorphs present in the mixture. An attempt was made to develop an analysis similar to that presented in Section III.A; however, the conditions of the sample precluded the single-crystal treatment of the data. Consequently, all 128 diffraction images were merged into a single one and treated with the DIOPTAS software [57], as is
customary with powder samples. Unfortunately, the data obtained from this polycristalline sample lacks the quality necessary to perform a proper powder refinement. Figure S1 of the SM shows the integrated signal of the XRD measurement matching with the diffraction contributions of the potential remaining compounds in the sample. In this spectrum, contributions from columbite $CoV_2O_6$ and Re from the gaskets are found as expected. There are also reflections of the predominant downstroke phase (brannerite) and others related to $\gamma-CoV_2O_6$. These two phases are to be expected as they can be synthetized at ambient pressure. [5,10].

In summary, the Raman spectra obtained following the release of all pressure indicate that brannerite-type $CoV_2O_6$ is the predominant remaining phase. The phase transition from columbite $CoV_2O_6$ occurs at pressures below 4.9(3) GPa during decompression. Although the XRD signal and its results are of a lower quality than those of the rest of this work, they indicate the presence of residual columbite $CoV_2O_6$ and $\gamma-CoV_2O_6$ in the sample, in addition to the predominant brannerite polymorph.
## IV. CONCLUSIONS

High pressure X-ray diffraction and Raman spectroscopy experiments have been carried out on single crystals of columbite-type $CoV_2O_6$ up to 50.0(1) and 54.9(1) GPa respectively. Both experiments show an isostructural phase transition to the columbite-II polymorph at 20 GPa. The XRD data reflect the symmetrisation of the $CoO_6$ and VO6 octahedra forming the structure, while the Raman spectra show a collective shift of all vibrational modes to higher wavenumbers together with the disappearance of the modes related to the V-O bonds. At 45.6(1) GPa the vibrational data change completely, indicating a second phase transition. The diffraction data show that this structure is a different monoclinic polymorph (space group *P2₁/c*) with increased coordination of the VO₇ polyhedra. On decompression, both phase transitions show reversibility in the same ranges. However, below 4.9(3) GPa the sample undergoes a further structural changes resulting in a phase admixture with $CoV_2O_6$ brannerite as the predominant polymorph. Anisotropic compressibility and EOS parameters (including bulk moduli) are obtained for both columbite polymorphs. All results for these structures are also supported by DFT calculations, which show excellent overall agreement.

## ACKNOWLEDGMENTS

The authors acknowledge financial support from the Spanish Research Agency (AEI) and Spanish Ministry of Science and Investigation (MCIN) under grants PID2022-138076NB-C41/C44, RED2022-134388-T (DOI: 10.13039/501100011033). This work was also supported by the Generalitat Valenciana under Grant No. PROMETEO CIPROM/2021/075-GREENMAT and MFA/2022/007. This study forms part of the Advanced Materials program and is supported by MCIN with funding from European Union Next Generation EU (PRTR-C17.I1) and by the Generalitat Valenciana. J. S.-M. acknowledges the Spanish Ministry of Science, Innovation and Universities for the PRE2020-092198 fellowship. The authors thank ESRF synchrotron for providing beam time for the HP XRD experiments (BLC14893).
Figures

<figure><img src="image_2.png" /><figcaption>FIG 1. The orthorhombic (Pbcn) crystal structure of CoV<sub>2</sub>O<sub>6</sub> before (19.0(1) GPa) and after (20.0(1) GPa) the isostructural phase transition. Both structures are columbite-type, but the atom positions and unit-cell parameters vary significantly. Dashed lines illustrate the relationship with the TiO<sub>2</sub> phase.</figcaption></figure>

TO BE PRESENTED IN DOUBLE COLUMN
<figure><img src="image_3.png" /><figcaption>FIG 2. Pressure evolution of internal parameters of the CoV<sub>2</sub>O<sub>6</sub> crystal. (a) Co-O bond distances, (b) V-O bond distances, (c) polyhedral volumes, (d) polyhedral distortion indexes and (e, inset) effective coordination of the polyhedra. Black, blue and red data correspond to columbite, columbite-II and δ-CoV<sub>2</sub>O<sub>6</sub> structures respectively. Experimental data are shown with dots/squares (V, Co respectively), while DFT calculated result are represented with lines.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>FIG 3. Crystal structure of monoclinic δ-CoV₂O₆. Co, V and O atoms are in blue, red and green respectively. The related monoclinic TiO₂ unit cell is also sketched with dashed lines.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>FIG 4. Pressure dependence of CoV~2~O~6~ unit-cell parameters (top) and volume/Z (bottom). Black, blue and red dots correspond to experimental results on columbite, columbite-II and δ-CoV~2~O~6~ polymorphs respectively. Solid lines are DFT calculated results, while dashed lines represent EOS fits. The shadowed region mark coexistence of columbite-II and δ-CoV~2~O~6~ phases (44.5(1) to 47.5(1) GPa). The inset shows the β parameter of the monoclinic structure. Please note that the *a* parameter of this structure is multiplied by 2.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>FIG 5. Raman spectra of single crystal CoV<sub>2</sub>O<sub>6</sub> at selected pressures indicated on the label to the right of the spectra. Black, blue and red correspond to columbite, columbite-II and monoclinic δ-CoV<sub>2</sub>O<sub>6</sub> structures, respectively. Letter “d” indicates downstroke measurements. These spectra are zoomed to improve their visibility. Peaks labelled with asterisk belong to a residual brannerite phase.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>FIG 6. Pressure dependence of the Raman modes of columbite CoV₂O₆. Colored dots are experimental, while grey lines are DFT calculated. Columbite modes match in colour after the first phase transition. Black squares mark brannerite CoV₂O₆ impurities. Additional modes after the first phase transition are in solid black, while the ones for the monoclinic δ-CoV₂O₆ structure are hollow black dots. Hollow triangles correspond to downstroke pressure measurements.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>FIG 7 Phonon vibration pattern of the $A_g^{13}$ mode, following ab-initio calculations. Arrow lengths are proportional to amplitude vibration. a) Columbite phase at 0 GPa. b) Columbite-II phase at 22 GPa.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>FIG 8. Raman spectrum of the CoV~2~O~6~ polycrystal after de HP experiment in two different sections. Area A is fitted and labelled with expected modes of brannerite CoV~2~O~6~.</figcaption></figure>
TABLES

<table><thead><tr><th>CoV<sub>2</sub>O<sub>6</sub></th><th>V<sub>0</sub> (Å<sup>3</sup>)</th><th>B<sub>0</sub> (GPa)</th></tr></thead><tbody><tr><td>Experimental Columbite</td><td>360(3)</td><td>162(3)</td></tr><tr><td>DFT calculated Columbite</td><td>357.8(2)</td><td>164.7(5)</td></tr><tr><td>Experimental Columbite-II (from 20 GPa)</td><td>311(2)</td><td>318(5)</td></tr><tr><td>DFT calculated Columbite-II (from 20 GPa)</td><td>310.0(2)</td><td>310.2(2)</td></tr></tbody></table>

Table 1. EOS parameters (unit-cell volume and bulk modulus) for CoV₂O₆ obtained through the fitting of second order Birch-Murnaghan model (B' is fixed at 4). For columbite-II, V₀ corresponds to 20 GPa.

<table><thead><tr><th>CoV<sub>2</sub>O<sub>6</sub> (×10<sup>-3</sup> GPa<sup>-1</sup>)</th><th>κ<sub>a</sub></th><th>κ<sub>b</sub></th><th>κ<sub>c</sub></th></tr></thead><tbody><tr><td>Experimental Columbite</td><td>1.74(9)</td><td>1.80(2)</td><td>1.44(3)</td></tr><tr><td>DFT calculated Columbite</td><td>1.92(3)</td><td>1.63(2)</td><td>1.28(1)</td></tr><tr><td>Experimental Columbite-II (from 20 GPa)</td><td>1.22(7)</td><td>0.73(1)</td><td>0.81(1)</td></tr><tr><td>DFT calculated Columbite-II (from 20 GPa)</td><td>1.416(3)</td><td>0.681(2)</td><td>0.730(4)</td></tr></tbody></table>

Table 2. Isothermal compressibility coefficients of the principal unit-cell axes of CoV₂O₆.

<table><thead><tr><th rowspan="2">mode</th><th colspan="2">DFT</th><th colspan="2">this work</th><th>Ref. [9]</th></tr><tr><th>ω<sub>0</sub></th><th>∂ω/∂P</th><th>ω<sub>0</sub></th><th>∂ω/∂P</th><th>ω<sub>0</sub></th></tr></thead><tbody><tr><td>B<sup>1</sup><sub>3g</sub></td><td>85</td><td>2.1(1)</td><td>72(2)</td><td>2.6(2)</td><td>71 (A<sub>g</sub>)</td></tr><tr><td>A<sup>1</sup><sub>g</sub></td><td>100</td><td>0.8(1)</td><td></td><td></td><td>60 (A<sub>g</sub>)</td></tr><tr><td>B<sup>2</sup><sub>3g</sub></td><td>147</td><td>2.7(1)</td><td>129(3)</td><td>3.4(1)</td><td>130 (A<sub>g</sub>)</td></tr><tr><td>B<sup>1</sup><sub>2g</sub></td><td>149</td><td>1.3(1)</td><td></td><td></td><td></td></tr><tr><td>B<sup>1</sup><sub>1g</sub></td><td>156</td><td>4.8(1)</td><td>146(2)</td><td>3.0(1)</td><td>144 (B<sub>1g</sub>)</td></tr><tr><td>B<sup>2</sup><sub>2g</sub></td><td>161</td><td>1.5(1)</td><td></td><td></td><td></td></tr><tr><td>B<sup>2</sup><sub>1g</sub></td><td>162</td><td>2.7(1)</td><td></td><td></td><td></td></tr><tr><td>A<sup>2</sup><sub>g</sub></td><td>167</td><td>1.9(1)</td><td>168(2)</td><td>3.0(7)</td><td></td></tr><tr><td>B<sup>3</sup><sub>3g</sub></td><td>183</td><td>1.6(1)</td><td>183(5)</td><td>Powder</td><td></td></tr><tr><td>B<sup>3</sup><sub>1g</sub></td><td>202</td><td>1.2(2)</td><td>207(4)</td><td>1.4(3)</td><td></td></tr><tr><td>B<sup>4</sup><sub>3g</sub></td><td>208</td><td>2.0(1)</td><td>201(4)</td><td>2.1(4)</td><td>200 (B<sub>2g,3g</sub>)</td></tr><tr><td>B<sup>3</sup><sub>2g</sub></td><td>219</td><td>2.5(1)</td><td></td><td></td><td></td></tr><tr><td>A<sup>3</sup><sub>g</sub></td><td>236</td><td>2.6(3)</td><td>239(3)</td><td>2.5(2)</td><td>238 (A<sub>g</sub>)</td></tr><tr><td>B<sup>4</sup><sub>1g</sub></td><td>236</td><td>2.7(1)</td><td>235(4)</td><td>2.7(4)</td><td></td></tr><tr><td>B<sup>5</sup><sub>3g</sub></td><td>236</td><td>1.6(1)</td><td></td><td></td><td></td></tr></tbody></table>
<table><tr><td>A<sub>g</sub><sup>4</sup></td><td>250</td><td>2.0(1)</td><td></td><td></td><td></td></tr><tr><td>A<sub>g</sub><sup>5</sup></td><td>275</td><td>2.8(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>2g</sub><sup>4</sup></td><td>284</td><td>2.6(1)</td><td>279(2)</td><td>2.3(1)</td><td>279 (A<sub>g</sub>)</td></tr><tr><td>B<sub>1g</sub><sup>5</sup></td><td>289</td><td>3.1(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>3g</sub><sup>6</sup></td><td>292</td><td>0.8(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>2g</sub><sup>5</sup></td><td>295</td><td>9.9(4)</td><td></td><td></td><td></td></tr><tr><td>B<sub>1g</sub><sup>6</sup></td><td>308</td><td>0.9(1)</td><td>301(4)</td><td>1.3(4)</td><td></td></tr><tr><td>B<sub>3g</sub><sup>7</sup></td><td>312</td><td>5.7(1)</td><td></td><td></td><td></td></tr><tr><td>A<sub>g</sub><sup>6</sup></td><td>323</td><td>1.4(1)</td><td>317(4)</td><td>2.9(1)</td><td>320 (B<sub>1g</sub>)</td></tr><tr><td>A<sub>g</sub><sup>7</sup></td><td>328</td><td>2.6(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>2g</sub><sup>6</sup></td><td>328</td><td>1.5(8)</td><td></td><td></td><td></td></tr><tr><td>B<sub>1g</sub><sup>7</sup></td><td>330</td><td>0.9(1)</td><td>338(4)</td><td>1.0(6)</td><td></td></tr><tr><td>B<sub>2g</sub><sup>7</sup></td><td>351</td><td>2.7(1)</td><td>354(3)</td><td>3.5(5)</td><td>356 (A<sub>g</sub>)</td></tr><tr><td>B<sub>1g</sub><sup>8</sup></td><td>365</td><td>2.3(1)</td><td></td><td></td><td></td></tr><tr><td>A<sub>g</sub><sup>8</sup></td><td>367</td><td>2.4(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>3g</sub><sup>8</sup></td><td>383</td><td>2.4(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>3g</sub><sup>9</sup></td><td>389</td><td>4.0(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>1g</sub><sup>9</sup></td><td>395</td><td>4.7(1)</td><td>400(3)</td><td>3.2(1)</td><td>399 (A<sub>g</sub>)</td></tr><tr><td>A<sub>g</sub><sup>9</sup></td><td>414</td><td>3.3(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>2g</sub><sup>8</sup></td><td>416</td><td>1.3(1)</td><td>428(5)</td><td>1.0(5)</td><td></td></tr><tr><td>B<sub>1g</sub><sup>10</sup></td><td>441</td><td>2.7(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>3g</sub><sup>10</sup></td><td>447</td><td>2.8(1)</td><td>454(3)</td><td>2.6(1)</td><td></td></tr><tr><td>A<sub>g</sub><sup>10</sup></td><td>467</td><td>5.5(1)</td><td>467(5)</td><td>5.6(1)</td><td></td></tr><tr><td>B<sub>2g</sub><sup>9</sup></td><td>469</td><td>3.5(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>2g</sub><sup>10</sup></td><td>477</td><td>4.5(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>3g</sub><sup>11</sup></td><td>479</td><td>3.2(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>1g</sub><sup>11</sup></td><td>491</td><td>5.2(1)</td><td>511(4)</td><td>Powder</td><td></td></tr><tr><td>A<sub>g</sub><sup>11</sup></td><td>535</td><td>2.4(1)</td><td>572(2)</td><td>1.4(1)</td><td>571 (A<sub>g</sub>)</td></tr><tr><td>B<sub>1g</sub><sup>12</sup></td><td>565</td><td>3.6(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>3g</sub><sup>12</sup></td><td>593</td><td>4.4(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>1g</sub><sup>11</sup></td><td>647</td><td>2.1(1)</td><td>631(5)</td><td>2.0(5)</td><td></td></tr><tr><td>A<sub>g</sub><sup>12</sup></td><td>666</td><td>2.5(1)</td><td></td><td></td><td></td></tr><tr><td>B<sub>1g</sub><sup>13</sup></td><td>693</td><td>2.9(1)</td><td>697(4)</td><td>1.9(1)</td><td>692 (B<sub>2g</sub>)</td></tr><tr><td>B<sub>3g</sub><sup>13</sup></td><td>712</td><td>1.6(1)</td><td>727(3)</td><td>2.3(2)</td><td>723 (A<sub>g</sub>)</td></tr><tr><td>B<sub>1g</sub><sup>14</sup></td><td>784</td><td>3.3(1)</td><td></td><td></td><td></td></tr></table>
<table><caption>Table 3. Raman modes, wavenumbers and pressure coefficients corresponding to the zone-center Raman active modes under ambient conditions for columbite CoV<sub>2</sub>O<sub>6</sub>. ω<sub>0</sub> is expressed in cm<sup>-1</sup>, and P, in GPa. The DFT-calculated ω<sub>0</sub> has a related uncertainty of ±5%.</caption><tr><td>B<sup>14</sup><sub>3g</sub></td><td>791</td><td>3.6(1)</td><td>810(4)</td><td>6.6(1)</td><td>811 (B<sub>1g,2g,3g</sub>)</td></tr><tr><td>B<sup>12</sup><sub>2g</sub></td><td>804</td><td>3.5(1)</td><td>830(5)</td><td>1.6(8)</td><td></td></tr><tr><td>A<sup>13</sup><sub>g</sub></td><td>838</td><td>2.2(1)</td><td>851(2)</td><td>2.2(1)</td><td>850 (A<sub>g</sub>)</td></tr><tr><td>B<sup>13</sup><sub>2g</sub></td><td>842</td><td>1.4(1)</td><td></td><td></td><td></td></tr></table>
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