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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Stability of FeVO4 under Pressure: An X‑ray Diffraction and FirstPrinciples Study Sinhué López-Moreno,*,† Daniel Errandonea,*,‡ Julio Pellicer-Porres,‡ Domingo Martínez-García,‡ Sadeque J. Patwe,§ S. Nagabhusan Achary,§ Avesh Kumar Tyagi,§ Placida Rodríguez-Hernández,∥ Alfonso Muñoz,∥ and Catalin Popescu⊥ †

CONACYT - División de Materiales Avanzados, IPICYT, Camino a la presa San José 20155, San Luis Potosí, S. L. P. 78216, Mexico Departamento de Física Aplicada-ICMUV, Universidad de Valencia, MALTA Consolider Team, Edificio de Investigación, C. Dr. Moliner 50, 46100 Burjassot, Spain § Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India ∥ Departamento de Física, Instituto de Materiales y Nanotecnología, MALTA Consolider Team, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain ⊥ CELLS-ALBA Synchrotron Light Facility, Cerdanyola, 08290 Barcelona, Spain ‡

ABSTRACT: The high-pressure behavior of the crystalline structure FeVO4 has been studied by means of X-ray diffraction using a diamond-anvil cell and first-principles calculations. The experiments were carried out up to a pressure of 12.3 GPa, until now the highest pressure reached to study an FeVO4 compound. High-pressure X-ray diffraction measurements show that the triclinic P1̅ (FeVO4-I) phase remains stable up to ≈3 GPa; then a first-order phase transition to a new monoclinic polymorph of FeVO4 (FeVO4-II′) with space group C2/m is observed, having an α-MnMoO4-type structure. A second first-order phase transition is observed around 5 GPa toward the monoclinic (P2/c) wolframite-type FeVO4-IV structure, which is stable up to 12.3 GPa in coexistence with FeVO4-II′. The unit cell volume reductions for the first and second phase transitions are ΔV = −8.5% and −13.1%. It was observed that phase transitions are irreversible and both high-pressure phases remain stable once the pressure is released. Calculations were performed at the level of the generalized gradient approximation plus Hubbard correction (GGA+U) and with the hybrid Heyd−Scuseria−Ernzerhof (HSE06) exchange-correlation functional in order to have a good representation of the pressure behavior of FeVO4. We found that theoretical results follow the pressure evolution of structural parameters of FeVO4, in good agreement with the experimental results. Also, we analyze FeVO4-II (orthorhombic Cmcm, CrVO4-type structure) and -III (orthorhombic Pbcn, α-PbO2-type structure) phases and compare our results with the literature. Going beyond the experimental results, we study some possible post-wolframite phases reported for other compounds and we found a phase transition for FeVO4-IV to raspite (monoclinic P21/c) type structure (FeVO4-V) at 36 GPa (ΔV = −8.1%) and a further phase transition to the AgMnO4-type (monoclinic P21/c) structure (FeVO4-VI) at 66.5 GPa (ΔV = −3.7%), similar to the phase transition sequence reported for InVO4. transition metal ions.1−3,5−7,11,12 Besides, the vanadates of the Fe3+ and Cr3+ have attracted studies under ambient condition in order to understand the diverse properties2,3,5,11,13−15 and structures formed under different preparation conditions.16−25 In an earlier study, the structural properties of orthorhombic CrVO4-type [space group (SG) Cmcm] InVO4 have been extensively studied under pressure, along with the polymorphic transitions to different phases, like an unknown phase and a monoclinic wolframite-type (SG P2/c) phase at higher pressure.16 Further theoretical investigations revealed that the orthorhombic phase of InVO4 is likely to transform to

1. INTRODUCTION ABO4-type oxides (where A = Cr, Fe, In, Ti, Tl; B = As, P, V) are of interest due to their wide physical properties relevant to the photovoltaic cells for solar energy utilization, catalysts for water splitting, electrolyte for lithium-ion batteries, gas sensors, etc.1−7 Furthermore, such materials are known to exhibit structural diversity depending on the nature and ionic radius of the A site cation as well as external parameters like pressure and/or temperature.1,8−10 The primary physical properties of such materials are originated from the d-electrons of the transition metal ions, while the active participation of the tetrahedral rigid groups of BO4 has less influence on the physical properties. This is because they are isolated structure fragments which serve as bridges between the polyhedra of © XXXX American Chemical Society

Received: April 11, 2018

A

DOI: 10.1021/acs.inorgchem.8b00984 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 1. Crystal structures of FeVO4 phases: (a) FeVO4-I (P1̅), (b) FeVO4-II (CrVO4-type), (c) FeVO4-II′ (α-MnMoO4-type), (d) FeVO4-III (αPbO2-type), (e) FeVO4-IV (wolframite), (f) FeVO4-V (raspite), and (g) FeVO4-VI (AgMnO4-type). Arrows indicate the direction of the magnetic moment of Fe atoms for the magnetic state of lowest energy, where a, b, and c are the lattice parameters.

temperature and pressure.18,24 The orthorhombic FeVO4-II phase has been prepared under high temperature and pressure18 but also by the hydrothermal method.35 The prototype of this phase is the CrVO4-type structure which has been already reviewed by Baran.8 The other vanadates that crystallize in this structure are CrVO4,8 InVO4,16,17 and TlVO4.36 The orthorhombic FeVO4-III, the α-PbO2-type structure, was first synthesized at 750 °C and 6 GPa,37 but it was also observed in the high-pressure studies performed by Hotta et al.24 According to ref 18, the monoclinic wolframite FeVO4-IV phase was first found at 800 °C and 8 GPa. On the other hand, this phase was observed at ≈4 and ≈5.5 GPa at 800 °C.24 There is an important list of ABO4 compounds that crystallize in this structure, being the most representative the AWO4 compounds.38−41 The crystal structure of the FeVO4 polymorphs is shown in Figure 1. Besides, the structural changes in FeVO4 by chemical substitution have also been investigated in order to understand its structural variation as well as to tune its physical and chemical properties.21,23,42,43 It has been observed that a small substitution of other cations transforms the triclinic FeVO4-I to the monoclinic (SG C2/m) α-MnMoO4-type structure and it remains stable in a wider range of compositions. To the best of our knowledge, the only study about the phase transitions driven by pressure in FeVO4 was reported back in the 1980s.24 A piston-cylinder apparatus (for pressures below 2 GPa) and a cubic anvil (from 2 to 5.5 GPa) were used to study this compound in conjunction with X-ray and differential scanning calorimetry (DSC) measurements. In this study, three successive pressure induced phase transitions as FeVO4-I → -II → -III → -IV have been identified from a limited number of measurements, while no structural details were presented. Phase I was found to be stable at ambient pressure and at ≈0.5 GPa in coexistence with phase II, whereas phase II appears

wolframite, raspite (SG P21/a), and AgMnO4-type (SG P21/n) structure under pressure, whereas, at a lower pressure, the formation of α-MnMoO4-type (SG C2/m) may be observed.17 It can be noted here that the α-MnMoO4-type InVO4 can be prepared by controlled low temperature under ambient pressure or hydrothermal synthesis routes and is metastable and transforms to the stable orthorhombic (Cmcm) structure at a higher temperature through an intermediate phase.19,26 Moreover, it has been proposed that CrVO4-type AVO4 (A = Fe3+, Cr3+, In3+, etc.) oxides can undergo phase transitions under pressure to structures having higher coordination numbers of either or both cations.1,8,17,27−29 It is known that the cristobalite- and berlinite-type phosphates also transform to the CrVO4-type structure at higher pressure and subsequently to a distorted CaCl2-type structure by increasing the coordination of P5+ from 4 to 6.30 Structural transition sequence CrVO4-type to zircon to scheelite to wolframitetype phases for CrVO4-type InPO4 and TiPO4 has been predicted by DFT.31 The post-CrVO4-type structures with an increased coordination number of vanadium in vanadates have been observed at relatively lower pressure, whereas in the analogous phosphates where the coordination number of phosphorus remains four up to 70 GPa.31 Despite several studies reported for orthorhombic AVO4type vanadates, analogous studies on FeVO4 are not available in the literature. This can be due to the structural complexity of FeVO4 compared to other transition metal orthovanadates. FeVO4 possesses a unique triclinic crystal structure at ambient conditions (phase FeVO4-I), which has zigzag chains formed by edge shared FeO6 octahedra and FeO5 trigonal pyramids, and these chains are connected by VO4 tetrahedra.18,25,32,33 Another vanadate with the same structure is AlVO4.34 Besides the triclinic FeVO4-I structure, it was reported that FeVO4 can exist in three other polymorphs under different conditions of B


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Inorganic Chemistry alone at ≈1 and ≈1.5 GPa. In this study, the authors only reported on the existence of phase III at ≈2 GPa and phase IV at ≈4 and ≈5.5 GPa. As we can see, the information available about the structural behavior of FeVO4 under pressure is pretty limited, as well as the range of pressure for which each phase is stable. Since there is not systematic high-pressure study on triclinic FeVO4 available in the literature, the transition sequence and pressure induced lattice distortion are unexplained to date. In this study, we have carried out a systematic investigation on the effect of pressure on triclinic FeVO4-I and concluded that the structure irreversibly transforms to denser phases with increasing coordination of V5+ sequentially from 4 to 6 (4 + 2) to 6. Also, it is observed that transitions are reconstructive and occur with large volume collapses. Ab initio density functional calculations have also supported the structures of high-pressure phases and phase transition pressures. The room temperature equations of state corresponding to all phases have been obtained for the ambient as well as high-pressure phases. On the other hand, first-principles calculations have been used to study the stability of the all known phases of FeVO4, but also to propose possible post-wolframite phases far beyond the range of the pressures reached by experiments. The paper is organized as follows: In sections 2 and 3, we give a detailed description of the experimental methods and the computational procedure, respectively. The experimental and theoretical results at ambient conditions and under pressure are discussed in section 4. The concluding remarks are given in section 5.

projector-augmented wave (PAW)49,50 method as implemented in the Vienna Ab initio Simulation Package (VASP).51−54 A plane-wave energy cutoff of 520 eV was used to ensure a high precision in our calculations. The exchange-correlation energy was described with the HSE0655−58 hybrid functional. We also performed the calculations within the generalized gradient approximation (GGA) in the GGA+U method with the Perdew−Burke−Ernzerhof (PBE)59 and in the PBE for solids (PBEsol)60 in order to study other possible high-pressure phases at a lower computational effort in comparison with the calculations with HSE06 functional. The GGA+U was used to account for the strong correlation between the electrons in the d shell, on the basis of the Dudarev’s method.61 In this method, the Coulomb interaction U and the onsite exchange interaction JH are treated together as Ueff = U − JH. For our GGA+U calculations, we choose U = 6 eV and JH = 0.95 eV. Similar values were previously used with success in the study of other iron and vanadate compounds.39,62−65 To ensure the quality of the chosen Ueff value, we calculated the dependence of the phase transition as a function of the Ueff in a range from 3 to 7 eV. We found that the results observed with the HSE06 functional are well represented with the GGA+U approximation with a value of Ueff = 5.05 eV. The Monkhorst−Pack scheme66 was employed to discretize the Brillouin zone (BZ) integrations with meshes 3 × 2 × 2, 4 × 3 × 3, 2 × 5 × 3, 4 × 4 × 4, 4 × 4 × 4, 4 × 4 × 2, and 3 × 4 × 2 which correspond to a set of 6, 8, 9, 8, 16, 8, and 6 special k points in the irreducible BZ for the FeVO4-I (SG P1̅, No. 2, Z = 6), FeVO4-II (CrVO4-type, SG Cmcm, No. 63, Z = 4), FeVO4II′ (α-MnMoO4-type, SG C2/m, No. 12, Z = 4), FeVO4-III (αPbO2-type, SG Pnc2, No. 30, Z = 2), FeVO4-IV (wolframite, SG P2/c, No. 13, Z = 2), FeVO4-V (raspite, SG P21/c, No. 14, Z = 4), and FeVO4-VI (AgMnO4-type, SG P21/c, No. 14, Z = 4) phases, respectively. For the other phases considered in the high-pressure regime, we have used the most suitable mesh for each case. In the relaxed equilibrium configuration, the forces are less than 2 meV/Å per atom in each of the Cartesian directions.

2. EXPERIMENTAL METHODS Polycrystalline FeVO4 was synthesized by solid-state reaction of appropriate amounts of Fe2O3 (99.9%, Aldrich) and V2O5 (99.5%, Riedel-de Haën) at 750 °C. Pellets of a homogeneous mixture of the reactants were heated slowly to 750 °C and held for 12 h. This procedure was repeated until the complete formation of phase pure product. The dark brown sintered product obtained was characterized by powder XRD data recorded on a rotating anode based powder XRD using Cu Kα radiation. On the basis of powder XRD, a good crystallinity and phase high purity was determined for the synthesized FeVO4. No composition analysis was performed. However, the excellent fit obtained from Rietveld refinements at ambient conditions indicates a negligible presence of impurities (if any) at that composition of FeVO4 does not deviate from stoichiometry. HPXRD patterns were recorded by using diamond-anvil cells (DAC) with diamond culets of 300 μm in angle-dispersive mode. The pressure chamber was a 100 μm hole drilled on a 40 μm preindented inconel gasket, and pieces of sintered pellets of FeVO4 were loaded along with ruby for pressure measurements. 16:3:1 methanol−ethanol−water was used as a pressure-transmitting medium (PTM). XRD experiments were performed at the MSPD beamline at the ALBA synchrotron facility using Kirkpatrick−Baez mirrors to focus the monochromatic beam to the sample inside the DAC and a Rayonix CCD detector with a 165 mm diameter of active area to collect the scattered X-rays.44 Monochromatic radiation of a wavelength of 0.4246 Å was used for the high-pressure X-ray diffraction studies. The sample−detector distance was set to 280 mm. The images of diffraction data collected on the CCD detector were integrated by using FIT2D software.45 Pressure was determined by using ruby as pressure standard.46 Pressure uncertainties are smaller than 0.1 GPa in the pressure range covered by the experiments. Structural analyses were performed with the Fullprof 2000 package.47

4. RESULTS AND DISCUSSION 4.1. Structural Description at Ambient Pressure. The powder XRD pattern of the polycrystalline sample agrees well with the triclinic phase of FeVO4 (Phase I) reported earlier in the literature.21,32,33,67 Further, the structure and phase purity of the sample was confirmed by Rietveld refinement of the observed powder XRD data (see description of experimental techniques). Structural parameters reported by Robertson and Kostiner33 for FeVO4-I were used as initial model parameters for Rietveld refinement. The background of the XRD pattern was modeled by a fifth-order polynomial while the peaks were generated by using a pseudo-Voigt profile function. All the observed reflection, as well as intensity of the XRD data, could satisfactorily be explained by the considered model and residuals of refinements are Rp = 12.8%, Rwp = 15.9%, and χ2 = 2.07. The final Rietveld refinement plot for the ambient condition XRD data is shown in Figure 2. The refined structural parameters and interatomic distances of the triclinic phase are given in Tables 1−3. The refined unit cell parameters of the FeVO4-I under ambient pressure and temperature are: a = 6.7080(1) Å, b = 8.0533(2) Å, c = 9.3414(3) Å; α = 96.682(2)°, β = 106.585(2)°, γ = 101.515(2)°; V = 465.77(2) Å3, Z = 6. These refined unit cell parameters are in agreement (within 0.6%) with those reported earlier in the literature (a =

3. OVERVIEW OF THE CALCULATIONS Calculations of the total energy were performed within the framework of the density functional theory (DFT)48 and the C


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Inorganic Chemistry

the diamond-anvil cell is in agreement with the spectrum from the triclinic phase of FeVO4. The structural parameters of FeVO4-I obtained by Rietveld refinement of the XRD data are in a similar manner as in the cases of ambient condition XRD data. Only the XRD data recorded in between 2° and 18° in the high-pressure experiments could be used in Rietveld refinement. In these cases, the backgrounds of the XRD patterns were modeled by linear interpolation of selected points to describe the smooth varying background. The peak profiles were generated by using the pseudo-Voigt profile function. The refined position coordinates and unit cell parameters of triclinic FeVO4 under ambient conditions were used as an initial structural model. Initially, the unit cell parameters and profile parameters were refined along with the scale parameters. An appreciably good match in the observed and calculated diffraction patterns is observed (Rp: 14.7%, Rwp: 19.8%, χ2 = 3.42). It can be mentioned here that, in the refinement of highpressure XRD data, only one thermal parameter, i.e., the overall thermal parameter, was considered. Since the isotropic thermal parameters are sensitive to the background and data reductions in the HP-XRD patterns,16 no attempts to refine them were made in any of the XRD data recorded under pressure. The refined unit cell parameters of FeVO4-I at 0.1 GPa are: a = 6.708(1) Å, b = 8.047(1) Å, c = 9.329(1) Å; α = 96.71(1)°, β = 106.57(2)°, γ = 101.49(2)°; V = 464.9(1) Å3 and are reasonably close to those observed outside the DAC. The XRD patterns recorded at higher pressure up to 2.1 GPa are closely similar to that recorded at lower pressure except for the shifts in peak positions due to the change in unit cell volume due to pressure. However, the XRD pattern recorded at still higher pressure, viz. at 3.0 GPa, show onset of several new peaks which cannot be accounted for by the triclinic FeVO4-I one. These peaks, identified by asterisks in the figure, become intense at higher pressure, suggesting the transition of the FeVO4-I to another high-pressure phase (FeVO4-HP1), but the peaks due to FeVO4-I persist even at higher pressure, viz. 4.2 GPa (Figure 3). The most intense peaks, which unequivocally belong to the remnant phase I, are indicated by plus symbols in Figure 3. They decrease in intensity as the new peaks increase their intensity. Notice that only the most distinctive peaks of each phase are labeled because many peaks of the two lowsymmetry structures overlap, indicating a phase coexistence. The patterns measured from 3.0 to 4.2 GPa have been successfully refined by considering the coexistence of phase I and a new phase (previously unknown we have named here as FeVO4-II′) that we will describe in the next paragraphs.

Figure 2. X-ray diffraction pattern for phase FeVO4-I at ambient conditions. Experiment: dots. Rietveld refinement: solid line. Vertical ticks indicate the position of Bragg reflections. The residual of the refinement is also shown.

6.719(7) Å, b = 8.060(9) Å, c = 9.254(9) Å; α = 96.65(8)°, β = 106.57(8)°, γ = 101.60(8)°; V = 462.46 Å3).33 In the unit cell of FeVO4-I, there are 3 Fe atoms (Fe1, Fe2, and Fe3), 3 V atoms (V1, V2, and V3), and 12 O atoms (O1−O12) present. All the atoms are occupying 2i sites of the SG P1.̅ The Fe1 and Fe3 atoms have octahedral coordination, while the Fe2 has five coordinated (trigonal bipyramidal) configuration. All the three vanadium atoms (V1, V2, and V3) have tetrahedral coordination. The typical V−O bond lengths are ranging in between 1.60 and 1.88 Å, which indicates that all the VO4 units are distorted. The crystal structure of FeVO4-I can be explained by FeOn units that only share edges with other FeOn units forming “s”-shaped FeO6-FeO5-FeO6-FeO6-FeO5-FeO6 chains that propagate in the [111] direction of the unit cell (where each FeOn polyhedron shares corners with VO4 tetrahedral unit). So that the VO4 tetrahedra connect the shaped s-shaped FeO6FeO5-FeO6-FeO6-FeO5-FeO6 chains without having contact with other VO4 tetrahedra. A typical crystal structure of FeVO4I phase is shown in Figure 1. 4.2. High-Pressure XRD Studies and Structural Transition. The powder XRD patterns of FeVO4 were recorded at several pressures while increasing pressure from ambient to a maximum of about 12.3 GPa and also while releasing the pressure. Typical XRD patterns at some representative pressures are shown in Figure 3. The powder XRD pattern recorded at the lowest pressure (0.1 GPa) inside

Table 1. Experimental (exptl) and Calculated (PBE+U, PBEsol+U, HSE06) Structural Parameters for the Observed Experimental Phases of FeVO4 FeVO4-II′ - α-MnMoO4-type

FeVO4-I - P1̅ exptl

PBE+U

PBEsol+U

PBEsol+U

HSE06

P (GPa)

ambient

ambient

ambient

ambient

5.2

5.2

5.2

5.2

12.4

12.4

12.2

12.3

a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V0 (Å3) B0 (GPa) B0′

6.7933 8.1758 9.4944 96.7982 106.444 101.703 486.5 91.1 3.7

6.7203 8.0691 9.3750 96.618 106.595 101.652 469.0 87.3 4.0

6.7207 8.0868 9.3861 96.838 106.387 101.689 470.75 87.9 4.4

6.7080(1) 8.0533(2) 9.3414(3) 96.682(2) 106.585(2) 101.515(2) 465.77(2) 76(3) 4.0

12.1074 3.6930 6.5548

11.9829 3.6663 6.4422

11.9949 3.6426 6.5038

11.9195(11) 3.656(5) 6.4972(13)

4.4561 5.4458 4.7761

4.4091 5.3811 4.7290

4.4308 5.3831 4.7254

4.3952(8) 5.4361(8) 4.7819(7)

106.922

106.921

106.835

106.99(2)

90.493

90.511

90.804

89.87(2)

280.4 154.1 5.1

270.78 150.1 5.4

272.0 152.3 4.6

270.77(7) 145(4) 4.0

115.9 176.8 5.4

112.2 198.3 4.7

112.7 206.7 4.3

114.25(3) 174(8) 4.0

D

HSE06

FeVO4-IV - wolframite

PBE+U

exptl

PBE+U

PBEsol+U

HSE06

exptl


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Inorganic Chemistry Table 2. Theoretical (HSE06) and Experimental (exptl) Wyckoff Positions of FeVO4-I Phase at Ambient Pressure HSE06 Fe1 Fe2 Fe3 V1 V2 V3 O1 O2 O3 O4 O5 O6 O7 O8 O9 O10 O11 O12

exptl

site

x

y

z

x

y

z

2i 2i 2i 2i 2i 2i 2i 2i 2i 2i 2i 2i 2i 2i 2i 2i 2i 2i

0.75524 0.46606 0.97376 0.00540 0.19986 0.52252 0.64568 0.25173 0.05248 0.15850 0.45100 0.76327 0.52546 0.14896 0.35588 0.26756 0.95190 0.05759

0.69288 0.88926 0.30380 0.99787 0.60059 0.29849 0.48311 0.43900 0.69874 0.09808 0.73714 0.86991 0.12720 0.87341 0.73315 0.29649 0.14503 0.52844

0.41053 0.21045 0.01374 0.25721 0.34077 0.12761 0.25135 0.42565 0.42239 0.42794 0.35751 0.26632 0.21735 0.17660 0.01718 0.03952 0.15257 0.14672

0.7492(12) 0.4578(10) 0.9835(11) −0.0056(10) 0.1966(10) 0.5256(11) 0.648(3) 0.296(3) 0.022(3) 0.177(3) 0.445(3) 0.747(3) 0.518(3) 0.142(3) 0.320(3) 0.285(3) 0.941(3) 0.013(3)

0.6969(9) 0.8912(9) 0.3058(9) 0.9866(9) 0.6096(8) 0.2944(9) 0.476(2) 0.444(2) 0.688(2) 0.092(2) 0.724(3) 0.879(2) 0.116(3) 0.888(3) 0.716(2) 0.297(2) 0.146(2) 0.517(2)

0.4067(8) 0.2115(7) 0.0161(7) 0.2560(8) 0.3477(7) 0.1269(8) 0.242(2) 0.441(2) 0.4185(19) 0.418(2) 0.364(2) 0.243(2) 0.230(2) 0.178(2) 0.001(2) 0.054(2) 0.168(2) 0.1498(20)

Table 3. Theoretical (HSE06) and Experimental (exptl) Typical Interatomic Bond Distances (Å) in the FeVO4-I Phase at Ambient Pressure Fen−On

HSE06

exptl

Vn−On

HSE06

exptl

Fe1−O1 Fe1−O2 Fe1−O3 Fe1−O4 Fe1−O5 Fe1−O6 ⟨Fe1−O⟩

1.9803 1.9785 1.9596 1.9971 2.0788 2.0871 2.0135

2.0682(167) 1.9718(208) 1.8180(225) 2.0905(176) 2.0259(227) 2.2376(201) 2.0353(82)

V1−O4 V1−O6 V1−O8 V1−O11 ⟨V1−O⟩

1.6452 1.7729 1.7720 1.6620 1.7130

1.6549(159) 1.6751(199) 1.6531(249) 1.6471(195) 1.6576(102)

Fe2−O5 Fe2−O6 Fe2−O7 Fe2−O8 Fe2−O9 ⟨Fe2−O⟩

1.9652 1.9608 1.8736 2.0388 1.9366 1.9550

2.0784(238) 1.9017(216) 1.7510(247) 2.0455(218) 2.1232(166) 1.9800(98)

V2−O2 V2−O3 V2−O5 V2−O12 ⟨V2−O⟩

1.6553 1.6733 1.7738 1.7635 1.7164

1.8101(193) 1.6876(228) 1.6906(206) 1.8717(164) 1.7650(100)

Fe3−O8 Fe3−O9 Fe3−O10 Fe3−O11 Fe3−O12 Fe3−O12 ⟨Fe3−O⟩

2.0015 2.1048 1.9343 1.9488 1.9556 2.1393 2.01405

2.0923(194) 1.9663(219) 1.9685(217) 2.0640(203) 1.9293(181) 2.2273(200) 2.0413(83)

V3−O1 V3−O7 V3−O9 V3−O10 ⟨V3−O⟩

1.6755 1.7053 1.7913 1.6751 1.7118

1.6186(156) 1.8207(248) 1.7958(235) 1.5679(202) 1.7008(107)

the XRD patterns recorded in the pressure release cycle show the FeVO4-HP2 as a major phase in all the XRD patterns with a minority -HP1 component. In order to understand the structures of FeVO4-HP1 and FeVO4-HP2 phases, the XRD patterns at 5.15 and 12.3 GPa were thoroughly analyzed. It can be recalled here that FeVO4 can exist in a number phases, like CrVO4-type (SG Cmcm, FeVO4-II),35 rutile-type,68 α-PbO2-type (SG Pbcn and Pnc2, FeVO 4 -III), 1 8 , 2 4 , 3 7 , 6 8 , 6 9 α-MnMoO 4 -type (SG C2/ m),21,23,41,68,70 monoclinic (SG P21),66 FeWO4-type (SG P2/ c, FeVO4-IV),18,33,37,69 etc., depending of the preparation conditions. The observed peaks other than those attributable to FeVO4-I phase in the XRD pattern recorded at 5.2 GPa could

Furthermore, the XRD pattern recorded at 5.2 GPa shows the appearance of several other new peaks, denoted by dollar symbols in the figure, which suggests another structural transition in FeVO4 (FeVO4-HP2). Also from Figure 3, it can be seen that the FeVO4-HP1 coexists with FeVO4-HP2 up to about 12.3 GPa, the maximum pressure of this study (see the asterisks and dollars indicating typical peaks of each phase in the figure). However, it can be seen that, with increasing pressure, the contribution of FeVO4-HP1 decreases systematically while increasing the contribution of FeVO4-HP2. The FeVO4-HP2 phase becomes a major phase beyond 7.4 GPa. Thus, it can be concluded that the FeVO4-I irreversibly transforms to FeVO4-HP1 and then to FeVO4-HP2. Similarly, E


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Inorganic Chemistry

of the space group C2/m. Attempts to find the possible distribution of Fe3+ and V5+ resulted in only 2% variation and thus ignored in the refinements. Hence, all sites were considered as fully occupied and without any distribution of cations. In order to be consistent with the notation used in the literature to assign the phase number, from now on, we call this phase the FeVO4-II′ (α-MnMoO4-type) since phase II is the well-known CrVO4-type structure (being the Cmcm the minimal nonisomorphic supergroup of C2/m) and the FeVO4-I → -II′ phase transition takes place at a similar pressure of the FeVO4-I → -II phase transition. A typical Rietveld refinement plot of the XRD data recorded at 5.2 GPa is shown in Figure 4. The refined position coordinates and typical interatomic distances are given in Tables 4 and 5, respectively.

Figure 3. X-ray diffraction patterns of FeVO4 at selected pressures. The phases identified at each pressure are indicated. The plus symbols (+) denote peaks that belong only to phase I, asterisks (*) denote peaks that belong only to phase II′, and dollar symbols ($) denote peaks that belong only to phase IV. Figure 4. Rietveld refinement plot of powder XRD data recorded at 5.2 GPA. The vertical ticks are: upper row (FeVO4-II′ - α-MnMoO4type, 69(3) wt %) and lower row (FeVO4-IV - wolframite, 31(2) wt %). The residual of the refinement is also shown.

be indexed on a monoclinic lattice with unit cell parameters (a = 11.948(2) Å, b = 3.664(1) Å, c = 6.517(1) Å, β = 107.01(2)°, at 4.2 GPa; a = 11.919(2) Å, b = 3.657(1) Å, c = 6.497(1) Å, β = 106.95(2)°, at 5.2 GPa) similar to the monoclinic AlNbO4type lattice.71,72 It can be mentioned here that the unit cell parameters of AlNbO4-type are closely similar to those of αMnMoO4-type,73 except the b-axis is elongated to 3 times in the later. Typical unit cell parameters of α-MnMoO4-type are: a = 10.469(5) Å, b = 9.516(5) Å, c = 7.143(5) Å, β = 106.4(1)° (SG C2/m);73 AlNbO4-type: a = 12.1558(5) Å, b = 3.7345(2) Å, c = 6.4886(3) Å, and β = 107.613(4)° (SG C2/m).72 The differences between these two are also reflected in the coordination polyhedral around the B cation, viz. Mo6+ is tetrahedrally coordinated while Nb5+ is octahedrally coordinated in AlNbO4. Monoclinic FeV3O8 (Fe0.5V0.5VO4) phase reported by Muller et al.70 has a similar unit cell (a = 12.129 Å, b = 3.679 Å, c = 6.547 Å, β = 106.85°) where the Fe and one V (V1) are occupied statistically in the 4i site while another V (V2) is occupied fully in another set of 4i sites. The authors have explained the structure as isostructural to AlNbO4 and the high-temperature form of VO2.70 Further analysis of the structure of FeVO4-HP1 was carried out by refining the XRD data by using a model based on AlNbO4 (SG C2/m), FeV3O8 (SG C2/m), and α-MnMoO4-type (SG C2/m) structure. Rietveld refinements carried out by considering the AlNbO4and FeV3O8-type structures (SG C2/m) can simulate all the peaks assigned to the FeVO4-HP1 phase. Thus, the XRD patterns above 2.1 GPa were refined by considering an AlNbO4-type monoclinic structure, where one Fe1, and one V1, and four oxygen (O1 to O4) atoms are considered at 4i sites

The analyses of structural parameters of FeVO4-II′ (Tables 3 and 4) indicate that both Fe and V are octahedrally coordinated but with a larger dispersion of Fe−O and V−O bond lengths. Typical crystal structures indicating the FeO6 and VO6 units are shown in Figure 1. The Fe−O bond lengths in FeVO4-II′ are in the range of 1.91 and 2.32 Å, while five bonds are between 1.91 and 2.13 Å. However, the average Fe−O bonds in FeO6 octahedra are 2.034(4) Å which is almost similar to those observed in FeVO4-I (⟨Fe1−O⟩6 = 2.035(8) Å and ⟨Fe3−O⟩6 = 2.041(8) Å, while ⟨Fe2−O⟩5 = 1.99(1) Å at ambient pressure). Similar analyses of V−O bond lengths in FeVO4-II′ indicate that the VO6 octahedron is largely distorted and show wider dispersion (1.70(1)−2.08(1) Å), bond length distortion: 49.47 × 10−4. Thus, the structural transition from FeVO4-I to FeVO4II′ occurs with the shift of the oxygen atoms from the FeO5 or FeO6 toward the V5+ of triclinic FeVO4-I. Besides, the unit cell volume decreases significantly at the transition (molar volume (Vm = V/Z) decreases almost about 8.5% (100 × ΔVm/Vm) at the FeVO4-I to FeVO4-II′ transition (at ≈3 GPa); see Figure 7). The better packing of atoms in the FeVO4-II′ arises from a reconstruction of the structure and first-order phase transition. Similar analyses of the peaks in the XRD pattern recorded at 12.3 GPa were carried out to reveal the structure of FeVO4-IV phase. Several weak peaks attributable to the monoclinic FeVO4-II′ still appear in this XRD pattern. The peaks not accountable by the FeVO4-II′ phase observed in this XRD pattern could be indexed on a monoclinic lattice similar to the F


Article

Inorganic Chemistry

Table 4. Theoretical (HSE06) and Experimental (exptl) Wyckoff Positions for the Most Relevant High-Pressure Phases of FeVO4 HSE06 site

x

y

exptl z

x

y

z

FeVO4-II - CrVO4-type, a = 5.5941 Å, b = 8.3216 Å, c = 6.2252 Å, HSE06 a = 5.6284(7) Å, b = 8.2724(7) Å, c = 6.1118(2) Å, exptl35 Fe1 4a 0 0 0 0 0 0 V1 4c 0 0.35523 0.25 0 0.3599(1) 0.25 O1 8f 0 0.76389 0.96276 0 0.7587(4) 0.9676(5) O2 8g 0.26412 0.47369 0.25 0.26470(5) 0.4801(4) 0.25 FeVO4-II′ - α-MnMoO4-type, a = 11.9949 Å, b = 3.6426 Å, c = 6.5038 Å, β = 106.8347°, HSE06 a = 11.9195(11) Å, b = 3.656(5) Å, c = 6.4972(13) Å, exptl Fe1 4i 0.30011 0 0.69765 0.2979(5) 0 0.7025(9) V1 4i 0.40083 0 0.27064 0.4010(5) 0 0.2642(9) O1 4i 0.36575 0 0.00714 0.3893(15) 0 0.9972(12) O2 4i 0.24970 0 0.35220 0.2498(9) 0 0.3292(15) O3 4i 0.45149 0 0.64882 0.4516(7) 0 0.5985(13) O4 4i 0.12617 0 0.69479 0.1256(10) 0 0.690(4) FeVO4-III - α-PbO2-type, a = 4.8066 Å, b = 4.4676 Å, c = 5.6206 Å, HSE06 a = 4.491(11) Å, b = 4.9(5) Å, c = 5.53(13) Å, exptl65 Fe1 2a 0 0 0.83279 0 0.11 0.25 V1 2b 0.5 0 0.19138 0 0.11 0.25 O1 4c 0.65664 0.25293 0.37458 0.25 0.36 0.125 O2 4c 0.18922 0.21370 0.11332 FeVO4-IV - wolframite, a = 4.4308 Å, b = 5.3831 Å, c = 4.7254 Å, β = 90.8044°, HSE06 a = 4.3952(8) Å, b = 5.4361(8) Å, c = 4.7819(7) Å, β = 89.87(2)° exptl Fe1 2f 0.5 0.68470 0.25 0.5 0.6674(13) 0.25 V1 2c 0 0.18232 0.25 0 0.1963(17) 0.25 O1 4g 0.22190 0.10775 0.93803 0.212(4) 0.072(3) 0.878(3) O2 4g 0.25860 0.38077 0.40314 0.254(5) 0.360(3) 0.437(3) FeVO4-V - raspite, a = 4.7284 Å, b = 4.5344 Å, c = 8.7685 Å, β = 90.2875°, HSE06 Fe1 4e 0.53627 0.26496 0.38480 V1 4e 0.04033 0.25442 0.11369 O1 4e 0.24873 0.50888 0.96815 O2 4e 0.22064 0.49318 0.47064 O3 4e 0.38353 0.18611 0.19357 O4 4e 0.10569 0.65820 0.21411 FeVO4-VI - AgMnO4-type, a = 6.7812 Å, b = 4.9717 Å, c = 8.8035 Å, β = 145.0562°, HSE06 Fe1 4e 0.36996 0.06767 0.13065 V1 4e 0.97116 0.00166 0.67061 O1 4e 0.73435 0.95371 0.40995 O2 4e 0.94675 0.35565 0.58764 O3 4e 0.33497 0.20221 0.88147 O4 4e 0.75562 0.20640 0.74608

an additional two long bonds with distances 2.11 Å, while the average bond lengths in VO6 (⟨V−O⟩6 = 1.88(1) Å) is almost similar to that in FeVO4-II′. But the bond length distortion increases significantly (49.47 × 10−4 in FeVO4-II′ and 90.31 × 10−4 in FeVO4-IV). The average Fe−O bond length in FeO6 octahedra of FeVO4-IV (2.01(1) Å) is lower compared to that in FeVO4-II′ (2.03(1) Å). This suggests that the FeO6 compress more in the FeVO4-II′ to FeVO4-IV transition. Thus, the octahedra around the V5+ in FeVO4-IV and that in FeVO4-II′ phases are distorted and formed by the displacement of oxygen atoms of the FeO6 octahedra. It may be noted here that, in both AlNbO4 (SG C2/m),72 FeV3O8 (SG C2/m)70 and presently observed FeVO4-II′, the B site cations are octahedrally coordinated and they are similar to the wolframite-type structures. The earlier reported highpressure and high-temperature phases of FeVO4 have also similar octahedral BO6 units.37,69 The typical phases of FeVO4 observed in different conditions are summarized in Figure 6. Further details of the structure of FeVO4 phases are explained later in this paper.

FeWO4 wolframite-type monoclinic lattice. Such phases in FeVO4 have been earlier reported by simultaneous application of pressure and temperature on FeVO4.24,37,69 Thus, the powder XRD pattern was analyzed by considering structural data of FeVO4-II′ and position coordinates of wolframite-type monoclinic phase. The Rietveld refinement plot of the XRD data is shown in Figure 5. The refined structural parameters of FeVO4-IV and typical interatomic distances are given in Tables 4 and 5, respectively. In the FeVO4-IV, both the V5+ and Fe3+ are octahedrally coordinated and arranged in a rutile-type arrangement of the octahedral units. Similar to the earlier case, no intermixing of cations is considered here. Thus the structure is formed by linking the chains of FeO6 and VO6 polyhedra by sharing the corner oxygen atoms. The unit cell volume also drops appreciably at this transition; see Figure 7. The molar volume difference between FeVO4-II′ and IV at around 3.2 GPa is 13.1%, and thus the later structure has better packing efficiency than the former. The analysis of bond lengths indicates that both octahedra are distorted and the VO6 octahedra is formed by typically four bonds within the range of 1.69−1.84 Å, while G


Article

Inorganic Chemistry

Table 5. Theoretical (HSE06) and Experimental (exptl) Typical Interatomic Bond Distances (Å) for the High-Pressure Phases of FeVO4 Fen−On

HSE06

Fe1−O1 × 2 Fe1−O2 × 4 ⟨Fe1−O⟩

1.9784 2.0521 2.0152

Fe1−O1 Fe1−O2 × 2 Fe1−O2 Fe1−O3 Fe1−O4 ⟨Fe1−O⟩

1.9374 1.9150 2.1510 1.9319 2.0811 1.9886

Fe1−O1 × 2 Fe1−O2 × 2 Fe1−O2 × 2 ⟨Fe1−O⟩

1.9993 1.9962 2.0555 2.0170

Fe1−O4 × 2 Fe1−O3 × 2 Fe1−O1 × 2 ⟨Fe1−O⟩

1.9763 2.0896 1.8939 1.9866

Fe1−O1 Fe1−O1 Fe1−O2 Fe1−O2 Fe1−O3 Fe1−O3 Fe1−O4 ⟨Fe1−O⟩

1.9836 2.0078 1.9688 2.0288 1.8571 2.0653 1.9666 1.9825

Fe1−O1 Fe1−O1 Fe1−O2 Fe1−O2 Fe1−O3 Fe1−O3 Fe1−O4 Fe1−O4 ⟨Fe1−O⟩

1.9694 2.0576 1.9176 2.0074 2.0403 2.0499 1.9822 1.9902 2.0018

Vn−On

exptl

P = ambient, phase FeVO4-II - CrVO4-type 2.006(3)35 V1−O1 × 2 2.029(3)35 V1−O2 × 2 2.0175 ⟨V1−O⟩ P = 5.2 GPa, phase FeVO4-II′ - α-MnMoO4-type 1.9063(97) V1−O1 1.9076(33) V1−O2 2.3235(111) V1−O3 2.1299(114) V1−O3 2.0315(150) V1−O4 × 2 2.0344(41) ⟨V1−O⟩ P = ambient, phase FeVO4-III - α-PbO2-type V1−O1 × 2 V1−O1 × 2 V1−O2 × 2 ⟨V1−O⟩ P = 12.2 GPa, phase FeVO4-IV - wolframite 1.9951(174) V1−O2 × 2 2.1810(184) V1−O1 × 2 1.8544(174) V1−O4 × 2 2.0101(72) ⟨V1−O⟩ P = 43.3 GPa, phase FeVO4-V - raspite V1−O1 V1−O1 V1−O2 V1−O2 V1−O3 V1−O4 V1−O4 ⟨V1−O⟩ P = 71.3 GPa, phase FeVO4-VI - AgMnO4-type V1−O1 V1−O1 V1−O2 V1−O2 V1−O3 V1−O3 V1−O4 V1−O4 ⟨V1−O⟩

HSE06

exptl

1.6544 1.7762 1.7153

1.652(3)35 1.792(3)35 1.722

1.6421 2.0309 2.3565 1.6954 1.8748 1.9124

1.6992(105) 1.9675(132) 2.0774(99) 1.7224(92) 1.8936(62) 1.8756(39)

1.7041 2.2262 1.8263 1.9188 2.0421 1.8280 1.7188 1.8629

2.1170(155) 1.8365(180) 1.6859(192) 1.8798(72)

1.8776 1.9858 1.8611 1.8899 1.7912 1.7197 2.0543 1.8828 1.7774 1.9422 1.8636 1.9078 1.9566 2.0181 1.8269 1.8685 1.8951

4.3. Evolution of Unit Cell Parameters and Equations of State. The evolution of unit cell parameters with pressure is shown in Figure 8a,b. The unit cell parameters of FeVO4-II′ and -IV observed in the pressure release cycle are included in the figure. The unit cell parameters for FeVO4-II′ and -IV are considered in the complete range of pressure due to their existence up to the ambient pressure in pressure release experiments. It can be seen that all the three axes of the unit cell of the FeVO4 phases decrease systematically with increasing pressure. In the case of FeVO4-I, the angles β and γ show a decreasing trend with pressure, whereas α shows an increasing trend with pressure. The typical linear expressions for the variation of unit cell parameters of FeVO4 phases with pressure are given in Table 6. Such redistribution of unit cell angles rotates the VO4 tetrahedron toward the FeOn (n = 5, 6) polyhedra, and in turn, the FeO5 transforms to the FeO6 and the VO4 are transformed to VO6 units of FeVO4-II′. Also, the distribution of cation polyhedra favors a better packing of the

Figure 5. Rietveld refinement plot of powder XRD data recorded at 12.3 GPa. The vertical ticks are: upper row (FeVO4-II′ - α-MnMoO4type, 7(1) wt %) and lower row (FeVO4-IV - wolframite, 93(3) wt %). H


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Inorganic Chemistry

Figure 6. Structure map of FeVO4 phases observed under different conditions. Each FeVO4 phase includes the respective space group, the cation coordination in red, and the structure type.

the FeVO4-IV (for FeVO4-II′, βa:βb:βc = 1.40:1.00:1.88 and for FeVO4-IV, βa:βb:βc = 1.06:1.09:1.00). The unit cell volume of FeVO4-I is most compressible than the latter two phases, as can be seen from the bulk compressibilities (βV) (Phase-I: 12.5 × 10−3 GPa−1, Phase-II′: 5.72 × 10−3 GPa−1; Phase-IV: 4.95 × 10−3 GPa−1). The larger compressibility of the FeVO4-I is a consequence of its relatively open structure and larger molar volume, which arise from the lower coordination number around one Fe atom (Fe2) and V atoms. At the FeVO4-I → -II′ phase transition, the molar volume decreases to about 8.5%, whereas, in the FeVO4-II′ → -IV transition, the volume decreases about 13%. As the molar volume decreases, the coordination number of the Fe and V atoms increases and hence the effective rearrangement of oxygen atoms occurs and thus becomes less compressible. The equations of state (EOS) of FeVO4 phases were obtained by fitting the pressure−volume data to the secondorder Birch−Murnaghan equation of states. The calculated pressure−volume data from the EOS parameters are included in Table 1. The obtained EOS parameters (B0 and B0′) for the FeVO4 phases are: 76(3) GPa, 4.00 (Phase-I), 145(4) GPa, 4.00 (Phase-II), and 174(8) GPa, 4.00 (Phase-IV). The rapid increase in bulk modulus is again an indication of effective packing in the unit cell in the FeVO4-II′ and IV phases. From the pressure evolution of unit cell parameters and equation of states, it is indicated that FeVO4 undergoes a structural transition at a relatively smaller pressure to denser phases (FeVO4-II′ and IV). Thus, the transition from FeVO4-I to FeVO4-IV is intervened by an intermediate phase which has also appreciable pressure stability. Earlier studies on FeVO4 have often indicated the formation of CrVO4-type or αMnMoO4-type in low-temperature synthesis, whereas wolframite or α-PbO2-type phases appear at high-temperature synthesis (Figure 6). On the other hand, the transition from FeVO4-I to CrVO4-type orthorhombic phase (FeVO4-II) on substitution of amounts (about 20%) of Cr3+ in the FeVO4-I lattice was observed.23,42 The authors have indicated a small decrease in the unit cell volume (≈1.7%) can transform the triclinic FeVO4 to monoclinic α-MnMoO4-type (SG C2/m) structure. However, similar solid solutions at lower concentration ( ⟨Fe−O⟩IV = 1.9866 Å > ⟨Fe− O⟩V = 1.9825 Å, and ⟨V−O⟩VI = 1.8951 Å > ⟨Fe−O⟩V = 1.8828 Å > ⟨Fe−O⟩IV = 1.8629 Å. According to Figure 11a,b, the

enthalpies from phases FeVO4-III and -IV (Figure 10), we could speculate that, at high-temperatures, the phase transitions would be similar to the reported by Hotta et al.: FeVO4-I → -II or -II′ → -III → -IV.24 In order to understand a little bit more on the pressure driven phase transitions in FeVO4, we measured the pressure evolution of the Fe−O and V−O interatomic bond distances. However, due to the number of the interatomic bond distances presented in the FeVO4 phases, it is easier to analyze the results by measuring the grade of the distortion in the coordination polyhedra by using the distortion parameter Δd,16,78 which is defined as follows Δd =

⎛1⎞ ⎜ ⎟ ⎝n⎠

n

⎡ di − d ⎤2 ⎥ d ⎦

∑ ⎢⎣ i=1

IV

where d is the average Fe−O (V−O) bond distance for specific polyhedra, di is the individual Fe−O (V−O) bond distance, and n is the coordination number of the different polyhedra. The pressure evolution of the distortion parameter for FeOn and VOn polyhedra of FeVO4 phases is shown in Figure 11a,b, where the curves are labeled according to their Wyckoff position. According to this figure, in general, it is observed that VOn polyhedra from the studied phases of FeVO4 are more distorted than the FeOn ones. Also, the distortion becomes smaller for the VOn polyhedra of all phases as pressure increases, whereas the behavior is the opposite for some FeOn phases. For FeVO4-I, the distortion in the three VO4, the FeO5, and one FeO6 polyhedron decreases as pressure increases, while Δd increases for other FeO6 polyhedra. The trend of Δd from phase II′ is the most particularly among the phases observed in the experiments of this work. On one side, the distortion parameter of VO4 tetrahedra has the biggest reduction in comparison with the other phases as pressure increases, whereas the opposite occurs for the FeO6 octahedron. On the other hand, the distortion diminishes for both polyhedra in wolframite, and the slope of the distortion becomes smaller as pressure increases. Thus, we observe that FeOn and VOn polyhedra from phases I and II are competing between order and disorder as pressure increases, whereas, in wolframite, the octahedra tend to be ordered with pressure. Going beyond the pressures reached in the experiments, we have performed a study of the possible post-wolframite phases by using the procedure previously described. To make a selection of the candidates to study, we have taken into account the Bastide’s diagram and previous theoretical and experimental studies of other ABX4 compounds.1,8,12,17,31,39,78−86 Thus, we have considered the potentially existing phases: the monoclinic raspite (SG P21/a, No. 14, Z = 4) which has been reported as a post-wolframite phase of InVO4,17 the α-PbO2-type structure (SG Pnc2, No. 30, Z = 2) reported as a high-pressure phase of FeVO4 in refs 18, 24, and 69, and the monoclinic AgMnO4-type structure (SG P21/n, No. 14, Z = 4) that appeared as a highpressure phase of SrCrO4,83 CaSeO4,78 and InVO4,17 to name a few compounds. We also considered some possible highpressure phases reported for wolframates like SrWO480 and N


Article

Inorganic Chemistry Notes

distortion parameters for phases V and VI are smaller than that in wolframite, and it is also observed that the change with the pressure of Δd for FeVO4-VI is the smallest of all, remaining almost constant in the range of pressure studied.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research was supported by the Spanish Ministerio de Economiá y Competitividad, the Spanish Research Agency, and the European Fund for Regional Development under Grant Nos: MAT2016-75586-C4-1-P/3-P and MAT201571070-REDC. A.M. and P.R.-H. acknowledge computing time provided by Red Española de Supercomputación (RES) and MALTA-Cluster. S.N.A. acknowledges the support provided by Universitat de Valencia during his visit to it. S.L.-M. thanks CONACYT of México for financial support through the program “Catedras para Jóvenes Investigadores”. The authors gratefully acknowledge the computing time granted by LANCAD and CONACYT on the supercomputer Miztli at LSVP DGTIC UNAM. Also, some of the computing for this project was performed with the resources of the IPICYT Supercomputing National Center for Education & Research, grant TKII-R2018-SLM1.

5. CONCLUDING REMARKS A systematic in situ high-pressure XRD study on triclinic FeVO4 was carried out under nearly hydrostatic conditions and revealed two successive transitions from the triclinic P1̅ (FeVO4-I) to the monoclinic α-MnMoO4-type (FeVO4-II′) and then to the monoclinic wolframite (FeVO4-IV) with increasing coordination of tetrahedral V5+ to octahedral VO6. A large change in unit cell volume and about 90% increase in bulk modulus occur at a moderate pressure (≈2 GPa). Also, all the transitions are found to be irreversible and first-order in nature. The axial compressibilities and equation of states for all the phases were obtained from the pressure evolution of unit cell parameters. The axial compressibility of FeVO4-I is highly anisotropic (βa:βb:βc = 1.00:1.00:1.51) and becomes nearly isotropic in FeVO4-IV (βa:βb:βc: 1.06:1.09:1.00). Bulk moduli obtained from experiments for FeVO4 phases are 76(3), 145(4), and 174(8) GPa for the FeVO4-I, -II′, and IV, respectively. The phase transitions were supported by ab initio density functional theory calculations. In addition, from theoretical studies, the possible formation of the orthorhombic CrVO4type (FeVO4-II) and the α-PbO2-type (FeVO4-III) structures in the range pressures reached in the experiments was analyzed. Beyond that, the theoretical results predict two possible postwolframite phase transitions: wolframite → raspite (36 GPa) → AgMnO4-type (66.5 GPa), similar to the phase transition sequence reported for InVO4. The cation coordination changes from 6 in wolframite to 7 for raspite and 8 for AgMnO4-type structure, which is reflected in the bulk modulus of the postwolframite phases.

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ASSOCIATED CONTENT

Accession Codes

CCDC 1838364, 1838368, and 1838401 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

(1) Errandonea, D.; Manjón, F. J. Pressure effects on the structural and electronic properties of ABX4 scintillating crystals. Prog. Mater. Sci. 2008, 53, 711−773. (2) Balamurugan, M.; Yun, G.; Ahn, K.-S.; Kang, S. H. Revealing the beneficial effects of FeVO4 nanoshell layer on the BiVO4 inverse opal core layer for photoelectrochemical water oxidation. J. Phys. Chem. C 2017, 121, 7625−7634. (3) Marberger, A.; Elsener, M.; Ferri, D.; Sagar, A.; Schermanz, K.; Kröcher, O. Generation of NH3 selective catalytic reduction active catalysts from decomposition of supported FeVO4. ACS Catal. 2015, 5, 4180−4188. (4) Butcher, D. P., Jr.; Gewirth, A. A. Photoelectrochemical response of TlVO4 and InVO4:TlVO4 composite. Chem. Mater. 2010, 22, 2555. (5) Zhao, C.; Tan, G.; Yang, W.; Xu, C.; Liu, T.; Su, Y.; Ren, H.; Xia, A. Fast interfacial charge transfer in a-Fe2O3‑δCδ/FeVO4‑x+δCx‑δ@C bulk heterojunctions with controllable phase content. Sci. Rep. 2016, 6, 38603. (6) Yi, Z.; Ye, J.; Kikugawa, N.; Kako, T.; Ouyang, S.; StuartWilliams, H.; Yang, H.; Cao, J.; Luo, W.; Li, Z.; Liu, Y.; Withers, R. L. An orthophosphate semiconductor with photooxidation properties under visible-light irradiation. Nat. Mater. 2010, 9, 559−564. (7) Yu, Y.; Ju, P.; Zhang, D.; Han, X.; Yin, X.; Zheng, L.; Sun, C. Peroxidase-like activity of FeVO4 nanobelts and its analytical application for optical detection of hydrogen peroxide. Sens. Actuators, B 2016, 233, 162−172. (8) Baran, E. J. Materials belonging to the CrVO4 structure type: preparation, crystal chemistry and physicochemical properties. J. Mater. Sci. 1998, 33, 2479−2497. (9) Orel, B.; Vuk, A. S.; Krašovec, U. O.; Drazic, G. Electrochromic and structural investigation of InVO4 and some other vanadia-based oxide films. Electrochim. Acta 2001, 46, 2059−2068. (10) Van de Krol, R.; Ségalini, J.; Enache, C. S. Influence of point defects on the performance of InVO4 photoanodes. J. Photonics Energy 2011, 7770, 7770. (11) Dixit, A.; Chen, P.; Lawes, G.; Musfeldt, J. L. Electronic structure and polaronic excitation in FeVO4. Appl. Phys. Lett. 2011, 99, 141908. (12) Errandonea, D. High-pressure phase transitions and properties of MTO4 compounds with the monazite-type structure. Phys. Status Solidi B 2017, 254, 1700016. (13) Kumarasiri, A.; Abdelhamid, E.; Dixit, A.; Lawes, G. Effect of transition metal doping on multiferroic ordering in FeVO4. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 014420.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.L.-M.). *E-mail: [email protected] (D.E.). ORCID

Sinhué López-Moreno: 0000-0001-6292-8275 Daniel Errandonea: 0000-0003-0189-4221 S. Nagabhusan Achary: 0000-0002-2103-1063 Placida Rodríguez-Hernández: 0000-0002-4148-6516 Alfonso Muñoz: 0000-0003-3347-6518 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. O


Article

Inorganic Chemistry

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under Pressure - American Chemical Society

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