The Structure, Vibrational Spectra, and Thermal Expansion Study of AVO4 (A=Bi, Fe, Cr) and Co2V2O7.

Vanadate is an important functional material. It has been widely studied and applied in luminescence and photocatalysis. Vanadium compounds have been synthesized to investigate the thermal expansion properties and structure. Both BiVO4 and Co2V2O7 are monoclinic at room temperature, FeVO4’s crystal structure is triclinic, and CrVO4 is orthorhombic. The relatively linear, thermal-expansion, and temperature-dependent Raman spectroscopy results showed that the phase transition of BiVO4 occurred at 200 to 300 °C. The coefficient of thermal expansion (CTE) of Co2V2O7 was larger than that of the monoclinic structure BiVO4. The CTE of the tetragonal structure of BiVO4 was 15.27 × 10−6 °C−1 which was the largest CTE in our measurement results, and the CTE of anorthic structure FeVO4 was 2.84 × 10−6 °C−1 and was the smallest.


Introduction
Due to the multivalent of vanadium, vanadate has rich physical and chemical properties. Vanadate is a kind of important functional material; it has been widely studied and applied in luminescence and photocatalysis. The bandgap of BiVO 4 is approximately 2.0 eV which means that it is a classic semiconductor. Bidmuth vanadate (BiVO 4 ) is a polycrystalline compound, among which there are three kinds of crystal structures: monoclinic, orthorhombic, and tetragonal structure. The tetragonal structure has absorption band in the ultraviolet region, while the monoclinic structure has absorption band in the visible region as well as in the ultraviolet region. Vanadate (BiVO 4 ) has emerged as a very photoanode for photoelectrochemical water splitting [1][2][3][4][5]. Although the hole/electron pair, produced by the excitation of BiVO 4 , has strong redox ability; it also has some disadvantages for practical application: a high electron hole recombination rate, low photocatalytic efficiency, small particles which are easy to lose and difficult to recover, etc. As an n-type semiconductor material, the bandgap of FeVO 4 is approximately 1.9-2.7 eV. There are four crystal types of FeVO 4 , only triclinic structure material is easy to obtain [6]. The triclinic structure of FeVO 4 remains up to approximately 3 GPa, and then a first-order phase transition to a new monoclinic with space group C2/m is observed [7]. As a kind of transition metal oxide, FeVO 4 can be used as electrode materials for ion batteries and supercapacitors [8][9][10][11]. The compound materials of FeVO 4 /BiVO 4 and FeVO 4 /V 2 O 5 has higher than pure FeVO 4 photocatalytic activity [12,13]. Chrome vanadate has three different crystal forms tetragonal, monoclinic, and orthorhombic structures [7]. The ambient-pressure stable polymorph of CrVO 4 is orthorhombic space group D 17 2h -Cmcm-, with Z=4 at room temperature [14]. Cobalt vanadates and their composites have drawn a tremendous amount of attention because of their outstanding cycling stability [15,16]. The Co 2 V 2 O 7 was recently reported to exhibit amazing magnetic field-induced magnetization plateaus and ferroelectricity, but its magnetic ground state remains ambiguous due to the fact of its structural complexity [17]. From the above discussion, we know that vanadium compounds have many structures, rich physical and chemical properties which makes them have potential application value in many aspects. Although there are many studies on the structure and application of vanadium compounds, the thermal expansion of materials has not been reported very intensively. It has been pointed out that ionic radius and electronegativity of the cations are important with respect to structure and phase transition temperature [18,19]. Herein, we have prepared some vanadate materials by a simple solid-phase sintering method, X-ray diffraction (XRD) was used to measure the structure of materials, Raman scattering was used to measure the lattice vibration, and dilatometers was used to measure the thermal expansion.

Experimental and Methods
The AVO 4 and Co 2 V 2 O 7 were synthesized by a solid-state method from Fe 2 O 3 (≥99.0%), Bi 2 O 3 (≥99.0%), Cr 2 O 3 (≥99.0%), Co 2 O 3 (≥99.5%), and V 2 O 5 (≥99.0%). The raw materials were mixed according to stoichiometric amounts (1:1) of A 2 O 3 and 2%-5% excess V 2 O 5 of desirable material except (in order to compensate for the loss in the sintering process) and ground in a mortar for 2 h. Then, alcohol was poured over the raw material and grind again until dry. Lastly, it was pressed into tablets with a length of approximately 7 mm and a diameter of approximately 6 mm, followed by sintering at 750 • C for 4 h and cooling naturally to room temperature.
The XRD measurements were carried out with an X'Pert PRO X-ray Diffractometer (Bruker D8, Bruker, Karlsruhe, Germany). Raman spectroscopy (Renishaw MR-2000 Raman spectrometer, Gloucestershire, UK) with a TMS 94 heating/freezing stage with an accuracy of ±0.1 • C was used to characterize the vibrational property of lattice. The linear thermal expansion coefficients were measured on dilatometers (LINSEIS DIL L76, Linseis, Selb, Germany), with heating and cooling rates of 5 • C/min. Figure 1 shows the XRD patterns of as-prepared materials. Figure 1a is the pattern of BiVO 4 , all diffraction peaks corresponded to BiVO 4 (PDF No. 01-083-1699) which means that the material crystals were in monoclinic structure space group I2/b, with Z = 4. The lattice constants of BiVO 4 were a = 5.196 Å, b = 5.094 Å, c = 11.703 Å and α = β = 90 • , γ = 90.380 • . Figure 1b is the pattern of CrVO 4 , the primary diffraction peaks corresponded to CrVO 4 (PDF No. 00-038-1376, space group Amam) except for weak peaks indicated as "∇" for secondary phase Cr 2 O 3 and " * " for secondary phase V 2 O 5 which could relate to the fact that the reaction time was much shorter than that reported in the literature (122 h). There was a second and third phase which could lead to some situations, such as internal stress, many cracks on the tablet, etc. The material crystals in orthorhombic structure with space group Amam from the primary diffraction peaks. The lattice constants of CrVO 4 are a = 5.567 Å, b = 8.210 Å, c = 5.975 Å. The pattern of FeVO 4 is shown in Figure 1c. As seen, the diffraction peaks are corresponding to FeVO 4 (PDF No. 00-038-1372, space group P-1) which crystal in anorthic structure. The lattice constants of FeVO 4 are a = 6.720 Å, b = 8.059 Å, c = 9.256 Å, and α = 96.7 • , β = 106.4 • , γ = 101.6 • . Figure 1d is the pattern of Co 2 V 2 O 7 , all diffraction peaks corresponded to Co 2 V 2 O 7 (PDF No. 01-070-1189) with lattice constants a = 6.595 Å, b = 8.380 Å, and c = 9.470 Å which means that the material crystals were in monoclinic structure space group P21/c, with Z = 4. To visualize the coordination number associated with the structural transitions, crystal structures of monoclinic (BiVO4, Co2V2O7), triclinic (FeVO4), and orthorhombic (CrVO4) systems with polyhedral representation were drawn using a VESTA software as shown in Figure 2 (the "atomic coordinates" were obtained come from the joint conferences on pervasive computing (JCPC) references). The BiVO4 crystal structure was monoclinic. From Figure 2a,b, Bi and V atoms occupied the symmetry position 4e, and O atoms occupied 8f. The distance between the V and O atoms was evenly distributed (approximately 1.68 Å and 1.785 Å); however, the distance of the Bi and O was very variable. There are six symmetry VO4 tetrahedras and asymmetrical BiO6 octahedra in one primitive cell of BiVO4. The CrVO4 crystal structure was orthorhombic (Figure 2c,d). The distance between the V and O atoms (approximately 1.63342 Å and 1.70978 Å) was shorter than that of BiVO4, and the distances between the Cr and O atoms were 1.98422 Å and 2.04868 Å. It can be seen that there are four CrO6 octahedra around each tetrahedron; however, each CrO6 octahedron is not only connected with six tetrahedron vertices, but also connected with two other octahedron edges. The FeVO4 crystal structure was triclinic Figure 2d. For FeVO4, there were 18 symmetrical inequivalent atoms in a one-unit cell, and all the atoms occupied the symmetry position 2i (Figure 2e). The total number of atoms in a unit cell was 36. The distance between the V atom and the O atom was different. The unit cell contained three asymmetrical inequivalent VO4 tetrahedra, two asymmetrical inequivalent FeO6 octahedra, and one FeO5 polyhedron [20] (Figure 2f). The CoV2O7 crystal structure was monoclinic Figure 2g, the total number of atoms in a unit cell was 44. The distance between the V atom and the O atom was different. The unit cell contained three symmetrical VO4 of each tetrahedron and six asymmetrical CoO6 octahedra (Figure 2h). It can be seen that each VO4 tetrahedron was connected to four CoO6 octahedra by the O atom; however, each CrO6 octahedron was connected by the O atom to six VO4 tetrahedras and shared a common edge with two other octahedral. To visualize the coordination number associated with the structural transitions, crystal structures of monoclinic (BiVO 4 , Co 2 V 2 O 7 ), triclinic (FeVO 4 ), and orthorhombic (CrVO 4 ) systems with polyhedral representation were drawn using a VESTA software as shown in Figure 2 (the "atomic coordinates" were obtained come from the joint conferences on pervasive computing (JCPC) references). The BiVO 4 crystal structure was monoclinic. From Figure 2a,b, Bi and V atoms occupied the symmetry position 4e, and O atoms occupied 8f. The distance between the V and O atoms was evenly distributed (approximately 1.68 Å and 1.785 Å); however, the distance of the Bi and O was very variable. There are six symmetry VO 4 tetrahedras and asymmetrical BiO 6 octahedra in one primitive cell of BiVO 4 . The CrVO 4 crystal structure was orthorhombic (Figure 2c,d). The distance between the V and O atoms (approximately 1.63342 Å and 1.70978 Å) was shorter than that of BiVO 4 , and the distances between the Cr and O atoms were 1.98422 Å and 2.04868 Å. It can be seen that there are four CrO 6 octahedra around each tetrahedron; however, each CrO 6 octahedron is not only connected with six tetrahedron vertices, but also connected with two other octahedron edges. The FeVO 4 crystal structure was triclinic Figure 2d. For FeVO 4 , there were 18 symmetrical inequivalent atoms in a one-unit cell, and all the atoms occupied the symmetry position 2i (Figure 2e). The total number of atoms in a unit cell was 36. The distance between the V atom and the O atom was different. The unit cell contained three asymmetrical inequivalent VO 4 tetrahedra, two asymmetrical inequivalent FeO 6 octahedra, and one FeO 5 polyhedron [20] (Figure 2f). The CoV 2 O 7 crystal structure was monoclinic Figure 2g, the total number of atoms in a unit cell was 44. The distance between the V atom and the O atom was different. The unit cell contained three symmetrical VO 4 of each tetrahedron and six asymmetrical CoO 6 octahedra (Figure 2h). It can be seen that each VO 4 tetrahedron was connected to four CoO 6 octahedra by the O atom; however, each CrO 6 octahedron was connected by the O atom to six VO 4 tetrahedras and shared a common edge with two other octahedral.  Figure 3 shows the relative linear thermal expansion of BiVO4, FeVO4, and Co2V2O7. It was found that the samples have different relative linear thermal expansion. For BiVO4, there was a thermal expansion inflection point at about 237 °C which means that the material occurs phase transition at  the CTE of FeVO 4 was obtained to be (2.751 ± 0.004) × 10 −6 • C −1 from 20 to 350 • C and (5.245 ± 0.005) ×10 −6 • C −1 from 400 to 600 • C. Although both vanadium and iron are variable metals and thermal expansion is related to valence states [21], we prepared and measured the material in an air atmosphere, vanadium and iron should remain stable in the highest valence state. So, there should be no chemical expansion here. The thermal expansion of Co 2 V 2 O 7 was stable below 500 • C, and the CTE was (9.230 ± 0.004) × 10 −6 • C −1 from 20 to 500 • C. The inflection point above 500 • C is due to the softening of glass state above 500 • C which can be explained by the fact that Co 2 V 2 O 7 goes from the crystalline form to a glassy one. This phenomenon indicates that material intelligent stability exists with below 500 • C. Though the structure of Co 2 V 2 O 7 is similar to BiVO 4 , their CTE is very different. This could come from the different ionic radius of Co 3+ (63 pm) and Bi 3+ (108 pm). The ionic radius of Co 3+ (63 pm) equals that of Fe 3+ (64 pm); however, they had the largest difference in CTE in this study. This was due to the different structures.  [21], we prepared and measured the material in an air atmosphere, vanadium and iron should remain stable in the highest valence state. So, there should be no chemical expansion here. The thermal expansion of Co2V2O7 was stable below 500 °C, and the CTE was (9.230 ± 0.004) × 10 −6 °C −1 from 20 to 500 °C. The inflection point above 500 °C is due to the softening of glass state above 500 °C which can be explained by the fact that Co2V2O7 goes from the crystalline form to a glassy one. This phenomenon indicates that material intelligent stability exists with below 500 °C. Though the structure of Co2V2O7 is similar to BiVO4, their CTE is very different. This could come from the different ionic radius of Co 3+ (63 pm) and Bi 3+ (108 pm). The ionic radius of Co 3+ (63 pm) equals that of Fe 3+ (64 pm); however, they had the largest difference in CTE in this study. This was due to the different structures. Raman spectroscopy was applied to further demonstrate the existence of crystal. Raman spectra collected at room temperature is shown in Figure 4. The Raman spectra of BiVO4, CrVO4, FeVO4, and Co2V2O7 were in agreement with literature [10,20,22,23] and the spectra did not show the characteristic bands of V2O5. Hence, no effort was taken to consider the product selectivity in this work. For BiVO4, the primitive cell contained 28 atoms ( Figure 2) and, in principle, 81 vibrational modes were expected. The band at approximately 828 cm −1 corresponded to stretching modes of V-O bonds, and there was no splitting which means that degeneracy occurs in the symmetric stretching vibration of VO4 tetrahedron. The strongest peak of CrVO4 and FeVO4 was much higher, whereas the stretching modes of V-O give rise to intense bands, the difference in electronegativity of these metal (Bi, Co, Cr, and Fe). The Raman bands of FeVO4 were much more than that of BiVO4, CrVO4 and Co2V2O7, because the structure of FeVO4 is triclinic, and all vibrations are nondegenerate. The 36 atoms in the unit cell had 105 vibrational modes among which 54 optical modes were Raman active Ag modes, 51 were infrared active Au modes [23]. For all Raman spectra, the stretching modes of V-O combining M-O and V-O occurred above 650 cm −1 , and bending modes together with stretching modes appeared in the 630-420 cm −1 region. The lower wavenumber bands were external modes from Raman spectroscopy was applied to further demonstrate the existence of crystal. Raman spectra collected at room temperature is shown in Figure 4. The Raman spectra of BiVO 4 , CrVO 4 , FeVO 4 , and Co 2 V 2 O 7 were in agreement with literature [10,20,22,23] and the spectra did not show the characteristic bands of V 2 O 5 . Hence, no effort was taken to consider the product selectivity in this work. For BiVO 4 , the primitive cell contained 28 atoms ( Figure 2) and, in principle, 81 vibrational modes were expected. The band at approximately 828 cm −1 corresponded to stretching modes of V-O bonds, and there was no splitting which means that degeneracy occurs in the symmetric stretching vibration of VO 4 tetrahedron. The strongest peak of CrVO 4 and FeVO 4 was much higher, whereas the stretching modes of V-O give rise to intense bands, the difference in electronegativity of these metal (Bi, Co, Cr, and Fe). The Raman bands of FeVO 4 were much more than that of BiVO 4 , CrVO 4 and Co 2 V 2 O 7 , because the structure of FeVO 4 is triclinic, and all vibrations are nondegenerate. The 36 atoms in the unit cell had 105 vibrational modes among which 54 optical modes were Raman active A g modes, 51 were infrared active A u modes [23]. For all Raman spectra, the stretching modes of V-O combining M-O and V-O occurred above 650 cm −1 , and bending modes together with stretching modes appeared in the 630-420 cm −1 region. The lower wavenumber bands were external modes from lattice, translational, and vibrational motions [24,25]. From a crystalline perspective, all catalysts were composed of V-O polyhedrons and other metal-oxygen polyhedrons.

Thermal Expansion Property
In order to study the sudden change in thermal expansion of BiVO 4 , the Raman spectroscopy dependent temperature of BiVO 4 is shown in Figure 5. The Raman bands became weaker and weaker with the increasing temperature which reflects the increase of the degree of disordering of the crystal structure. The relative intensity of 368 and 324 cm −1 had obvious change at 200 • C, and they disappeared at 300 • C; meanwhile, there was a new band at approximately 345 cm −1 . There was not only one change. The bands at 127 and 211 cm −1 gradually became a wave packet; meanwhile, the 703 cm −1 band disappeared, and the 828 cm −1 band moved to 815 cm −1 , this might be caused by the bond expansion and weakening. All these mean that there was a phase transition between 200 • C and 300 • C. There was no change in the Raman spectra above 300 • C. The high temperature Raman spectra were in agreement with tetragonal structure which means that BiVO 4 crystal tetragonal as well [26]. Compared with Figure 3, we found that materials with high symmetry have larger CTE. The Raman band at 368 and 324 cm −1 could inhibit the thermal expansion of material. It means that the thermal expansion property was related to the structure of the material.
Materials 2020, 13, 1628 6 of 9 lattice, translational, and vibrational motions [24,25]. From a crystalline perspective, all catalysts were composed of V-O polyhedrons and other metal-oxygen polyhedrons. In order to study the sudden change in thermal expansion of BiVO4, the Raman spectroscopy dependent temperature of BiVO4 is shown in Figure 5. The Raman bands became weaker and weaker with the increasing temperature which reflects the increase of the degree of disordering of the crystal structure. The relative intensity of 368 and 324 cm −1 had obvious change at 200 °C, and they disappeared at 300 °C; meanwhile, there was a new band at approximately 345 cm −1 . There was not only one change. The bands at 127 and 211 cm -1 gradually became a wave packet; meanwhile, the 703 cm -1 band disappeared, and the 828 cm −1 band moved to 815 cm -1 , this might be caused by the bond expansion and weakening. All these mean that there was a phase transition between 200 °C and 300 °C. There was no change in the Raman spectra above 300 °C. The high temperature Raman spectra were in agreement with tetragonal structure which means that BiVO4 crystal tetragonal as well [26]. Compared with Figure 3, we found that materials with high symmetry have larger CTE. The Raman band at 368 and 324 cm -1 could inhibit the thermal expansion of material. It means that the thermal expansion property was related to the structure of the material.

Summary
Vanadium compounds were synthesized to investigate the thermal expansion properties and structure. The CTE of Co2V2O7 was bigger than monoclinic structure BiVO4 which means that the thermal expansion property was related to the ionic radius of metals. The CTE of the tetragonal structure of BiVO4 was 15.27 × 10 −6 °C −1 which was the biggest CTE in our measurement results, and the CTE of tetragonal structure FeVO4 was 2.84 × 10 −6 °C −1 which was the smallest. This indicates that

Summary
Vanadium compounds were synthesized to investigate the thermal expansion properties and structure. The CTE of Co 2 V 2 O 7 was bigger than monoclinic structure BiVO 4 which means that the thermal expansion property was related to the ionic radius of metals. The CTE of the tetragonal structure of BiVO 4 was 15.27 × 10 −6 • C −1 which was the biggest CTE in our measurement results, and the CTE of tetragonal structure FeVO 4 was 2.84 × 10 −6 • C −1 which was the smallest. This indicates that the thermal expansion property was related to the structure of the material.