Polymorphs of ZnV2O6 under Pressure: A First-Principle Investigation

This work presents first-principle calculations on the pressure dependence of the stabilities, structures, and electronic properties of several polymorphs of ZnV2O6 under the pressure range 0-30 GPa. These properties are analyzed and discussed in detail using the different parameterizations of the exchange-correlation functional (B3LYP, HSE06, and PBE), and the results are compared with available experimental data. An extensive search process was carried out on the potential energy surface for a set of 12 possible polymorphs. Ten of them are stationary points but only five have positive frequency values in the range of 0-30 GPa, that is, monoclinic brannerite (C2/m and C2), orthorhombic columbite (Pbcn), trigonal CaAs2O6-type (P321), and triclinic NiV2O6-type (P (1) over bar). The monoclinic ThTi2O6-type (C2/c) phase presents a very low imaginary frequency around 50 cm(-1). Orthorhombic SrV2O6-type and BaV2O6-type, tetragonal trirutile, and trigonal PbSb2O6-type structures show several imaginary negative frequencies between -400 and -100 cm(-1). These imaginary frequencies are indicative of structural instabilities. All attempts to try to converge the calculations to obtain the MoLa2O6-type and HgV2O6-type polymorphs, by using the three functionals, have been unsuccessful. For both brannerite, ThTi2O6-type, columbite, CaAs2O6-type, and NiV2O6-type structures, numerical and analytical fittings were performed to obtain the lattice parameters, the bulk modulus, B, and their pressure derivative, B', and the energy-volume relationship of phases are analyzed. Vibrational calculations were performed at each pressure for each polymorph and compared with available experimental data. This study reports, for the first time, a complex and unexpected structural and chemical behavior as a function of pressure. An analysis of the results shows that brannerite monoclinic polymorphs present similar energies, suggesting that both structures may coexist in the range of pressures that were studied. Theoretical prediction reveals that, as the pressure increases, the most stable polymorph of ZnV2O6 moves from the monoclinic phase, C2/m (and C2), to the orthorhombic columbite structure (Pbcn) and the corresponding transition pressure is computed to be 5 GPa at the B3LYP level; however, using the HSE06 and PBE functionals, the FeNb2O6-type structure is the most stable polymorph from ambient pressure up to 30 GPa. In addition, the calculations show that the ThTi2O6-type phase (C2/c) becomes more stable than the other two monoclinic structures above 15, 13, and 8 GPa using the B3LYP, HSE06, and PBE functionals, respectively. The findings reported here indicate that the method used to predict relative phase stabilities and phase transitions should be chosen carefully and that the results should be scrutinized with a critical eye. We analyze the variations of different vibrational frequencies and V-O and Zn-O bond lengths as pressure is applied, and the results confirm that Badger's lineal relationships were not found for the monoclinic polymorphs (C2/m, C2, and C2/c) because of the presence of a charge-transfer process from ZnO6 to VO6 polyhedra. In addition, we determined the stability of ZnV2O6 against binary oxides (VO2, V2O5, and ZnO) or metals (Zn and V) and oxygen, and the results reveal that the decomposition channels are not energetically feasible. The present work provides a new perspective on the variations of crystal structures and physical properties at high pressure in ZnV2O6 polymorphs. With the wealth of data presented, this study aims to encourage further basic and experimental research on an interesting case of ZnV2O6-based materials with novel properties that may be the basis of important innovations for industrial applications.