The Preparations Of Vanadates MV₂O₆and Their Physical Properties Under High Pressure

Vanadates have exhibited so many excellent properties, such as good catalyticperformance, and some of them have become important industrial catalysts; due to thehigher specific capacity and the negative suitable lithium deintercalation, vanadatesare also widely used in aqueous lithium-ion batteries. Their unique physical andchemical properties primarily come from the interaction of their polyhedral structureof vanadium oxide.Under high pressure, the material can present many exotic phenomena and newproperties. Combined with other measurement methods, the in-situ measurementunder high pressure is an efficient approach to investigate the properties of thesematerials. In this thesis, the physical properties and structural phase transitions ofvanadates were studied by in-situ high pressure Raman, high pressure synchrotronX-ray diffraction, and high pressure electrical measurements.Firstly, by using the sol-gel method, we have successfully synthesized MV2O6(M=Mg, Ca, Zn, Cd, Mn, Co, Sr, Ba). And we have synthesized a new phase ofMgV2O6by using high temperature and high pressure (HTHP) method. The results ofin-situ high pressure measurements of MV2O6(M=Mg, Ca, Zn, Cd, Mn, Co, Sr, Ba)are listed as follow:1) The structural phase transition of MgV2O6(sol-gel) under high pressure has beenstudied by Raman spectroscopy and synchrotron X-ray diffraction. The combinedanalyses of experimental results suggest that MgV2O6undergoes a reversiblestructural phase transition around20.0GPa, as evidenced by the appearance ofnew peaks and splitting peaks. The transformation process from the C2/m phaseto the C2/m+C2phase is confirmed and is attributed to the more distorted MgO6octahedra and VO6octahedra at higher pressures. The structural stability of thetwo different phases of MgV2O6synthesized by sol-gel and HPHT methods hasbeen investigated, and we found that the structure of MgV2O6(HPHT) is morestable, and the conductivity is better. 2) The structural properties of CaV2O6under high pressure have been studied byRaman spectroscopy and synchrotron X-ray diffraction. The combined analysesof experimental results suggest that CaV2O6undergoes two reversible structuralphase transitions around1.7and4.3GPa, respectively. The first one was anisostructural phase transition and the second one was deduced from the C2/mphase to the C2phase.3) The structural and electrical properties of ZnV2O6under high pressure have beenstudied by Raman spectroscopy, synchrotron X-ray diffraction, and alternatingcurrent (AC) impedance spectroscopy. The results of Raman spectra indicate thatZnV2O6undergoes a reversible structural change around16.6GPa, as evidencedby the appearance of new peaks. The results of Rietveld refinements from highpressure synchrotron X-ray data indicate that the monoclinic symmetry (C2/m) isretained up to16.0GPa, and the C2phase comes to coexist between16.0GPaand16.9GPa. Above16.9GPa, the high-pressure phase can be onlydistinguished as the C2structure. The transformation process from the C2/mphase to the C2phase is mainly caused by the more distorted ZnO6octahedra andVO6octahedra at higher pressures. The equal bond distances Zn-O2and V-O3inthe C2/m phase become unequal in the C2phase. Furthermore, the measurementsof the AC impedance spectroscopy of ZnV2O6reveal obvious change in itselectrical transport properties at14.1GPa which could correspond to the observedphase transition in the Raman and synchrotron X-ray measurements. Thecombined analyses of experimental results suggest the occurrence of a reversiblestructural phase transition of ZnV2O6around16.0GPa.4) The structural properties of MV2O6(M=Cd, Mn, Co) under high pressure havebeen studied by Raman spectroscopy and synchrotron X-ray diffraction. Therewas no sign of a structural phase transition in MV2O6(M=Cd, Mn, Co) within acertain pressure range, but with further increasing pressures, amorphization hasbeen found in both MnV2O6and CoV2O6.5) The structural properties of MV2O6(M=Sr, Ba) under high pressure have beenstudied by Raman spectroscopy and synchrotron X-ray diffraction. Apressure-induced structural phase transition in SrV2O6has been observed at3.9GPa, while the phase transition in BaV2O6happened at4.3GPa. With furtherincreasing pressures, amorphization has been found in both SrV2O6and BaV2O6at12.3and10.4GPa, respectively. The pressure-induced amorphization (PIA) of SrV2O6and BaV2O6was suggested to be associated with breaking up of theinfinite chains of corner-linked tetrahedral VO4. The recovery of the originalcrystalline phases and the relinking of the VO3chains have been observed afterheating the reclaimed samples.