Investigation On High Pressure Phase Transition And Properties Of BiVo₄ And CoMoO₄

Pressure can effectively reduce the distance between atoms,enhance the interaction between electrons.It plays an important role in inducing crystal structure transformation and modulating energy band structure.Pressure induced phase transition can greatly expand the abundance of materials and thus excavate many new crystal structures with excellent properties.Remarkable achievements have been made in superconductivity,thermoelectric and other energy materials.This thesis selects energy materials such as typical photocatalytic material BiVO₄and electrode material CoMoO₄as research objects,using high pressure experimental devices such as diamond anvil cell,combined with in-situ high pressure Raman and synchrotron XRD measurements,to study their structural evolution under pressure,and obtain their high pressure phase transition process.On this basis,the photocatalytic material and electrode material with excellent performance are developed by using high pressure synthesis technology.The main research contents are as follows:（1）We successfully prepared a series of fergusonite-and zircon-BiVO₄:x Eu³⁺photocatalyst by hydrothermal method,and studied the effects of doping and pressure on the both type of BiVO₄.XRD and Raman spectra showed that the doping solubility of Eu³⁺in fergusonite-BiVO₄was only0.5 mol%.When the doping concentration is higher than 0.5 mol%,structure transformation from fergusonite-BiVO₄to zircon-BiVO₄will happen.This is not conducive to improving their photocatalytic activity.In-situ high pressure XRD and Raman spectra results revealed two consecutive reversible phase transitions for fergusonite-BiVO₄,namely fergusonite to scheelite and scheelite to a new high pressure phase at 3.2 GPa and 15.0 GPa respectively.The new high pressure phase was successfully identified as theβ-fergusonite by using metadynamics simulation and refinement of XRD data for the first time.For zircon-BiVO₄,an irreversible zircon-to-scheelite phase transition starts at 1.3 GPa,and completes at 5.0 GPa.The second transition to the same new high pressure phase starts above 15.0 GPa and completes around 41.6 GPa.The doping of Eu³⁺does not change the phase transition sequence of BiVO₄.Therefore,zircon-BiVO₄:x Eu³⁺transformed to scheelite-BiVO₄:x Eu³⁺during compression and returned to fergusonite-BiVO₄:x Eu³⁺during decompression.Thus,the doping solubility of Eu³⁺in fergusonite-BiVO₄was improved by compression and decompression cycle.Similarly,we also investigated the Gd³⁺and La³⁺doped BiVO₄.The same results can be obtained as well.As results,the doping solubility of Re³⁺in fergusonite-BiVO₄can be improved by compression-decompression cycle and the band gap of fergusonite-BiVO₄may be further reduced,which is favourable to improve its photocatalytic activity.Preliminary experiments show that fergusonite-BiVO₄:8 mol%Gd³⁺(HPHT-BiVO₄:8mol%Gd³⁺)has good photocatalytic activity under visible light and its oxygen production rate is 2.3times than that of zircon-BiVO₄:8 mol%Gd³⁺.（2）We have successfully prepared electrode materialsβ-CoMoO₄andα-CoMoO₄by combining hydrothermal method and annealing treatment.Their high pressure phase transition processes were investigated.At 1 GPa,theβ-CoMoO₄transforms irreversibly toα-CoMoO₄,and then to an unknown high pressure phase at 14.4 GPa.Forα-CoMoO₄,it transforms directly to the same unknown high-pressure phase at 13.5 GPa.The above pressure-induced phase transition were proved by in-situ high pressure synchrotron XRD measurements and the unknown high pressure phase was well indexed toγ-CoMoO₄or II-CoMoO₄.Then,bulkγ-CoMoO₄samples were synthesized by Kawai-type high pressure apparatu at 25 GPa,and the electrochemical performance ofα-,β-,andγ-CoMoO₄as anode material for lithium ions battery was evaluated.At the current density of 100 m A g^-1,the specific capacity ofγ-CoMoO₄is maintained at 1048 m Ah g^-1(β-CoMoO₄,952 m Ah g^-1;α-CoMoO₄,152 m Ah g^-1)after 50 cycles.When the current density is increased to 2000 m A g^-1,its capacity can still reach 510 m Ah g^-1(β-CoMoO₄,33 m Ah g^-1)after1000 cycles.In addition,it exhibits good rate performance and delivers high capacities of 1021,904,732,543 and 919 m Ah g^-1at 100,200,500,1000 and 100 m A g^-1,respectively.These results show thatγ-CoMoO₄possesses excellent electrochemical performance among them,such as high specific capacity,excellent structural stability and fast charge and discharge ability.These studies provide a method to obtain new structures or novel properties via pressure induced structural phase transitions,and provide strategic guidance for condensed matter physics and materials research.