Theoretical Study On Structures And Properties Of Binary V-O/W-O Compounds Under High Pressure

Transition metal oxides show different properties from the main group metal oxides due to the presence of diverse d-orbital electronic configuration.For example,CrO₂ exhibits semi-metal ferromagnetic characteristic;and B1 type FeO has Mott insulator to metal transition,etc.Due to the existence of various forms of oxygen atoms,oxygen ions(O^2-),peroxide(O₂^2-),and superoxide?O₂^-?,promotes the huge of variety and property of transition metal oxides.Pressure becomes an effective means to adjust the structures and properties of compounds.It is worth noting that transition metal oxides are sensitive to pressure.It is easy to produce compounds with novel chemical compositions and various structural phase transitions under pressure.Therefore,exploring the phase transition and oxygen evolution behavior of transition metal oxides under high pressure is a meaningful research topic.Vanadium?V?and tungsten?W?are very active due to the partial occupation of the d orbitals,and can easily react with the electronegative light element oxygen?O?to form diverse compounds.Furthermore,both V-O and W-O compounds undergo complex structural phase transitions under high pressure,further study of their phase stability under high pressure and oxygen-riched environment will be of great benefit in understanding the chemical reaction characters,structures and properties of transition metal oxides.Based on first-principles calculations and CALYPSO structure search methods,this thesis performs a comprehensive and systematic study towards binary V-O and W-O systems under high pressure.The specific research contents are as follows:For systematically study the high-pressure behavior of the V-O system,A transition metal peroxide,VO₄,appeared in the O-rich region,it contains not only O^2-but also O₂^2-,becoming the first binary vanadium oxide containing O₂^2-.In addition,a superconducting V₂O compound becomes stable,with a superconducting transition temperature of 0.3 K at 80 GPa.In addition,this structure is dynamically stable under normal pressure and is expected to exist in a metastable state.These results broaden the understanding the structures and electronic properties of the V-O system.Although the high-pressure phase transitons of tungsten oxides have been reported in previous work,but its high-pressure structures and phase diagram are still not fully determined.This thesis systematically studies the high-pressure phase diagram of W-O system.The results show that below 300 GPa,only the O-rich compositions,WO₂ and WO₃are stable.With the increase of pressure,WO₂ and WO₃ exhibit complex phase transitions,accompanied by a significant increase in the coordination number of W.Pm-3n WO₃ has the maximum coordination number of 12 among all W-O compounds.The pressure-induced semiconductor-to-metal transitions are observed for WO₂ and WO₃.Their metallicity mainly comes from the contribution of the W 5d state.In addition,pressure-induced volume reduction is the main driving force for the phase transition of W-O compounds.