Structural Evolution And Physical Properties Of Wolframites AWO₄ And Thiospinels AB₂S₄ Nanocrystals Under High Pressure

The wolframites AWO4 and the thiospinels AB2S4 are widely distributed functional minerals in the Earth’s mantle.The wolframite AWO4 has excellent optical and luminescent properties,and the thiospinels AB2S4 has excellent electrical properties,which are widely used in fields such as scintillation detectors and electrode materials for energy storage capacitors,respectively.However,wolframites AWO4 suffers from a large bandgap and a large energy difference between the d-d transition energy and the bandgap,which affect its optical absorption efficiency,as well as the cation disorder in AB2S4 strongly affects its physical properties.How to optimally enhance their optical and electrical properties and rationalize the structure-property relationship is a hot issue in this research.Pressure,as a unique thermodynamic variable,provides a powerful means to study the structure and electronic behavior of materials.In this works,the wolframites AWO4(A = Fe,Co,Ni)and the thiospinels AB2S4(A= Co,Ni,Cu;B = Ni,Co)nanocrystals are studied.The effects of octahedral symmetry and crystal field splitting on the optical properties of the wolframite compounds,and cation inversion at the A and B sites on the electrical properties of the thiospinel compounds are explored under pressure.The following innovative research results were obtained:Ⅰ.Structural evolution and physical properties of the wolframites AWO4(A = Fe,Co,Ni).1.The pressure-induced structural phase transitions of the wolframites AWO4(A =Fe,Co,Ni)was discovered.High-pressure in situ XRD test results show that Fe WO4,Co WO4 and Ni WO4 undergo monoclinic(P2/c)to triclinic(P–1)structural phase transitions at 15.7,26.0 and 20.1 GPa,respectively.The A–O and W–O bond lengths decrease with increasing pressure in the monoclinic phase,suppressing the octahedral Jahn-Teller distortion and resulting in the A2+O6 and WO6 octahedral symmetry enhancement.Simultaneous high-pressure Raman spectroscopy measurements confirm their structural evolution and the WO6 octahedron is a rigid unit.2.Octahedral symmetry and crystal field splitting energy of 10 Dq affect their optical properties.In situ high-pressure absorption spectroscopy measurements show that the wolframites AWO4(A = Fe,Co,Ni)is an indirect bandgap semiconductor in the monoclinic phase,and the optical bandgap decreases with increasing pressure with pressure coefficients of –17.6,–17.9,and –12.0 me V/GPa.The energy difference between the optical bandgap of Co WO4 and Ni WO4 and the A2+ intraband d-d transition energies decreases with increasing pressure in the monoclinic phase.Combining the experimental results with the first-principles calculations analysis found that the high-pressure optical behavior of the wolframites AWO4(A = Fe,Co,Ni)are mainly influenced by the increased octahedral symmetry of A2+O6 and WO6 and the enhanced orbital coupling of each element.3.The pressure-induced bandgap reduction facilitates electrical transport conduction.The results of electrical transport measurements show that the Fe WO4 resistivity and relaxation time decrease with increasing pressure,making it easier to conduct electrons.The resistivity continues to decrease after at 14.6 GPa,but the trend of change is significantly slower,which is conducive to the improvement of its electrochemical performance as a supercapacitor electrode material.Ⅱ.Structural evolution and electrical transport properties of the thiospinels AB2S4(A = Co,Ni,Cu;B = Ni,Co)nanocrystals.1.The pressure-induced structural phase transitions of the thiospinels Co Ni2S4,Ni Co2S4 and Cu Co2S4 nanocrystals were discovered.In situ high-pressure XRD measeurment results show that(1)pressure-induced inverse spinel Co Ni2S4 undergoes a cubic phase(Fd-3m)to orthorhombic phase(Pbcm)structural transition at 14.9 GPa,and the orthorhombic phase is irreversible to ambient conditions.(2)Pressure-induced inversion of the A-and B-site cations in the mixed spinels Ni Co2S4 and Cu Co2S4 induces discontinuous changes in their S-ion atomic occupancy u,tetrahedral Vtet and octahedral Voct at 10.0 GPa,thus undergoing an isostructural phase transition from the mixed spinel structure to the normal and inverse spinel structures.(3)The mixed spinel Cu Co2S4 undergoes another structural phase transitions at 30.3 GPa.2.Pressure-induced structural phase transitions induces discontinuous changes in resistivity.The results of in situ high-pressure DC resistivity measurements showed that the resistivity of the thiospinels Co Ni2S4,Ni Co2S4 and Cu Co2S4 showed a trend of decreasing and then increasing during the first compression,with resistivity discontinuities occurring at 13.5,10.7 and 9.1 GPa,respectively,corresponding to the structural phase transitions.The resistivity changes continuously during the first decompression and second compression and decompression,which indirectly proves that the structural phase transitions were irreversible,which is conducive to improving their electrochemical cycling stability in energy storage capacitor devices.3.The structural phase transitions of the thiospinels Co Ni2S4,Ni Co2S4 and Cu Co2S4 was found to be accompanied by a change in the conductive properties and can be irreversible up to ambient conditions.(1)Co Ni2S4 nanocrystals exhibit metallic properties in the cubic phase and insulator properties in the orthorhombic phase,where the structural phase transition is accompanied by a metal-to-insulator transition.After decompression,the insulator properties of the orthorhombic phase are retained to ambient conditions.(2)Ni Co2S4 nanocrystals are semi-metallic in the range of 2.7–24.2 GPa and finally completely insulator at 27.8 GPa,where the pressure-induced structural phase transitions accompanied by the transformation of semi-metal to insulator.After decompression,the Ni Co2S4 nanoparticles are almost insulator.(3)Cu Co2S4 nanocrystals is insulator in the pressure range of 3.5–8.5 GPa,semimetallic in the pressure range of 11.1–23.2 GPa,and insulator in the pressure range of27.3 GPa above.the Cu Co2S4 nanocrystals undergoes the transition from insulator to semi-metal to insulator,and then to insulator under pressure.After decompression,Cu Co2S4 nanocrystals exhibit insulator properties.In summary,we have systematically studied the structural evolution of wolframites AWO4(A = Fe,Co,Ni)and thiospinels AB2S4(A = Co,Ni,Cu;B = Ni,Co)under high pressure and the modulating effects on optical and electrical properties.The constructed the connection between crystal structure and macroscopic physical properties,given an interpretation of the corresponding physical property change mechanism,which provides new ideas and basis for the design,performance improvement and practical applications of these two new functional materials,with important practical significance and theoretical value.