Pressure And Temperature-dependent Optical Properties Of TiTa₂O₇

Tantalates have excellent optical properties and usually good chemical resistance,these materials could be of interest for further applications,for example synthetic gemstones,optical coatings,or low-thermal-expansion materials which has obtained worldwide attractions.Tantalates generally have a wide band gap,they can only absorb and utilize ultraviolet light in the sunlight,and the light energy conversion efficiency is low.Therefore,it is one of the hot topics to control the optical band gap of tantalum compounds and realize the absorption and utilization of visible light.In recent years,the tantalates exhibit excellent photocatalysis property,for example:TiTa₂O₇,NaTaO₃ and Bi₃TaO₇ etc.Tantalate photocatalyst has unique electronic structure and band structure,and its catalytic performance is better than that of other types of compounds in decomposition of water to hydrogen.TiTa₂O₇ is also considered as a promising photocatalyst.Polycrystalline samples of TiTa₂O₇ were grown directly by solid-state reaction methods.The optical properties of the monoclinic structure of TiTa₂O₇ at high-pressure and different temperatures were measured and studied by in-situ high-pressure Raman scattering,high-pressure absorption,in-situ temperature dependent Raman scattering and temperature dependent photoluminescence?PL?.The results are shown below:1.In-situ high-pressure Raman spectroscopy and UV-Vis absorption spectroscopy were used to study the high-pressure behavior of TiTa₂O₇.The experimental pressure reached 25.17 GPa and 15 GPa.respectively.TiTa₂O₇ Raman vibration modes are identified by in-situ high-pressure Raman spectroscopy.We found that the 75 cm^-1 mode split into two peaks at 3.75 GPa,and with the increase of pressure,part of the?₂ vibration mode began to shift toward lower wavenumbers.The existence of this soft mode indicates that the Ta?Ti?-O octahedron bending vibration under pressure made the O-Ta-O/O-Ti-O bond longer,which also indicates the instability of the TiTa₂O₇ structure under pressure.When the pressure increased to 11GPa,irreversible amorphization of TiTa₂O₇ occurred.A variation relationship of the band gap energy of TiTa₂O₇ under high-pressure is given in the high-pressure UV-Vis absorption spectrum.In the pressure range of0-7.3 GPa,the absorption edge blue shifted as the pressure increased.After7.3 GPa,the absorption edge began to redshift.When the pressure increased to 11 GPa,the original absorption edge continued to blueshift,at the same time an inflection point appeared,accompanied by pressure-induced amorphization of TiTa₂O₇.The optical band gap of the whole high-pressure experimental process did not appear the expected narrow optical band gap,and no new absorption edge appeared.2.TiTa₂O₇ high-pressure behavior was studied by using in-situ temperature dependent Raman spectroscopy and PL spectroscopy,the lattice vibration and fluorescence properties of TiTa₂O₇ under the influence of temperature were studied.The lowest temperatures of both experiments were 83.15 K,and the highest temperatures reached 803.15 K and 283.15K,respectively.The negative thermal expansion of TiTa₂O₇ along the b-axis at high temperature can be explained as the structural change caused by the external force acting on the bond angle of the O-Ta-O/O-Ti-O bond.No new raman peaks appeared during the whole temperature change process,indicating that the TiTa₂O₇ structure is stable at extreme temperatures.The temperature dependent PL spectra of TiTa₂O₇ are given and the luminescence mechanism is studied.The PL spectra were fitted by Gaussian method,and the fitting peaks were located at 1.42 eV?A peak?and 1.51 eV?B peak?,respectively,which was mainly determined by the defect and regular Ta-O₆ octahedra.