Optical Properties Of(Mg_1-xA_x) Ta₂O₆ Ceramics(A=Co,Cr) And MgTa₂O₆ Crystal

With the rapid development of microwave communication,great progress has been made in mobile phones and electronic communication equipments^[1-3].High frequency dielectric materials play an important role in communication systems,so they are widely explored by researchers and manufacturers in the scientific and technical industries^[4].Because of the good dielectric properties,Tantalate A²⁺Ta₂O₆(A=Ni,Co,Fe,Mn,Zn,Mg,Ca,Cu,Cd)plays an important role in microwave dielectric materials.Especially magnesium tantalate MgTa₂O₆,it has been paid much attention by many research groups all over the world.In addition,trirutile MgTa₂O₆has excellent nonlinear optical properties and great potential in photocatalytic water decomposition.The common methods for preparing powder crystal of MgTa₂O₆include solid phase sintering,sol-gel method,hydrothermal method and flux growth method.In addition,there are microwave hydrothermal method,hot pressing,isostatic pressing sintering and other methods.In this paper,solid state reaction method was used to prepare the samples.The samples included trirutile MgTa₂O₆powder crystal at different sintering temperatures,the series of(Mg_1-xA_x)Ta₂O₆ceramics with different doping concentrations.X-ray diffraction was used to determine the structure type and structural stability of the samples.The Raman and PL spectra were used to analyse the optical properties of MgTa₂O₆.The MgTa₂O₆single crystals were obtained by optical floating zone technology.According to Raman,PL spectroscopy and UV-Vis absorption spectroscopy,we studied the sample's properties.The research results were summarized as follows:1.We confirmed the phenomenon of oxygen vacancy in MgTa₂O₆by combining with XRD and Raman test datas.The X-ray diffraction peaks shifted.The position of Raman spectra peaks shifted and the width of peaks broadened.These phenomenons were caused by oxygen vacancy.And this phenomenon led to a result that the PL spectra was fully quenched.2.As for different doping concentrations of(Mg_1-XA_X)Ta₂O₆ceramics.First,The Raman and PL spectroscopy of(Mg_1-XCo _X)Ta₂O₆and(Mg_1-XCr _X)Ta₂O₆were investigated and compared by us.The results showed that the doping of Co²⁺caused the blue shift of Raman peaks and the decrease of photoluminescence intensity.The doping of Cr³⁺led to the result that the photoluminescence intensity enhanced.Second,the Raman spectra of(Mg_0.99Cr_0.01)Ta₂O₆and(Mg_0.8Co_0.2)Ta₂O₆under different electric fields were tested.A new peak of(Mg_0.8Co_0.2)Ta₂O₆appeared under the action of electric field.Finally,the magnetic test of the sample was carried out.The fitting results of(Mg_1-XCo _X)Ta₂O₆showed that with the increase of doping concentration,Weiss temperature was gradually declined.This phenomenon showed that the antiferromagnetic exchange interactions gradually enhanced.The magnetic hysteresis loop of(Mg_0.95Cr_0.05)Ta₂O₆showed that with the temperature gradually declined the area of the hysteresis loop becomed larger and coercive force of(Mg_0.95Cr_0.05)Ta₂O₆increased.Among the antiferromagnetic exchange interactions the magnetic hysteresis loop appeared hysteresis phenomenon,It was the properity of ferrimagnetic materials.As the temperature increased,the ferromagnetic interaction became weaker.3.The absorbance spectrum of MgTa₂O₆crystal showed an absorption below 336nm.The direct and indirect transitions co-existed in the trirutile MgTa₂O₆.The bandgap of direct transitions was 4.27 e V.The bandgap of indirect transitions with phonon participation was 3.85 e V,and the energy of the phonon was 0.28 e V.Room temperature PL spectra showed two emission centers at 784.21 nm(1.58 e V)and 829.88 nm(1.49 e V),which could be attributed to the defect and regular Ta–O emission center,respectively^[6].The phonon located at 704 cm^-1participated in the regular Ta–O emission process^[6].The temperature-dependent PL spectra blue shifted,and the reasons were discussed^[6][7].