Growth And Magneto-optic Properties Of CdTb₂（MoO₄）₄ And CdTb₂（WO₄）₄ Crystals With Scheelite Structure

In this paper, we introduce a brief overview of the research background and characteristic of the rare earth molybdenum and tungstate systems. We select CdTb2(Mo04)4 and CdTb2(WO4)4 compounds with the scheelite type structure as the research goals.Through the TG-DTA analysis, we determined the sintering process of CdTb2(Mo04)4 and CdTb2(WO4)4 polycrystalline materials by the high-temperature solid-phase synthesis method. By employing the characterization methods of XRD and IR, and the results were used to analyze the structure of polycrystalline powder materials; The TG-DSC was used to identify the melting point of CdTb2(MoO4)4 and CdTb2(WO4)4 compounds. When the sintering temperature is between 750℃ to 1050℃, the synthetic CdTb2(WO4)4 belongs to the monoclinic wolframite structure. Irreversible phase transition occurs in the molten state, the phase of CdTb2(WO4)4 transitions from monoclinic to tetragonal scheelite structure.The CdTb2(MoO4)4 (CTM) and CdTb2(WO4)4 (CTW) single crystals were grown by the Czochralski method. The as-grown CdTb2(MoO4)4 crystal was black in color and has low transmittance in the 400-1500nm waveband. By annealing in air or oxygen atmosphere, the transmittance of CTM crystal could be effectively improved. XPS analysis were used to investigate the change of element valences in crystal and the effect of annealing on the optical property of CTM crystal was discussed. Annealing process has little influence on cation valences. The existence of CdMoO4-Tb2(MoO4)3 solid solution and oxygen deficiency may be the main reasons of low transmittance.The volatilization of CdWO4 during the CdTb2(WO4)4 growth period leads to the compositional deviations of the melt, which would give rise to the appearance of CdTb2(WO4)4 and Tb2(WO4)3 polycrystallines. Whereas, the main volatile substance is MoO3 during the growth process of CdTb2 (MoO4)4 crystal.XRD analysis of the crystal powder was carried out and the structure of CdTb2(MoO4)4 and CdTb2(WO4)4 were refined by Rietveld refinement method. CdTb2(MoO4)4 arid CdTb2(WO4)4 belong to tetragonal scheelite structure. Both space groups are I41/a. The physical properties such as density, Vichers hardness, thermal expansion coefficient and variable temperature magnetic susceptibility were measured. Because of the transition from CdMoO4Tb2(MoO4)3 solid solution to scheelite-type CdTb2(MoO4)4, CdTb2(MoO4)4 crystal along the<001> direction occurs negative thermal expansion in the range of 540-680℃. CdTb2(MoO4)4 and CdTb2(WO4)4 exhibits a clear crystal magnetic anisotropy. They have complex magnetic properties in the temperature range of 2-50K.We measured the crystal transmittance in 300-1500nm waveband and calculated the absorption coefficients. The Faraday rotation angle of CdTb2(MoO4)4 and CdTb2(WO4)4 crystal wafers along<001> orientation were tested at 532nm,635nm and 1064nm, and the Verdet constants at different wavelengths were calculated. Faraday rotation spectrum of CdTb2(MoO4)4 at 400-1500nm waveband was tested and it fit well with the Faraday rotation angle tested. CdTb2(MoO4)4 and CdTb2(WO4)4 have higher Verdet constants than that of TGG. Results show the two crystals may be candidate magneto-optical materials in the visible and near infrared bands.