Growth And Performance Of Scheelite Na₂Tb₄（MoO₄）₇ And Fluorite Tb₆MoO₁₂ Magneto-optical Crystals

The paper introduces a brief overview of the background and characteristics in the rare earth molybdates. At last, the magneto-optical of scheelite Na2Tb4(MoO4)7 and fluorite Tb6MoO12 were selected as the research goals.Through the comprehensive analysis of the phase of diagram and DSC-TGA, it identified the best sintering process of Na2Tb4(MoO4)7 and TbeMoO12 polycrystalline materials by the high-temperature solid-phase synthesis method.The Na2Tb4(MoO4)7 magneto-optical crystal was grown by melt Czochralski method, and the influencing factors of the growth atmosphere, the temperature field, the growth rate and the annealing process for crystals’ quality were studied. Then, the crystal structure, composition and defects were characterized by means of ICP, SEM-EDS, XPS and optical microscopy. The results showed that a Na2Tb4(MoO4)7 crystal of good quality can be obtained under air atmosphere, appropriate and stable temperature field, the growth rate at 0.5～1.0mm/h. The transmittance of crystal was low if it was grown in N2 atmosphere, and the main reason was that the light was absorbed by color centers and the low valence state of molybdenum ions. The Na2Tb4(MoO4)7 crystals were obtained with well-distribution and structural integrity. The characterization results of the basic physical properties for Na2Tb4(MoO4)7 crystals:the vickers hardness is 336.1VDH, the density is 5.5812g/cm3, the thermal expansion coefficient is 7.74×10-6K-1. Finally, the faraday rotation angle of<001> orientation Na2Tb4(MoO4)7 crystal wafer was tested at 532nm,633nm,1064nm, the results showed that the crystal had relatively good magneto-optical properties with about 1.5 times TGG at the verdet constant. It was hopeful to be a good candidate for magneto-optical materials in visible-near infrared region.The TbeMoO12 magneto-optical crystal was grown via melt slow cooling method, at the same time exploring molten salt method to grow crystals. The Tb6MoO12 crystal belongs to cubic fluorite structure (space group Fm-3m), and its symmetry and Tb3+concentration are higher than those of Na2Tb4(MoO4)7 crystal. Tb6MoO12 system is melt congruently and it has a good crystallinity, then violet black TbgMoO12 crystals were obtained by slow cooling method. The elements valence of crystal were characterized through the XPS, the poor translucency was possible due to Mo5+ concentration too high and severe oxygen deficiency in the crystal. A variety of crystal defects were studied via SEM-EDS instrument and chemical etching method, such as cracks, parcels, stairs, and so on. The characterization results of the basic physical properties for TbeMoO12 crystals:the vickers hardness is 392.5VDH, the density is 8.7216g/cm3, the thermal expansion coefficient is 5.77×10-6K-1. The characterization results of crystalline powder magnetic susceptibility testes at 2-300K. low temperature indicated that TbeMoO12 crystal was typical paramagnetic materials with the Curie temperature of-13.6K and effective magnetic moment of terbium atom was 9.43μh.