Growth And Structure Analysis Of Transparent Crystal AMoO₄ (A=Ca, Sr) With Scheelite Structure

The scheelite structure transparent crystals are an important class of chemical raw materials.It can be used to prepare alloys with superhard properties.It is also widely used in medicine,catalyst and aviation.The most remarkable characteristic of tungsten and molybdate in scheelite structure is that it has good luminescence.It only needs to irradiate X-rays,infrared rays,ultraviolet rays,etc.in the sample which is not doped with activator,and the sample can emit fluorescence.Thanks to scheelite,it has a special MO₄?M=W,Mo?tetrahedral structure,which makes the charge in the crystal easier to transfer,so it is easy to generate fluorescence under the excitation of electromagnetic waves.Therefore,the use of tungstate and molybdate as a raw material produces a low-cost,high-quality fluorescent material with good stability and simple process.In this paper,molybdate single crystal is the main research object,and the single crystal growth mechanism,crystal structure,luminescence and structural transformation under reducing atmosphere of molybdate are studied.Firstly,the powder polycrystals of CaMoO₄ and SrMoO₄ were prepared by high-temperature solid-phase method.Then,the CaMoO₄ and SrMoO₄ single crystals were grown by the cosolvent method with CaCl₂-2H₂O and SrCl₂-6H₂O as cosolvents.The crystal structure and lattice constant of the sample were determined by powder and single crystal X-ray diffraction?XRD,PANalytical X-ray Diffractometer X'pert3?,and the data obtained after the measurement were compared with CaMoO₄ by X'Pert Plus software according to the Rietveld method.The crystal structure of SrMoO₄ was refined.Using the space group I4₁/a of the scheelite structure,it can be seen from the refinement results that no meaningful vacancies are detected at any grid point,and the result of the refinement is very good,which indicates that we have obtained samples of CaMoO₄ and SrMoO₄ in scheelite structure.At the same time,the single crystal rocking curve shows that the full width at half maximum is very small,indicating that the quality of the single crystal is high.At the same time,based on the perovskite structure Mo-based oxide has a wide range of applied physical properties,we have tried to obtain a perovskite-structured SrMoO₃and CaMoO₃ single crystal by H₂/Ar?5%?reduction of CaMoO₄ and SrMoO₄.We attempted to reduce CaMoO₄ and SrMoO₄ by H₂/Ar?5%?gas mixture to obtain SrMoO₃and CaMoO₃ single crystals of perovskite structure.However,although both SrMoO₄and CaMoO₄ belong to the scheelite structure molybdate,the former can obtain the perovskite structure of SrMoO₃,while the latter decomposes into CaO and Mo in the H₂/Ar?5%?mixed gas.The reason for the fact that the two reactants of the same structure produce different products may be because the highly distorted MoO₄tetrahedron in CaMoO₄ hinders the structural transformation.By performing photoluminescence emission spectroscopy tests and analysis on SrMoO₄ and CaMoO₄,CaMoO₄ and SrMoO₄ showed green emission at 498 nm and470 nm,respectively.The shift in the peak position indicates that there is a difference in the degree of distortion of the structure between the MoO₄ groups in CaMoO₄ and SrMoO₄,and the distortion of the corresponding local crystal field further leads to the splitting of the energy level and provides the polarization dependence of the spectrum.Thus,it was confirmed that the MoO₄ tetrahedron in CaMoO₄ can produce a greater degree of distortion than SrMoO₄ and it will hinder its structural transition from I4_1/a to Pm-3m.