Crystal Growth,Structure And Characterization Of New Molybdate Crystals

The study of molybdate crystal began in the 1970s.Kaminskii et al.first reported the stimulated emission characteristics of molybdate crystal.With further research,molybdate crystals can be used as the laser gain medium,ferroelectric matrix material,nonlinear optical crystal,Raman laser crystal and excellent oxygen ion conductor.In recent years,our research group has devoted huge efforts to grow tellurium molybdate crystals and studied their properties.Several kinds of crystals with excellent properties were grown.To broaden the research of molybdate crystals with high thermal conductivity,the new research has explored them,in order to grow molybdate crystals with high thermal conductivity,active metallic element was added in molybdate.Lithium is the lightest metal element with strong ionicity and high energy density,and is widely used in mobile phones,computers,digital cameras and other fields.In metallurgical industry,Li can form alloy which have small density and low melting point with other metals.To grow new molybdate crystals with higher thermal conductivity,adding lithium into the molybdate crystal is a good option.Tank et al.made the initial exploration in LiNbMo06(LNM)powder and find its abnormal dielectric properties in high temperature(300℃～600 0C).In generally,oxygen vacancy leads to the dielectric in molybdate.However,there is not any research on the physical properties of LNM crystal,especially the dielectric constant value.The cause for oxygen vacancies in crystal is not clear.In this thesis,LNM crystal was grown,and its optical and electrical properties were investigated in details.On the other hand,both vanadate and molybdate crystals were used as excellent Raman laser crystals.Vanadate crystals,such as YV04、GdVO4、LuVO4,are novel Raman crystals with high Raman gain coefficients,good thermal properties and wide transmission bands because of[VO4]group.Among the molybdate crystals.BaMoO4、SrMoO4、PbMoO4 with nice Raman gain coefficients are potential Raman crystals due to the[M0O4]group.The new compounds,containing[VO4]and[MoO4],are probable have high thermal conductivity and excellent Raman properties.The single crystals of LiVMoO6 were obtained for the first time.The crystal structure and physical properties were investigated in details.In this thesis,crystal growth,structure and physical properties of some new molybdates were carried out.The main contents and conclusions are as follows:1.The definition and characterizations of crystals are summarized.According to the different function,the molybdate crystals are classified.Additionally,the structure,properties and application areas of typical molybdate crystals are list and summarized.A simple introduction is given to the variety of crystal growth methods,and the ideal flux system was summarized detailly.2.Synthesis,bulk growth,and characterization of LiNbMoO6(LNM)Polycrystalline I.NM is synthesized by traditional solid state reaction and the range of optimum reaction temperature is 600 ℃～650℃.The millimeter sized*single crystal was grown by the flux method.Single-crystal XRD was used to resolve and refine the structure of crystal.The SHG measurements using 1064 nm radiation indicated that LNM shows a weak SHC response of about 1.2 × KDP and is obvious type I phase-matchable.The results of SHG measurements prove that its structure is acentric.The thermal,optical and electronic properties of LNM were investigated.The crystal LNM decomposed at 647.82 ℃,and the residue indicated it was incongruent melling compound.The LNM crystal exhibits a transmission window ranging from 479 nm to 5200 nm.The transmission of LNM is up to 75%in the Uv-Nir-mid-IR without coating.At a frequency of 1KHz,the results of dielectric measurement were 157 to 10000 in the range of temperature from 25 ℃ to 500 ℃.At room temperature,the intrinsic dielectric of LNM is 157.The crystal structure and mixed occupancy of positive ions are studied intensively at different high tcmperature by in-situ XRD and the crystal structure analysis.The crystal expands as the temperature increases and contracts as the temperature decreases.The results show the possible reason for appearance of oxygen vacancies in the crystal structure is the change of the ratio in mixed occupancy of positive ions.According to the analyses of crystal structure,in LiNbMoO6 the M2 site was occupied by mixed Nb5+ and Mo6+,also with the constrain that the sum of probable occupancy is 1000%.With consideration of respecting the valence equilibrium,such mixed-site occupancy in M2 is beneficial to emerging the oxygen vacancy.The conductivity of the crystal was investigated,and it is up to 1.01 ×10-3 S·cm-1 at 733K.In addition,the measurecd results indicated that oxygen vacancies and Li+both induce high conductivity of LNM.At high temperature,the Warburg finite diffusion in dominant in LNM is confirmed by the complex impedance spectroscopy and the Kinetic conduction of internal electron.The results indicated LNM crystal is an excellent oxide ionic conductor and the potential material for Solid Oxide Fuel Cell.LNM with crystal form,using as oxygen ion conductor,provide a new point of view for the development of SOFC.3.Bulk growth and characterizations of LiVMoO6Polycrystalline LiVMo06 was prepared using the conventional high temperature solid-state reaction method.The LiV03-MoO3 and LiVO3-MoO3-Li2O3 systems were investigated.The effects of growth parameters on the crystal quality were studied.Its results show that the quality of crystal with cooling rate 0.2 ℃/d is significantly better than the one with cooling rate 0.5 ℃/d.The crystals exhibit higher quality using vertical[001]and b oriented seed than that of the crystal grow in horizontal[001]directions.Comparing the quality of the crystal grown in the two different flux ratio by studying viscosity,the quality of the grown crystal with LiVO3:MoO3:Li2C03=1:2:0.2 was significantly higher than the other crystal grown with LiVO3:MoO3=1:2.Single crystals of LiVMoO6 were obtained for the first time.The crystalline quality of the as-grown LiVMoO6 crystal was checked by HRXRD.The FWHM of the rocking curve of the(020)diffraction plane is 47.22 The crystalline quality is good enough to measure physical properties.The physical properties of crystal,including thermal properties and spontaneous Raman spectra,were studied.The thermal expansion coefficient of LiVMoO6 was measured to be αa*= 3.4x 10-8 K-1,α101*= 9.388×10-8 K-1,αb ＝1.433×10-8 K-1,and ac*= 6.5 × 10-8 K-1.The specific heat increased with the increasing of temperature and was measured to be 0.849 g-1·K-1 at 300 ℃.The thermal conductivity is ka*=1.33 W/(m·K)and kb=3.53 W/(m K)at 25 ℃.The conductivity slightly decreases with the raising of temperature.The spontaneous Raman spectra of LiVMoO6 were investigated and analyzed.The strongest Raman shifts are around 963 cm-1 and 830.5 cm-1.The results indicated LiVMo06 crystal was a potential Raman crystal.4.The growth of new molybdate crystal form by hydrothermal methodThe new molybdate crystal forms were studied by hydrothemal method.The new crystal form of CS2Mo4O13 was obtained and compared with the existing crystal form reported in literature.The new crystal crystallizes in triclinic system,space group of P-1.The other new crystal was BaTe2Mo2O10F2 The crystal crystalizes in the orthorhombic system with space group Cmca.5.Bulk growth of MnTeMoO6The effects of growth conditions such as concentration,seed orientation,the rateof cooling and temperature field were studied in details.The experimental results show that the quality of crystal with cooling rate 0.1 ℃C/d and long constanttemperature zone is significantly good.The crystal grown at 15 rmp at initial stage and changed to 30 rmp at late stage was high quality.Centimetre-sized single crystals were grown using c-axis seeds.