Study On The Growth And Property Characterizations Of Novel Tungstate/Molybdate Functional Crystal Materials

As the unique structural characteristics of being periodic and symmetric,crystal materials can realize the interactions and transformations of different forms of energy,such as light,electricity,heat,sound,magnetism,etc.The multifunctionality and excellent stability make crystals the key materials in various fields,like laser,semiconductor,computer and atomic energy,and become more and more important in the industrial manufacture and the civil life.From another perspective,crystals are the minimized intrinsic energy state of materials.Study about crystals can reveal the intrinsic properties of matter and help understand the composition-strcuture-property relationships.Therefore,explorations of new functional crystals and having their intrinsic physical properties systematically characterized are still of great significance from both the engineering and theoretical point of view.Tungstate and molybdate crystals have long draw considerable research interest because of their excellent performance as laser host media,Raman gain crystals,second-order nonlinear optical crystals and piezoelectric crystals.At present,the research on tungstate and molybdate crystals as laser host media and Raman gain crystals is mainly focused on those with scheelite or related structures.When crystals with this kind of structures are employed as laser host media,they usually have the advantages like large absorption and emission bandwidth,being able to be pumed by the laser diode（LD）,and allowing polarized laser output while they exhibit high Raman gain and narrow linewidth when used as Raman crystals.Recent years,a kind of tungsten and molybdenum tellurites,such as β-BaTeMo2O9（β-BTM）,attracted intense attention of scientists for their superior application prospects as second-order nonlinear optical crystals and piezoelectric crystals.Besides,crystals in this family were also found to be excellent candidates as Raman gain crystals.Therefore,explorations for novel tungstate and molybdate functional crystals are still meaningful and of great significance.The family of bismuth tungstate or molybdate has various compositions and abundant structure types.Thus,crystals in this family may also be outstanding functional materials.Starting with the explorations,growth and characterizations of novel functional crystals,this thesis firstly reviewed the research on tungstates and molybdates as laser host media,second-order nonlinear optical and Raman crystals.Afterwards,taking the bismuth tungstate and molybdate as the research subject,the thesis systematically studied the synthesis,growth,structure and physical properties of scheelite-like mixed Bi₂Mo_2.66W_0.34O₁₂ crystals and the bismuth layered Aurivillius Bi2W2O9 crystals.Spontaneous Raman spectra under different configurations were also measured and analyzed for all the crystals and experiment of Raman laser output was carried out based on the Bi₂Mo_2.66W_0.34O₁₂ crystal.Additionally,the Nd-doped Bi₂Mo_2.66W_0.34O₁₂ single crystals were grown and the polarized absorption and fluorescence spectra of the crystal were measured and analyzed using the Judd-Ofelt（JO）theory.The continuous wave（CW）laser at 1064 nm was demonstrated using the Nd-doped Bi₂Mo_2.66W_0.34O₁₂ single crystals.Complex phase transitions of Bi2W2O9 were also explored and discussed by the first time.Besides,considering the promising application prospect of Na2TeW2O9（NTW）as second-order optical crystals and piezoelectric crystals,further research on NTW,including the optimizations of growth parameters and the flux system,was carried out.Large sized single crystals of NTW were successfully grown and a full set of electro-elastic properties were determined.The structure-property relationships were also analyzed and discussed.The main contents and conclusions of the thesis are as follows:Ⅰ.Growth,structure and physical properties of Bi₂Mo_2.66W_0.34O₁₂ crystals.The Bi₂O₃-MoO₃-WO₃ pseudo ternary system was systematically explored,and the proper ratio for the growth of Bi₂Mo_2.66W_0.34O₁₂ crystal was obtained.Bulk single crystal of Bi₂Mo_2.66W_0.34O₁₂ with dimensions up to 70 mm × 55 mm × 45 mm was grown by the top-seeded solution growth（TSSG）method.The rocking curve of the（040）diffraction plane has a sharp and symmetrical shape with the full width at half maximum（FWHM）of 54",which indicates a high-quality of the as-grown crystal.The optimized growth parameters are:c-oriented crystal seed,a cooling rate around 0.1 ℃/d and the crystal rotation speed of 32～18 r/min.Single-crystal X-ray diffraction data of Bi2Mo2.66Wo.34O12 was collected and the crystal structure was refined.Bi2Mo2.66Wo.34012 belongs to the monoclinic P21/c（No.14）space group and can be viewed as scheelite-like structure.Bi atoms occupy two unique lattice sites and are coordinated with eight 0 atoms while Mo or W atoms occupy three unique sites with different Mo:W ratios.Specifically,W occupies 12.3%,2.6%,and 19.5%of the Mo/W（1）,Mo/W（2）,and Mo/W（3）sites,respectively.The thermal and optical properties of Bi2Mo2.66Wo.34O12 crystals were investigated in details.The crystal melts incongruently at 682 ℃ and its specific heat values increase with the rising temperature.The three principal thermal expansion coefficients are determined to be α1 =-7.8（2）× 10-6 K-1,α2= 19.9（1）× 10-6 K-1,and α3 = 14.1（2）× 10-6 K-1,indicating a quite large anisotropy.The relationship between the crystallographic orientation and the conventional orientations was also calculated.The thermal conductivity has a limited change with the increasing temperature.The thermal conductivity along a,b,and c-axes is 1.50,0.53,and 0.94 W/（m·K）,respectively,at 300C.The transparency range of Bi₂Mo_2.66W_0.34O₁₂ crystal was determined to be 480～5000 nm.Refractive indices of the crystal were measured and the Sellmeier equations were obtained.Bi₂Mo_2.66W_0.34O₁₂ is optically positive biaxial crystal and its large refractive indices and birefringence（0.24～0.14 in the range of 480～1530 nm）indicate that Bi2Mo2.66W0.34012 crystals may also have potential applications as prisms.Ⅱ.Raman laser output based on Bi₂Mo_2.66W_0.34O₁₂ single crystals.The spontaneous Raman spectra along the three optical principal axes under different configurations were systematically measured and the strongest Raman shift located at around 901.8 cm-1.The Raman gain coefficient of Bi₂Mo_2.66W_0.34O₁₂ crystal when pumped at 1064 nm was calculated to be 1.1 times that of YVO4 by comparing the Raman peak at 901.8 cm-1 of Bi₂Mo_2.66W_0.34O₁₂ crystal with the peak at 890 cm-1 of YVO4 under the same experimental circumstances.The Raman laser output characteristics of Bi₂Mo_2.66W_0.34O₁₂ crystal were also studied.Based on the fundmental wavelength of 1064 nm,1st-order Stokes Raman laser at 1176 nm was realized.The maximum output power of 1.12 mW was obtained at the pump power of 16 mW,corresponding to an optical-to-optical conversion efficiency of 7.0%and slope efficiency of 15.6%.2nd-order Raman laser of 1315 nm was also detected under this configuration.Ⅲ.Growth,spectral analysis and laser properties of Nd:Bi₂Mo_2.66W_0.34O₁₂ crystals.Nd:Bi2MO2.66W0.34O12 crystal with dimensions up to 80 mm × 57 mm × 35 mm was grown using a c-oriented crystal seed.The concentration of Nd3+ Gas determined to be 3.7 at.%（1.54 × 1020 ions/cm3）by the X-ray Fluorescence（XRF）analysis and the doping of Nd3+ does not change the Mo/W ratio in the crystal.The polarized absorption and fluorescence spectra of the Nd:Bi2MO2.66W0.34O12 crystal were carefully investigated at room temperature in details.The FWHM of the absorption band around 808 nm was up to 19 nm,nearly three time larger than that of Nd:Bi2Mo3O12,which means the substitution of Mo by W fairly broadens the absorption bandwidth and makes the crystal quite favourable for the LD pumping.Meanwhile,based on JO theory,important spectroscopic parameters were calculated.The effective intensity parameters for the three polarizations are calculated to be Ω2 =8.83 × 10-20 cm2,Ω4 = 4.55 × 10-20 cm2,and Ω6 = 3.98 × 10-20 cm2.The radiative lifetime of the 4F3/2 multiplet was calculated to be 108 μs and the fluorescence lifetime was determined to be about 118 μs,which results in a high fluorescence quantum efficiency（close to unity）.In addition,a maximum output of 113 mW was achieved with the optical-to-optical conversion efficiency of 17.4%and the slope efficiency of 21.6%using a Y-cut sample in the CW laser experiment.Ⅳ.Growth,structure and physical properties of Bi2W2O9 crystals.Single crystals of Bi2W2O9 with dimensions up to 29 mm × 23 mm × 11 mm were grown with a[100]oriented seed using the mixture of Li2B4O7 and WO₃ as a flux（Bi2W2O9:Li2B4O7:WO₃ = 1:1:1）.The cooling rate was set between 0.3～1 0C/d.Contrary to the previously reported results,single crystal X-ray diffraction data reveal that Bi2W2O9 crystallizes in the centrosymmetric Pbcn（No.60）space group and exhits typical Aurivillius layered structure with the intergrowth of the fluorite-like Bi202 sheets and the perovskite-like W2O7 double octahedra layers.Rocking curves of the（800）,（020）,and（002）diffraction planes are sharp and symmetric with FWHMs of 86",47",and 54",respectively,indicating a high crystal quality.Bi2W2O9 melts incongruently at 882 ℃ and the thermal expansion coefficients in the 30～500 0C were measured to be αa=11.8 × 10-6 K-1,αb=8.0 × 10-6 K-1,αc = 4.8× 10-6 K-1.The specific heat at 30 ℃ was 0.311 J/（g·K）and changed little with the increasing temperature below 660 ℃.The thermal conductivity shows apparent anisotropy at room temperature as the conductivity along b,and c-axes is quite close while that along a-axis is obviously smaller.As the temperature rises,the thermal conductivity along b,and c-axes decreases while that along a-axis remains nearly unchanged.Bi2W2O9 has a bandgap of 2.48 eV and a broad transparency range of 410～5000 nm.The spontaneous Raman spectra under different configurations ware measured.Abundant Raman shift peaks were detected and the two highest peaks located at 800 cm-1 and 323 cm-1,respectively,indicating a promising prospect of Bi2W2O9 as a novel Ramarr crystal.A complete survey of the anisotropic and variable-temperature thermal and dielectric properties was carried out with the aim of gaining detailed insights into the polymorphism evolution process of Bi2W2O9.Anisotropic anomalies of dielectric permittivity were detected around 792 ℃ and 715 ℃.Similar anomalies were also observed on the specific heat curve.Therefore,it is reasonable to speculate that phase transitions occur at the corresponding temperatures.Weak anomalies of dielectric permittivity and losses were also observed between 250～500 ℃.By the analysis of in-situ powder X-ray diffractions,thermal expansion and the in-situ Raman and infrared spectra,and comparing with Bi2WO6 and Bi2MoO6,it is believed that the weak and broad anomalies between 250～500 0C have a relaxation origin rather than a phase transition.Besides,anomalies in dielectric permittivity and losses between 17～26 0C were also detected and discussed.Ⅴ.Optimized growth and electro-elastic properties of Na2TeW2O9 crystals.Large sized crystals of NTW with dimensions up to 75 mm × 35 mm × 25 mm were grown successfully using a ⊥（001）oriented seed from the flux system of NTW:TeO2:NaF:Na2WO4 = 1:0.2:0.05:0.05.A cooling rate of 0.01～0.025℃/d may help get a high-qaulity crystal.A complete set of room temperature dielectric,elastic,and piezoelectric constants of NTW was determined using resonance techniques and impedence analysis.The results show that NTW exhibits excellent piezoelectric properties（d15 =-23.93 pC/N,d24 =-20.34 pC/N）which are comparable to LiTaO₃ and β-BTM crystals,making the crystal a promising candidate for piezoelectric applications.Spatial orientation dependence of the piezoelectric constants was also analyzed systematically and a maximum of 30.16 pC/N was found by rotating the piezoelectric constant d15 by 69°around the Y axis.Besides,the distortion parameters and local dipole moments of the asymmetric building blocks（WO6 octahedra and TeO₃ polyhedra）in NTW are calculated according to the crystal structure information.The net dipole moment of the unit cell was also obtained.The relationships of structure-property are analyzed and discussed.