Growth Exploration And Properties Of Molybdate And Borate Crystals

As one of the most important innovations of the twentieth century, laser has been widely used in the fields of science, industry, communication, medical treatment, military and amusement. However, the advent and applications of laser materials, especially the laser crystals, have been played a vital role in the development of laser technique. Nowadays, with the development and applications of laser, high level demands are put forward on the characters and performance of the laser crystals.Molybdate laser crystals were investigated during1970's, the research was focused on stimulated emission characteristics of LiRE(MoO4)2(RE=Y, La, Gd) crystals. As the thoughtful investigations and successfully applications for Y3Al5O12(YAG) crystals, the research interest was transferred to YAG from molybdate laser crystals. Along with the widely applications of laser diode pumped solid state lasers, re-evaluation of the molybdate crystals has garnered a tremendous amount of attention because of its highly disordered structure. Alkali metal-rare earth-double molybdate crystals with the formula AX(MoO4)2(A=Li, Na; X=Y, La, Gd) have been investigated as laser gain hosts. Up to now, there were few report on the laser experiment of AX(MoO4)2-Laser properties of Yb3+-doped NaLa(MoO4)2and LiGd(MoO4)2crystals have been reported. The laser output was302mW from Nd3+KLa(MoO4)2crystal pumped with a diode laser. However, there is no report on laser properties of Nd:LiGd(MoO4)2crystals. Compared to AX(MoO4)2crystals, AX2(MoO4)4(A=Ba; X=Gd), belonging to monoclinic crystal system, have been also attracted many attentions. The results suggested that the rare-earth doped AX2(MoO4)4crystals might be regarded as a potential free processing microchip laser material for diode laser pumping. Considering the advantage of molybdate crystals, growth, thermal properties, optical spectra and laser performace of LiNdxGd1-x(MoO4)2(x=0.005,0.01,1) and RE:BaGd2(MoO4)4(RE=Er, Nd, Pr) have been investigated in this work. In addition, considering the important applications of borate laser crystals, exploration of new crystal from the BaO-Bi2O3-B2O3system has also been investigated in this work. The research work of the thesis can be overviewed as follows:Ⅰ. Growth and physical properties characterizations of LiNdxGd1-x(MoO4)2(x=0.005,0.01,1) and RE:BaGd2(MoO4)4(RE=Er, Nd, Pr) crystalsLiNdxGd1-x(MoO4)2crystals with different Nd3+concentrations and RE: BaGd2(MoO4)4(RE=Er, Nd, Pr) crystals have been grown by the Czochralski method. The main factors that influence the crystal growth and quality have been discussed. The high quality seed is very important to improve the crystal quality. The synthesis of high quality polycrystalline material and design of reasonable temperature field are the precondition for the growth of single crystal. In addition, controlling appropriate parameters is the key to crystal growth.The structures of the as-grown crystals were determined by X-ray powder diffraction. The results confirm that the as-grown crystals LiNdxGd1-x(MoO4)2belong to the tetragonal crystal system and RE:BaGd2(MoO4)4(RE=Er, Nd, Pr) belong to monoclinic crystal system. The lattice constants of these crystals were calculated by the DICVOL programme. The lattice constants of LiNdxGd1-x(MoO4)2increase with the increasment of x. The density of the as-grown crystal was measured by the buoyancy method. The results of the Vickers microhardness measurement showed that the microhardness of a-cut wafer is larger than that of c-cut wafer. The Mohs' hardness value is on the order of5.0. The microdefects of Nd:LiGd(MoO4)2crystal were observed by chemical etching method. The result showed that the main defect of the crystal were dislocation and grain boundaries. For LiNdxGd1-x(MoO4)2crystals, the segregation coefficient of Nd3+increase with increasing doping concentration. Compared with Nd3+and Pr3+ions, Er3+ion is easier to be doped into the lattice of BaGd2(MoO4)4.Ⅱ. Thermal properties of LiNdxGd1-x(MoO4)2(x=0.005,0.01,1) and RE: BaGd2(MoO4)4(RE=Er, Nd, Pr) crystalsThe thermal properties of the LiNdxGd1-x(MoO4)2(x=0.005,0.01,1) single crystals were investigated systematically, including specific heat, thermal expansion, thermal diffusion coefficients and thermal conductivity. These properties on the function of temperature and Nd3+doped concentration were analysed. The influence of these properties on crystal growth and applications were also discussed.The specific heats of LiNdxGd1-x(MoO4)2(x=0.005,0.01,1) and RE: BaGd2(MoO4)4(RE=Er, Nd, Pr) crystals were measured by differential thermal scanning calorimeter. For LiNdxGd1-x(MoO4)2, the specific heat increases with increasing x value. At50℃, specific heats of x=0.005,0.01,1LiNdxGd1-x(MoO4)2are0.476,0.504,0.538J·g-1·K-1, respectively. The result shows that these crystals may have higher resistance to laser damage. At room temperature, the specific heats of Er3+, Nd3+, Pr3+doped BaGd2(MoO4)4crystal are:0.471,0.454,0.395J·g-1·K-1, respectively.The thermal expansions of LiNdxGd1-x(MoO4)2with x=0.005,0.01,1were measured by thermodilatometer in the temperature range of25-500℃. The values of thermal expansion along c axis are larger than that of a axis from the experimental results and this shows that the crystals exhibit an anisotropic thermal expansion. Difference of the thermal expansion coefficients between the two directions increases with the increasing temperature.The thermal diffusion coefficients of LiNdxGd1-x(MoO4)2with x=0.005,0.01,1in the temperature range of25-600℃were measured by the laser flash method. The values of thermal diffusion coefficients along c axis are larger than that of a axis, and the principal thermal diffusion coefficients of LiNdxGd1-x(MoO4)2with x=0.005,0.01and1at room temperature are λ1=0.504,0.426,0.117mm2·s-1and λ3=0.518,0.448,0.448mm2·s-1.The thermal conductivity of LiNdxGd1-x(MoO4)2was calculated. The results show that the thermal conductivity increases with the increasing temperature, which is attributed to the disordered structure. At room temperature, the thermal conductivity of LiNdxGd1-x(MoO4)2with x=0.005,0.01,1are κ1=1.123,1.134,0.314W·m-1·K-1and κ3=1.204,1.193,1.202W·m-1·K-1.Ⅲ. Optical properties of LiNdxGd1-x(MoO4)2(x=0.005,0.01,1) crystalsThe refractive index, polarization absorption spectrum and fluorescence spectrum of LiNdxGd1-x(MoO4)2crystals were measured. The refractive indice of LiNd(MoO4)2was measured by the minimum deviation method. The result showed that the refractive index of LiNd(MoO4)2is decreasing with the increasing of the wavelength. The smaller difference between the refractive indices values of no and ne indicated that the crystal is an optically uniaxial negative crystal.The polarized absorption spectra were measured for LiNdxGd1-x(MoO4)2in the wavelength range of400-1200nm. The result showed that the absorption peaks for σ-polarization had slightly red shif than that of π-polarization. Upon excitation of the LiNdxGd1-x(MoO4)2at806nm, the luminescence emission spectrum was measured. Based on these spectra and J-O theory, the optical spectral parameters of LiNdxGd1-x(MoO4)2have been calculated. The results show that the difference among the parameters of LiNdxGd1-x(MoO4)2(x=0.005,0.01) is insignificant. The strongest emission is located near1061nm and it is easy to obtain laser output. For LiNdxGd1-x(MoO4)2(x=0.005,0.01), the emission cross-sections at1061nm are18.76×10-20cm2and20.04×10-20cm2.The emission cross-section at1536nm for Er:BaGd2(MoO4)4is4.50×10-20cm2, which is advantageous for generation of laser operation. The emission cross-section at1060nm for Nd:BaGd2(MoO4)4is24.68×10-20cm2.The emission cross-sections at651nm for Pr:BaGd2(MoO4)4is1.793×10-18cm, which is beneficial for generation of red laser.Ⅳ. Laser performance of LiNdxGd1-x(MoO4)2(x=0.005,0.01,1) crystalsThe continuous-wave (CW) laser experiments of LiNdxGd1-x(MoO4)2crystals operating at1060nm were carried out by using a LD pump source centered at808nm. This is the first time for reporting the CW laser output of Nd:LiGd(MoO4)2crystals. It has been found that there is no laser output from LiNd(MoO4)2due to the concentration quenching effect of the crystal. For0.5at.%and1at.%doped Nd: LiGd(MoO4)2crystals, the laser output power of c-cut was lower than that of a-cut. And c-cut crystal had a higher pump threshold.A-cut sample with the size of3mmx3mmx10mm of1at.%doped Nd:LiGd(MoO4)2crystal had the best laser output power. Its pump threshold is0.276W, the maximum output power is920mW and the optical conversion efficiency is11.84%.Ⅴ. Exploration of new crystal from BaO-Bi2O3-B2O3system by the flux methodSelf-flux systems, such as B2O3, Bi2O3, BaO, for growing BaBiBO4were explored. The results showed that it is difficult to obtain BaBiBO4crystal from self-flux systems. Due to the density difference of B2O3and Bi2O3and the volatile of B2O3, it is difficult to obtain homogeneous melt. Using a Li2Mo3O10flux system, two different phase crystals, BaMoO4polycrystal and single crystal LiBaB9O15, were obtained by the spontaneous nucleation and top-seeded growth method.A large LiBaB9O15single crystal has been grown by top-seeded solution growth method using a Li2Mo3O10flux system. The structure of LiBaB9O15crystal was obtained by single crystal X-ray diffraction. The compound crystallized in the trigonal system. The crystal cell parameters are a=b=10.9679A, c=17.0457A, V=1775.79A3. The crystal quality was characterized by high-resolution X ray diffraction and chemical etching method. The results show that the crystal obtained is of excellent quality and the main defect is dislocation.The thermal expansion was measured by thermodilatometer in the temperature range of25～500℃. The results show that the crystal has an anisotropic thermal expansion. The material exhibits a positive thermal expansion along the a-axis, which is coupled with negative thermal expansion along the c-axis over the measured temperature range from25to500℃. The average thermal expansion coefficients are αa=6.56×10-6K-1, αc=-4.82×10-6K-1. The combination of the positive and negative thermal expansion of this crystal leads to a small net volume expansion. The thermal diffusion coefficient in the temperature range of25～600℃was measured by the laser flash method. The results show that thermal diffusion coefficients along c axis are much larger than that of a axis, and the principal thermal diffusion coefficients at room temperature are λ1=0.991mm2·s-1and λ2=2.977mm2·s-1. The thermal conductivity was calculated, and it was decreased with the increasing temperature. At room temperature, its principal thermal conductivities are:κ1=1.825W·m-1·K-1and κ2=5.132W·m-1·K-1.The transmittance spectrum was measured at room temperature using a Hitachi U-3500spectrophotometer and a NEXUS670FTIR Spectrophotometer over the ranges of120-2200nm and2200-3200nm. The deep-UV transparency cutoff wavelength of the as-grown crystal occurs at165nm. The transmittance over the wavelength range of165-200nm increases sharply from0to above90%and remains at about90%over the wavelength range of200-2200nm. The ordinary and extraordinary refractive indices n0and ne, of the LiBaB9O15crystal were measured at room temperature by the minimum deviation method at13different wavelengths from250to2400nm. LiBaB9O15is confirmed to be a negative uniaxial optical crystal.