| Crystal materials show the characteristics of periodic repetition and symmetry in structure,and have excellent physical properties.They can achieve the interaction and conversion among electricity,magnetism,acoustics,light and stress,and are indispensable and important materials in the development of modern science and technology.In the 1950s,the appearance and application of silicon(Si)and germanium(Ge)single crystals caused a revolution in microelectronics field.In the 1980s,the emergence of solid-state lasers such as ruby(Ti:Al2O3)and neodymium doped yttrium aluminum garnet(Nd:YAG,Nd:YAP)crystals has made great progress in laser technology.To this day,crystal materials as an important frontier play a dominant role in the development of materials science.It is closely related to the development of new technologies such as laser,infrared,electronics,aerospace and new energy development.With the development of modern science and technology,especially the development of optoelectronic information technology,natural crystals are far from being able to meet the demand in terms of quantity and quality,which puts forward higher requirements for the types and performance of crystal materials,thus greatly promoting the development of synthetic crystals.In recent years,niobate crystals,such as LiNbO3,KNbO3 and ReNbO4(La～Lu),have attracted extensive attention due to their excellent proton conductivity,electro-optic and nonlinear optical effects,which shows great application value in fuel cells,electro-optic modulator,multiplier of laser and photoluminescence.And studies found these crystals have excellent chemical stability and thermal properties,but it is difficult to grow high quality bulk single crystals.Therefore,it is great significance to explore new niobate crystals and their applications,and to improve their preparation process.In this paper,the growth of GaNbO4 single crystal by Czochralski method was studied for the first time.On the basis of obtaining the centimeter-level GaNbO4 single crystal,the physical properties were systematically studied and its possible application prospects were explored.The main research work and achievements of this paper are as follows:(1)In this paper,the Czochralski method is used for single crystal growth.According to the growth characteristics of GaNbO4 crystals,the platinum crucible was used for growth,and the synthesis of polycrystalline materials was designed.Combining the crystal growth kinetics and thermodynamic factors as well as the actual situation,the process parameters(rotation,pulling and cooling rate,etc.),temperature field structure and annealing process in the process of crystal growth were continuously adjusted.The causes of cracking phenomenon in the crystal were analyzed and improved.Finally,the large size GaNbO4 single crystal without macro defects was successfully grown.The results of quality characterization showed that the structure of GaNbO4 crystal was complete and the quality of crystal was excellent,(2)The structure of high quality GaNbO4 crystal was analyzed.GaNb04 crystal belongs to the monoclinic system,C2/m space group.The GaNbO4 crystal has a centrosymmetric structure and is composed of highly distorted GaO6 and NbO6 octahedra,which are connected by co-top and co-edge to form a zigzag chain along the[010]direction.Moreover,there is an infinite tunnel space along the b-axis in the structure,so GaNbO4 crystal doped with other elements is feasible theoretically,and the maximum gap radius is 1.34 A.(3)The basic physical properties of GaNbO4 crystal,including density,hardness and chemical stability,were characterized.The experimental density of GaNbO4 crystal is 5.064 g/cm3,which is close to the theoretical density of 5.055 g/cm3.The hardness of GaNbO4 crystal along(100)and(010)directions are 7.91 and 7.37,respectively,which are relatively high,but this do not affect the process of crystal.At room temperature,the GaNbO4 crystal is non-hygroscopic and has excellent acid resistance and chemical stability.(4)The X-ray photoelectron spectroscopy of GaNbO4 crystal has been studied.The results show that there are no impurity ions and mixed valence ions in GaNbO4 crystal,and the purity of crystal is very high.According to the structure of GaNbO4 crystal,the first-principles calculation shows that GaNbO4 is an indirect bandgap semiconductor with a large bandgap value of 3.48 eV.(5)The thermal properties of GaNbO4 crystal,including specific heat,thermal expansion,thermal diffusion and thermal conductivity,were characterized comprehensively.The specific heat increases with increasing of temperature,from 0.482 J·g-1·K-1 at room temperature to 0.687 J·g-1·K-1 at 300 ℃.The thermal expansion coefficient has obvious anisotropy,and it shows negative thermal expansion along the crystallographic b-axis,which is related to the shortest unit cell parameter b in the structure.The thermal expansion coefficients of the three main axes were obtained by measuring and calculating.The values were α1=6.040 × 10-6 K-1,α2=-2.444 × 10-6 K-1 and α3=9.498 × 10-6 K-1,respectively.The relationship between the crystallographic axis and the thermal expansion axis was determined,and the orientation of zero coefficient of thermal expansion was determined by space transformation.The thermal diffusion and thermal conductivity decrease with increasing of temperature.The thermal diffusion coefficient and thermal conductivity along the b-axis are the largest,which is attributed to the large voids in the b direction,and the small collision probability between phonons and the larger free path.At room temperature,the thermal conductivity in the b direction is 10.71 W/(m-K).The excellent thermal properties of GaNbO4 crystals make it have potential applications in laser host crystals and high-power electronic devices.(6)The uv-vis reflective spectrum,transmission spectrumlaser damage threshold and Raman spectrum of GaNbO4 crystal were systematically studied.The results show that GaNbO4 crystal has a wide transmission range(340～5200 nm),and the transmittance reaches 70%in the transmission range.The laser damage threshold of GaNbO4 crystal is 148 MW/cm2.The maximum Raman frequency shift peak of GaNbO4 crystal is 918 cm-1 with a high intensity,and its corresponding linewidth is about 9.0 cm-1,which indicates that this crystal is a potential Raman crystal material.Moreover,due to its excellent thermal properties,it will have a more stable output as a Raman crystal.(7)All the dielectric constants of GaNbO4 crystal are obtained by designing different cut types,and the values are ε11=15.59,ε22=19.20,ε33=13.34 andε13=15.64,respectively,which have small anisotropy.And GaNbO4 crystal has high resistivity and good resistance characteristics at high temperature.The Hall effect of GaNbO4 crystal indicates that there is electronic conductivity in the crystal.The results show that GaNbO4 crystal belongs to N-type conduction,and donor defects play a leading role in the crystal.(8)The dislocation morphology of GaNbO4 crystal with different faces was studied by chemical etching method.The dislocation morphology has obvious anisotropy,and which corresponds to its structural characteristics. |
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