Mechanical Properties, Quantum Mechanical Calculations, and Crystallographic/Spectroscopic Characterization of GaNbO4, Ga(Ta,Nb)O4, and GaTaO4

Single crystals as well as polycrystalline samples of GaNbO4, Ga­(Ta,Nb)­O4, and GaTaO4 were grown from the melt and by solid-state reactions, respectively, at various temperatures between 1698 and 1983 K. The chemical composition of the crystals was confirmed by wavelength-dispersive electron microprobe analysis, and the crystal structures were determined by single-crystal X-ray diffraction. In addition, a high-P–T synthesis of GaNbO4 was performed at a pressure of 2 GPa and a temperature of 1273 K. Raman spectroscopy of all compounds as well as Rietveld refinement analysis of the powder X-ray diffraction pattern of GaNbO4 were carried out to complement the structural investigations. Density functional theory (DFT) calculations enabled the assignment of the Raman bands to specific vibrational modes within the structure of GaNbO4. To determine the hardness (H) and elastic moduli (E) of the compounds, nanoindentation experiments have been performed with a Berkovich diamond indenter tip. Analyses of the load–displacement curves resulted in a high hardness of H = 11.9 ± 0.6 GPa and a reduced elastic modulus of Er = 202 ± 9 GPa for GaTaO4. GaNbO4 showed a lower hardness of H = 9.6 ± 0.5 GPa and a reduced elastic modulus of Er = 168 ± 5 GPa. Spectroscopic ellipsometry of the polished GaTa0.5Nb0.5O4 ceramic sample was employed for the determination of the optical constants n and k. GaTa0.5Nb0.5O4 exhibits a high average refractive index of nD = 2.20, at λ = 589 nm. Furthermore, in situ high-temperature powder X-ray diffraction experiments enabled the study of the thermal expansion tensors of GaTaO4 and GaNbO4, as well as the ability to relate them with structural features.

分别针对GaNbO₄、Ga(Ta,Nb)O₄及GaTaO₄，采用熔体法生长单晶样品、固相反应法制备多晶样品，两类样品的制备温度区间均为1698至1983 K。采用波长色散电子探针显微分析验证了单晶样品的化学组成，并通过单晶X射线衍射解析了其晶体结构。此外，本研究还在2 GPa压力与1273 K温度条件下，完成了GaNbO₄的高压高温合成。为补充结构表征工作，本研究对所有化合物开展了拉曼光谱测试，并针对GaNbO₄的粉末X射线衍射图谱完成了里特维尔德精修分析。借助密度泛函理论（Density Functional Theory，DFT）计算，将GaNbO₄的拉曼特征峰归属至其晶体结构内的特定振动模式。为测定各样品的硬度（H）与弹性模量（E），本研究采用贝科维奇金刚石压头开展了纳米压痕实验。通过对载荷-位移曲线的分析，测得GaTaO₄的硬度为H=11.9±0.6 GPa，约化弹性模量为Er=202±9 GPa；GaNbO₄的硬度相对更低，为H=9.6±0.5 GPa，约化弹性模量为Er=168±5 GPa。针对抛光后的GaTa0.5Nb0.5O4陶瓷样品，本研究采用光谱椭偏术测定了其光学常数n与k。GaTa0.5Nb0.5O4在589 nm波长处的D线平均折射率可达2.20。此外，通过原位高温粉末X射线衍射实验，本研究得以解析GaTaO₄与GaNbO₄的热膨胀张量，并将其与晶体结构特征建立关联。