Mechanical Properties, Quantum Mechanical Calculations, and Crystallographic/Spectroscopic Characterization of GaNbO<sub>4</sub>, Ga(Ta,Nb)O<sub>4</sub>, and GaTaO<sub>4</sub>

Abstract

Single crystals as well as polycrystalline samples of GaNbO<sub>4</sub>, Ga­(Ta,Nb)­O<sub>4</sub>, and GaTaO<sub>4</sub> 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 GaNbO<sub>4</sub> 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 GaNbO<sub>4</sub> 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 GaNbO<sub>4</sub>. To determine the hardness (<i>H</i>) and elastic moduli (<i>E</i>) 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 <i>H</i> = 11.9 ± 0.6 GPa and a reduced elastic modulus of <i>E</i><sub>r</sub> = 202 ± 9 GPa for GaTaO<sub>4</sub>. GaNbO<sub>4</sub> showed a lower hardness of <i>H</i> = 9.6 ± 0.5 GPa and a reduced elastic modulus of <i>E</i><sub>r</sub> = 168 ± 5 GPa. Spectroscopic ellipsometry of the polished GaTa<sub>0.5</sub>Nb<sub>0.5</sub>O<sub>4</sub> ceramic sample was employed for the determination of the optical constants <i>n</i> and <i>k</i>. GaTa<sub>0.5</sub>Nb<sub>0.5</sub>O<sub>4</sub> exhibits a high average refractive index of <i>n</i><sub>D</sub> = 2.20, at λ = 589 nm. Furthermore, <i>in situ</i> high-temperature powder X-ray diffraction experiments enabled the study of the thermal expansion tensors of GaTaO<sub>4</sub> and GaNbO<sub>4</sub>, as well as the ability to relate them with structural features

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