Thermal expansion, mechanical and optical properties of gallium and aluminum substituted Zn2TiO4 spinels

Polycrystalline samples of new Zn_1.75Ti_0.75Ga_0.5O₄, Zn_1.5GaTi_0.5O₄, and Zn_1.75Ti_0.75Al_0.5O₄ spinel were prepared by solid-state reactions. Phase analysis and structural investigations were carried out by powder X-ray diffraction and Raman spectroscopy. In situ high-temperature powder X-ray diffraction experiments enabled the investigation of the thermal expansion of the spinel-type compounds. To determine the mechanical properties, nanoindentation experiments were performed with a Berkovich diamond indenter tip. Analyses of the load–displacement curves revealed a high hardness ranging from 10.2 ± 0.4 GPa to 11.8 ± 0.5 GPa and a reduced elastic modulus of 162 ± 5 GPa up to 178 ± 6 GPa. Furthermore, the optical properties of the spinel-type materials were studied by spectroscopic ellipsometry in the wavelength range of 250–1000 nm. All materials exhibit a high refractive index ranging from n = 2.20 to n = 2.10 at 430.8 nm (Fraunhofer G line) with a similar low dispersion (0.09 to 0.07).