Transparent semiconducting oxides are materials suitable for a wide variety of optoelectronic applications, such as UV-LEDs, Schottky diodes, high voltage transistors and transparent thin film transistors. Spinel structure MgGa2O4 was recently shown to have an appropriate carrier concentration and bandgap for such applications and it can be grown using various techniques from the melt as relatively large crystals. Furthermore, it has a high chemo-physical stability and its conductivity can be controlled by adjusting the atmosphere during the growth process, making it an excellent candidate for future applications. However, spinel-like materials often exhibit anomalies in their physical properties at higher temperatures due to changes of their cation ordering (normal/inverse spinel). In fact, both heat capacity and thermal diffusivity of MgGa2O4 were shown to behave anomalously as a function of temperature.
We studied this issue by direct investigation of the cation ordering of annealed samples using single crystal X-ray diffraction. Additionally, we used inductive gauge dilatometry and resonant ultrasound spectroscopy to determine thermal expansion and the complete set of elastic constants, respectively, from 103 K to 1673 K to assess correlations between the changing structure and properties.
Thermal expansion and most stiffness coefficients show a discontinuity in their temperature dependence at about 820 K. Furthermore, the cation disorder is independent of temperature up to about 820 K and increases gradually at higher temperatures. Thus, the cation disorder in MgGa2O4 is most likely in equilibrium at high temperatures and undergoes a transition to a nonequilibrium state below 820 K, where the disorder cannot relax in laboratory timescales; a glass-like transition. This transition is likely also related to the anomalies in heat capacity and thermal diffusivity. The bond-valence model can qualitatively explain the relation between changing cation order and thermoelastic properties.References: