Crystal chemistry of the epitaxy of cristobalite (SiO2) on basal plane sapphire

Epitaxial thin films of α-cristobalite were synthesized on c-plane sapphire using the chemical solution deposition method. The films grow with (1 0 1) orientation at temperatures as low as 800 °C and crystallize only in the presence of Na, either by intentional doping or contamination. X-ray diffraction reveals an in-plane orientation of [1¯01]cris||[101¯0]sapp, with three cristobalite in-plane variants which correspond to the three-fold symmetry of the sapphire. The use of crystal chemistry concepts enabled the accurate prediction of the in-plane orientation between the two structures prior to their experimental determination. To that end, we propose that the interface consists of silicon-centered tetrahedrons that properly coordinate all neighboring cations and provide charge balance between the two structures.