Rearrangement of the ultrasmooth surface of La$_3$Ga$_5$SiO$_{14}$ crystals at heating

The morphology and phase composition of the surface of La$_3$Ga$_5$SiO$_{14}$ (langasite) crystals at annealing in a temperature range 1000–1200$^\circ$C have been studied using electron and atomic force microscopy. It has been shown that trigonal lanthanum oxide (La$_2$O$_3$) crystals with sizes to 3–4 $\mu$m, as well as a microstructure with sizes to 50 $\mu$m with gallium excess, with the approximate composition of 15 mol % La$_2$O$_3$, 65 mol % Ga$_2$O$_3$, and 20 mol % SiO$_2$ are formed on the surface of langasite crystals annealed in air at temperatures above 1100$^\circ$C. Possible reasons for thermal destruction of the compound can be a significant rearrangement of the disordered crystal structure of langasite caused by the interaction with air oxygen and under the intense surface diffusion of atoms of the crystal, as well as the incongruent character of melting of the La$_3$Ga$_5$SiO$_{14}$ compound. The revealed thermal destruction of the surface of langasite crystals should be taken into account when using this material to fabricate piezoelectric elements for operation at high temperatures.