Flux-Grown Piezoelectric Materials: Application to α-Quartz Analogues

Using the slow-cooling method in selected MoO3-based fluxes, single-crystals of GeO2 and GaPO4 materials with an a-quartz-like structure were grown at high temperatures (T >= 950 degrees C). These piezoelectric materials were obtained in millimeter-size as well-faceted, visually colorless and transparent crystals. Compared to crystals grown by hydrothermal methods, infrared and Raman measurements revealed flux-grown samples without significant hydroxyl group contamination and thermal analyses demonstrated a total reversibility of the alpha-quartz <-> beta-cristobalite phase transition for GaPO4 and an absence of phase transition before melting for alpha-GeO2. The elastic constants C-IJ (with I, J indices from 1 to 6) of these flux-grown piezoelectric crystals were experimentally determined at room and high temperatures. The ambient results for as-grown alpha-GaPO4 were in good agreement with those obtained from hydrothermally-grown samples and the two longitudinal elastic constants measured versus temperature up to 850 degrees C showed a monotonous evolution. The extraction of the ambient piezoelectric stress contribution e(11) from the C-11(D) to C-11(E) difference gave for the piezoelectric strain coefficient d(11) of flux-grown alpha-GeO2 crystal a value of 5.7(2) pC/N, which is more than twice that of alpha-quartz. As the alpha-quartz structure of GeO2 remained stable up to melting, a piezoelectric activity was observed up to 1000 degrees C.