Growth of Piezoelectric Water-Free GeO₂ and SiO₂-Substituted GeO₂ Single-Crystals

Abstract

Using the slow-cooling method in selected fluxes, we have grown spontaneously nucleated single-crystals of pure GeO(2) and SiO(2)-substituted GeO(2) materials with the α-quartz structure. These piezoelectric materials were obtained in millimeter size as well-faceted, visually colorless, and transparent crystals. Cubic-like or hexagonal prism-like morphology was identified depending on the chemical composition of the single-crystals and on the nature of the flux. Both the silicon substitution rate and the homogeneity of its distribution were estimated by Energy Dispersive X-ray spectroscopy. The cell parameters of the flux-grown GeO(2) and Ge(1-x)Si(x)O(2) (0.038 ≤ x ≤ 0.089) solid-solution were deduced from their X-ray powder diffraction pattern. As expected, the cell volumes decrease as the silicon content substitution increases. A room temperature Infrared spectroscopy study confirms the absence of hydroxyl groups in the as-grown crystals. Unlike what was observed for hydrothermally grown GeO(2) crystals, these flux-grown oxide materials did not present any phase transition before melting as pointed out by a Differential Scanning Calorimetry study. Neither a α-quartz/β-quartz transition as encountered in SiO(2) near 573 °C nor a α-quartz to rutile transformation were detected for these GeO(2) and Ge(1-x)Si(x)O(2) single-crystals.

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

“…As for hydrothermally-grown α-GaPO 4 single crystals, OH contamination of the lattice was reported for hydrothermally-grown α-GeO 2 [43,79,80,94]. In a non-polarized infrared spectrum collected in the transmission mode of a hydrothermally-grown α-GeO 2 material containing significant OH-groups, Figure 2, a well-pronounced broad band between 2500 and 3800 cm −1 is observed with maxima at about 3455, 3500 and 3562 cm −1 attributed to Ge-OH vibration groups [43,79,81,94].…”

“…The α-GeO 2 millimeter-size crystals, up to 3 mm, presented either an unshaped morphology or a pseudo-cubic shape as already found for natural α-quartz [95]. In the case of pseudo-cubic crystals, the positive r rhombohedral faces were prevalent with the restricted presence of the negative z rhombohedral faces while m faces were absent [93,94]. The crystal structure and quality of these flux-grown α-GeO 2 materials were studied by both powder and single-crystal X-ray diffraction at room temperature [16,93,94].…”

“…Recently, unseeded slow cooling growth experiments were performed from 970 to 600 °C at a cooling rate of 1 °C · h −1 with a solute to flux ratio of 10/90 by weight in a Pt covered crucible (not sealed) [93,94]. Using K 2 W 2 O 7 flux, the majority of the as-grown α-GeO 2 crystals contained yellow flux inclusions.…”

“…Using K 2 W 2 O 7 flux, the majority of the as-grown α-GeO 2 crystals contained yellow flux inclusions. Thus, other inorganic solvents were investigated in the MoO 3 -based systems since they present low melting temperature and low evaporation at high temperatures (loss of mass <1% after two weeks at T > 700 °C for K 2 Mo 4 O 13 flux) [93,94] [16,93,94]. The α-GeO 2 millimeter-size crystals, up to 3 mm, presented either an unshaped morphology or a pseudo-cubic shape as already found for natural α-quartz [95].…”

“…In the case of pseudo-cubic crystals, the positive r rhombohedral faces were prevalent with the restricted presence of the negative z rhombohedral faces while m faces were absent [93,94]. The crystal structure and quality of these flux-grown α-GeO 2 materials were studied by both powder and single-crystal X-ray diffraction at room temperature [16,93,94]. The refinements confirmed the crystallization of the α-quartz-type structure and the lattice parameters and volumes were in good agreement with the literature data [4,10,11,32,80].…”