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.

The spontaneous nucleation by the high-temperature flux method of GeO 2 and SiO 2 -substituted GeO 2 (Ge 1Àx Si x O 2 ) compounds was improved to give single crystals free of hydroxy groups. The crystal structure and quality of these -quartz-like piezoelectric materials were studied by single-crystal X-ray diffraction at room temperature. The refinements gave excellent final reliability factors, which are an indication of single crystals with a low level of defects. A good correlation was found between the silicon content in Ge 1Àx Si x O 2 crystals determined through extrapolation from the inter-tetrahedral bridging angle and that found from energy-dispersive X-ray spectroscopy. The effect of germanium replacement by silicon on the distortion of the -quartz-type GeO 2 structure was followed by the evolution of the intra-tetrahedral angle and other structural parameters. The TO 4 (T = Si, Ge) distortion was found to be larger in -GeO 2 than in -SiO 2 and, as expected, the irregularity of the TO 4 tetrahedra decreased linearly as the substitution of Si for Ge increased. † The record of GeO 2 data was optimized according to its Laue class (3m), which gave a higher number of unique reflections.

We calculated the lattice dynamics, the second-order nonlinear susceptibility, and the electro-optic response of the germanium dioxide in its αquartz-type form (α-GeO 2 ) from first-principles calculations based on density functional theory. No theoretical or experimental investigations of these nonlinear optical properties have been previously reported in the literature. The calculation of the infrared and Raman spectra of α-GeO 2 allowed us to assign its experimental phonon modes, contributing to the clarification of a long-standing debate in the literature. The second-order nonlinear susceptibility and the electro-optic coefficients of α-GeO 2 are predicted to be significantly higher than those reported for α-quartz. Thus, α-GeO 2 should be a promissing candidate for nonlinear optical applications when compared to α-quartz.

A high-transparency and large-size single crystal, up to 0.5 cm 3 , of the piezoelectric phase of GeO 2 was grown by the top seeded solution growth method from a high-temperature solution using K 2 Mo 4 O 13 as solvent. The obtained volume makes this flux-grown GeO 2 single crystal, with the metastable α-quartz like structure, the largest reported in the literature to our knowledge. Several oriented plates, X-, Y-, and Z-cut according to the dielectric frame, were obtained from the grown crystal, which exhibits a typical hexagonal morphology. The presence of hydroxyl groups as chemical impurities, known to damage the piezoelectric property, was not detected by infrared spectroscopy in transmission mode or Raman spectroscopy on the resulting oriented plates of α-GeO 2 . The effect of a prolonged annealing (up to five months) at high-temperature (800−900 °C) was followed by Raman spectroscopy: no structural evolution as well as no macroscopic modification of the transparency or the morphology of the α-GeO 2 single crystal were observed. These results were consistent with a high optical quality crystal as checked by UV−vis−NIR spectroscopy.

From high-precision Brillouin spectroscopy measurements, six elastic constants (C11, C33, C44, C66, C12, and C14) of a flux-grown GeO2 single crystal with the α-quartz-like structure are obtained in the 298-1273 K temperature range. High-temperature powder X-ray diffraction data is collected to determine the temperature dependence of the lattice parameters and the volume thermal expansion coefficients. The temperature dependence of the mass density, ρ, is evaluated and used to estimate the thermal dependence of its refractive indices (ordinary and extraordinary), according to the Lorentz-Lorenz equation. The extraction of the ambient piezoelectric stress contribution, e11, from the C'11-C11 difference gives, for the piezoelectric strain coefficient d11 , a value of 5.7(2) pC N(-1), which is more than twice that of α-quartz. As the quartz structure of α-GeO2 remains stable until melting, piezoelectric activity is observed until 1273 K.

We report an experimental and theoretical vibrational study of the high-performance piezoelectric GeO2 material. Polarized and variable-temperature Raman spectroscopic measurements on high-quality, water-free, flux-grown α-quartz GeO2 single crystals combined with state-of-the-art first-principles calculations allow the controversies on the mode symmetry assignment to be solved, the nature of the vibrations to be described in detail, and the origin of the high thermal stability of this material to be explained. The low-degree of dynamic disorder at high-temperature, which makes α-GeO2 one of the most promising piezoelectric materials for extreme temperature applications, is found to originate from the absence of a libration mode of the GeO4 tetrahedra.

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Using the slow-cooling method in selected MoO 3 -based fluxes, single-crystals of GeO 2 and GaPO 4 materials with an α-quartz-like structure were grown at high temperatures (T ≥ 950 °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 α-quartz ↔ β-cristobalite phase transition for GaPO 4 and an absence of phase transition before melting for α-GeO 2 . 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 α-GaPO 4 were in good agreement with those obtained from hydrothermally-grown samples and the two longitudinal elastic constants measured versus temperature up to 850 °C showed a monotonous evolution. The extraction of the ambient piezoelectric stress contribution e 11 from the C D 11 to C E 11 difference gave for the piezoelectric strain coefficient d 11 of flux-grown α-GeO 2 crystal a value of 5.7(2) pC/N, which is more than twice that of α-quartz. As the α-quartz structure of GeO 2 remained stable up to melting, a piezoelectric activity was observed up to 1000 °C.