Vibrational Origin of the Thermal Stability in the Highly Distorted α-Quartz-Type Material GeO₂: An Experimental and Theoretical Study

Abstract

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.

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

“…In contrast with what has been observed for other α-quartz homeotypes like SiO 2 or AlPO 4 , which undergo an α-quartz to β-quartz phase transition [19,28], neither phase transitions nor a second phase were detected by this Raman study from room temperature to 1100 °C. First-principle calculations with the ABINIT code [93,106,107] revealed the absence of the tetrahedral libration mode in the α-quartz-like structure of GeO 2 which explained the very low degree of thermally-induced dynamic disorder registered in the 30-1100 °C range and further confirmed that the piezoelectric properties of flux-grown α-GeO 2 should not be degraded significantly up to its melting point (1116 °C).…”

“…Room temperature non-polarized Raman spectra of commercial α-GeO 2 powder or α-GeO 2 stain-etch sample exhibited some features around 760-780 cm −1 which were found to disappear once the sample was annealed in air above 400 °C [39,93,[104][105][106]. These features have been assigned either to oxygen vacancy complexes or to water bound to Ge-O entities or to a Ge-O stretching vibration of a water-distorted GeO 4 entity [39,104,105].…”

“…These features have been assigned either to oxygen vacancy complexes or to water bound to Ge-O entities or to a Ge-O stretching vibration of a water-distorted GeO 4 entity [39,104,105]. The non-polarized Raman signal of a MoO 3 -based flux-grown α-quartz GeO 2 single crystal was collected in the 50-4000 cm −1 range and no other modes than the four non-degenerate A1 ones and the eight doubly-degenerate E modes predicted by group theory for α-quartz-type GeO 2 (D 3 point group) were observed [93,[106][107][108]. In agreement with infrared measurements [27,93], neither hydroxyl groups nor water inclusions were detected by Raman spectroscopy.…”

“…Furthermore, these MoO 3 -based flux-grown α-GeO 2 samples were heated up to 1000 °C several times without presenting the well-known milky hue attributed to the release of water from hydroxyl impurities with increasing temperature [109]. Ambient polarized Raman measurements performed on flux-grown α-quartz GeO 2 single crystals along with state of the art density functional theory (DFT) based calculations were reported [93,106,107]. An excellent agreement was obtained between experimental and theoretical Raman lines for both wavenumbers and relative intensities which permitted to unambiguously assign the symmetry and the nature of α-quartz GeO 2 modes.…”

“…Variable-temperature Raman spectroscopy measurements performed on high temperature flux-grown α-quartz GeO 2 single crystals in MoO 3 -based solvents were reported [93,106]. Vibrations in α-GeO 2 were shown to be very slightly anharmonic as evidenced by the very low wavenumber shifts and the weak damping of the modes between room temperature and 1100 °C.…”