Modulation of quartz-like GeO2 structure by Si substitution: an X-ray diffraction study of Ge1-xSixO2 (0 ≤ x < 0.2) flux-grown single crystals

The spontaneous nucleation by the high-temperature flux method of GeO2 and SiO2-substituted GeO2 (Ge1-xSixO2) compounds was improved to give single crystals free of hydroxy groups. The crystal structure and quality of these alpha-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 Ge1-xSixO2 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 alpha-quartz-type GeO2 structure was followed by the evolution of the intra-tetrahedral angle and other structural parameters. The TO4 (T = Si, Ge) distortion was found to be larger in alpha-GeO2 than in alpha-SiO2 and, as expected, the irregularity of the TO4 tetrahedra decreased linearly as the substitution of Si for Ge increased.