Modulation of quartz-like GeO₂structure by Si substitution: an X-ray diffraction study of Ge_1−xSi_xO₂(0 ≤x< 0.2) flux-grown single crystals

Abstract

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.

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

“…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]. The excellent final reliability factors of the refinements indicated that the as-grown α-GeO 2 crystals were of high quality precluding the presence of any considerable amount of twinning [16]. The value of the Flack coefficient which is an indicator of the presence of growth portions containing mirror-image was found close to zero.…”

“…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].…”

“…In the XO 2 (X = Si, Ge) and MPO 4 (M = Fe, Al, Ga, B) family, the α-quartz-like structure is composed of either only XO 4 corner-shared tetrahedra or of both MO 4 and PO 4 tetrahedra forming a trigonal system [3][4][5][6][7][8][9][10][11][12][13][14][15][16]. The α-phase (P3 1 21 or P3 2 21, respectively left-handed or right-handed, Z = 3) is derived from the β-phase (P6 2 22 or P6 4 22) by a symmetry loss induced by a tilt of the tetrahedra around the b-axis.…”

“…Under ambient conditions, GeO 2 exhibits two persistent forms of differing anion coordination around the central cation: the α-quartz-type modification (trigonal) with 4:2 coordination [16] and the rutile-type modification (tetragonal P4 2 /mnm, Z = 2) with 6:3 coordination [4,11,[30][31][32][33][34]. Naturally occurring, GeO 2 is known to be more stable in the rutile structure than in the α-quartz structure.…”