Crystal structures of Zn2SiO4 III and IV synthesized at 6.5–8 GPa and 1,273 K

Abstract

We report the crystal structures determined under ambient condition for two Zn 2 SiO 4 polymorphs synthesized at 6.5 GPa and 1,273 K (phase III) and 8 GPa and 1,273 K (phase IV) and also compare their 29 Si MAS NMR spectroscopic characteristics with those of other Zn 2 SiO 4 polymorphs (phases I, II and V). Electron microprobe analysis revealed that both of phases III and IV are stoichiometric like the lower-pressure polymorphs (phases I and II), contrary to previous report. The crystal structures were solved using an ab initio structure determination technique from synchrotron powder X-ray diffraction data utilizing local structural information from 29 Si MAS NMR as constraints and were further refined with the Rietveld technique. Phase III is orthorhombic (Pnma) with a = 10.2897(5), b = 6.6711(3), c = 5.0691(2) Å . It is isostructural with the high-temperature (Zn 1.1 Li 0.6 Si 0.3 )-SiO 4 phase and may be regarded as a 'tetrahedral olivine' type that resembles the 'octahedral olivine' structure in the (approximately hexagonally close packed) oxygen arrangement and tetrahedral Si positions, but has Zn in tetrahedral, rather than octahedral coordination. Phase IV is orthorhombic (Pbca) with a = 10.9179(4), b = 9.6728(4), c = 6.1184(2) Å . It also consists of tetrahedrally coordinated Zn and Si and features unique edge-shared Zn 2 O 6 dimers. The volumes per formula under ambient condition for phases III and IV are both somewhat larger than that of the lower-pressure polymorph, phase II, suggesting that the two phases may have undergone structural changes during temperature quench and/or pressure release.

Crystal structures of Zn₂GeO₄ cubic/tetragonal spinel and Zn₂SiO₄ modified spinel phases

“…Partial occupancy of octahedral sites by Si is expected if Zn/Si disorder occurs. Previously reported 29 Si MAS NMR results with the same sample revealed no clear octahedral Si peak other than a minor coexisting ZnSiO 3 ilmenite phase (Liu et al, 2013), supporting the Rietveld result. However, a small degree of disorder cannot be ruled out for this system.…”

“…The cubic spinel phase was synthesized at 3 GPa and 1600°C for 1 h, whereas the tetragonal spinel phase was synthesized at 5 GPa and 1200°C for 1.7 h, following the studies of Ringwood and Major (1967) and Syono et al (1971). The modified spinel phase of Zn 2 SiO 4 used for this study is identical to that reported in our previous study using 29 Si MAS NMR spectroscopy (Liu et al, 2013). This sample was synthesized at 15 GPa and 1200°C for 1 h using the same press.…”

“…Instead, other high-pressure phases II-V have been reported . Phase I is willemite (phenacite structure), phase II has a dense crystal structure as reported by Marumo and Syono (1971), and the crystal structures of phases III and IV have recently been reported (Liu et al, 2013). In all these phases (I-IV), Zn 2+ occupies tetrahedral sites despite its relatively large ionic radius (0.74 Å; Shannon, 1976).…”

“…The measurement procedure was identical to that of our previous studies (Kanzaki and Xue, 2012;Liu et al, 2013). The wavelength of the Xray beam used was 0.500052 Å, and the exposure time was 10 min.…”

“…The crystal structures of these phases were refined with the Rietveld refinement technique using the 'RIETAN-FP' program (Izumi and Momma, 2007). For refinement of the modified spinel phase of Zn 2 SiO 4 , the ZnSiO 3 ilmenite phase, which was detected by 29 Si MAS NMR (Liu et al, 2013), was included. Quantitative phase analysis by the Rietveld method revealed that the sample contains 0.8 wt% ZnSiO 3 ilmenite.…”