Synthesis And Structural Characterization Of Zn₂SiO₄Polymorphs At High-Pressure High-Temperature

We synthesized a series of Zn2SiO4polymorphs using a5000-ton Kawai-type double-stage uniaxial split-sphere multi-anvil apparatus installed at the Institute for Study of the Earth’s Interior (ISEI) of Okayama University, Japan. The starting materials for phase Ⅰ (willemite) are regent-grade ZnO and SiO2in a molar ratio of2:1. It is used as starting material for other high pressure polymorphs (phase Ⅱ, Ⅲ and IV).Phase Ⅰ, willemite was obtained by mixture of regent-grade ZnO and SiO2in a molar ratio of2:1at ambient condition. Phase Ⅱ, Ⅲ and IV was synthesized at6GPa and1473K,6.5GPa and1273K and8GPa and1273K, respectively. We solved the crystal structures determined under ambient condition for phase Ⅲ, and phase IV. After that, the four polymorphs of Zn2SiO4have been characterized by solid-state29Si Nuclear Magnetic Resonance spectroscopy (NMR), scattering Raman technique, and Electron Micro-probe Analysis (EMPA).(1) The crystal structures were solved using an ab initio structure determination technique from synchrotron powder X-ray diffraction data utilizing local structural information from29Si MAS NMR as constraints, and were further refined with the Rietveld technique using RIETAN-FP program.i. Phase Ⅲ is orthorhombic (Pnma) with a=10.2897(4), b=6.6718(3), c=5.0693(2) A. It is isostructural with the high-temperature (Zn1.1Li0.6Si0.3)SiO4phase, 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.ii. Phase Ⅳ is orthorhombic (Pbca) with a=6.1133(3), b=10.9091(5), c=9.6645(4) A. It also consists of tetrahedrally coordinated Zn and Si, and features unique edge-shared Zn2O6dimers. The volumes per formula of phase Ⅲ and Ⅳ are both somewhat larger than that of the lower-pressure polymorph, phase Ⅱ under ambient condition, suggesting that the two phases may have undergone structural changes during temperature quench and/or pressure release.(2) We compare their29Si MAS NMR spectroscopic characteristics with those of other Zn2SiO4polymorphs (phases Ⅰ, Ⅱ and Ⅴ). The29Si chemical shifts of phase Ⅱ and Ⅳ are very close (within0.5ppm) which is corresponding with their similarity on topology after our solution of the crystal structure of phase Ⅳ.(3) Electron microprobe point-wise and mapping analysis revealed that both of phases Ⅲ and Ⅳ are stoichiometric like the lower-pressure polymorphs (phases Ⅰ and Ⅱ), contrary to previous report. In our study, careful EMPA mapping and point-wise analyses revealed that there were no extra ZnO phase in samples#1to#5and that one of the phases Ⅰ to Ⅳ was the only or the predominant constituent phase in each sample, which indicate it is more likely that the apparent non-stoichiometry for phase Ⅲ and IV reported by Syono et al. was an artifact of analytical uncertainties. Both of them are stoichiometric, but not an exceptional mode in M2SiO4groups as previous study reported.(4) We also reported the Micro-Raman spectra for the powder of these four Zn2SiO4polymorphs. The olivine-related structure is also indicated by its Raman spectra which have peaks near862,885,920and942cm-1are correspond or closed to856,882,920,966cm-1which appear in forsterite’s Raman spectra. This similarity may interpret the fact of hexagonal closet packing of oxygen atoms in phase Ⅲ.(5) The phase "transformation" of Zn2SiO4is quite unique and so different from the other M2SiO4or Zn2GeO4which has similar formula with it, which is caused by the fact of the Zn2+ion’s strong tetrahedral preference. The discrepancy between the high pressure polymorphs of zinc silicates and germinates also suggests that the analogy between silicates and germinates should be applied carefully in discussing high pressure phase transformations. Some other factors influence and control the crystal structure type such as ionic radius ratio, electrostatic energy and crystal field should be considered.(6) In our study, careful EMPA mapping and point-wise analyses revealed that there were no extra ZnO phase in samples#1to#5and that one of the phases Ⅰ to Ⅳ was the only or the predominant constituent phase in each sample, which indicate it is more likely that the apparent non-stoichiometry for phase Ⅲ and Ⅳ reported by Syono et al. was an artifact of analytical uncertainties. Both of them are stoichiometric, but not an exceptional mode in M2Si04groups.(7) It is tempting to speculate that phase Ⅲ could have assumed an olivine-like structure under the pressure and temperature condition for its synthesis, and Zn cations could have migrated from octahedral sites to adjacent tetrahedral sites during recovery to ambient condition as a result of its strong preference for tetrahedral coordination. It is likely that phase IV could also have existed with a different structure under high pressure and temperature, and the Zn cations could have changed from octahedral to tetrahedral coordination accompanying structural changes during recovery to ambient condition. The structure of phase IV under high pressure and temperature could be either a new type unknown thus far, or related to known structures such as the thenardite (Na2SO4) structure. There is another possibility that phase IV has previously been suggested as another candidate for high-pressure Zn2SiO4polymorphs on the basis of packing density (7) arguments. Further in-situ high pressure and temperature measurements and first-principles calculations would be necessary and helpful in resolving these puzzles.(8) This study has demonstrated the usefulness of the combined application of ab initio structure determination from powder X-ray diffraction and solid-state NMR spectroscopy for the structural characterization of high pressure samples and structure determination. Solid-state NMR is a very useful tool for high pressure phase characterization. Therefore, the combination application of ab initio structure determination via X-ray powder diffraction and solid-state NMR is becoming a very powerful technique allowing the rapid solution of complex inorganic crystal structures, especially for the unknown high pressure inorganic phases.