Crystal structure of Ca(Fe0.4Si0.6)O2.8 oxygen-deficient perovskite

The crystal structure of Ca(Fe3+0.4Si0.6)O2.8 oxygen-deficient perovskite phase synthesized at 12 GPa and 1400 °C was studied using synchrotron powder X-ray diffraction. The phase is isostructural to low-pressure phase of Ca(Al0.4Si0.6)O2.8. The structure was refined by the Rietveld method and is consists of a perovskite-like triple-layer of corner-shared (Fe3+,Si)O6 octahedra and a double-layer of SiO4 tetrahedra those are stacked alternatively in the [111] direction of ideal cubic perovskite. Small degree of Fe3+/Si disorder was detected between two octahedral sites. The structure is compared with other oxygen-deficient perovskites

Crystal structure of Ca(Fe0.4Si0.6)O2.8 oxygen-deficient perovskite

The crystal structure of Ca(Fe3+0.4Si0.6)O2.8 oxygen-deficient perovskite phase synthesized at 12 GPa and 1400 °C was studied using synchrotron powder X-ray diffraction. The phase is isostructural to low-pressure phase of Ca(Al0.4Si0.6)O2.8. The structure was refined by the Rietveld method and is consists of a perovskite-like triple-layer of corner-shared (Fe3+,Si)O6 octahedra and a double-layer of SiO4 tetrahedra those are stacked alternatively in the [111] direction of ideal cubic perovskite. Small degree of Fe3+/Si disorder was detected between two octahedral sites. The structure is compared with other oxygen-deficient perovskites

Structural Characterization of Moganite-Type AlPO₄ by NMR and Powder X-ray Diffraction

Structural characterization of a new high-pressure AlPO₄ phase synthesized at 5 GPa and 1500 °C is reported. The phase is monoclinic (<i>P</i>2/<i>a</i>) with <i>a</i> = 8.7437(1) Å, <i>b</i> = 4.8584(1) Å, <i>c</i> = 10.8600(2) Å, β = 90.124(1)° (<i>Z</i> = 6). ³¹P MAS NMR and two-dimensional (2D) ²⁷Al triple-quantum (3Q) MAS NMR revealed that it contains two tetrahedral P sites of 1:2 abundance ratio, and two tetrahedral Al sites with 1:2 ratio. 2D ³¹P dipolar-recoupled double-quantum (DQ) MAS NMR and ²⁷Al → ³¹P dipolar-based (through-space) and J coupling-based (through-bond) 3Q-heteronuclear correlation (HETCOR) experiments provided direct information on the linkages of these sites. The crystal structure was solved and refined from synchrotron powder X-ray diffraction data utilizing the information from NMR. The phase is isostructural to moganite, a rare SiO₂ polymorph, and its structure can be derived from the latter via an ordered replacement of tetrahedral Si sites by Al and P. The NMR parameters of the phase were also calculated by first-principles method, which are consistent with those observed. Contrary to the other moganite phases known to date (i.e., SiO₂ and PON), moganite-AlPO₄ has a higher-pressure stability field than the corresponding quartz phase. This is the first moganite-type phase found in the ABX₄ system

Raman and NMR spectroscopic characterization of high-pressure K-cymrite (KAlSi<SUB>3</SUB>O<SUB>8</SUB><SUP>.</SUP>H<SUB>2</SUB>O) and its anhydrous form (kokchetavite)

International audienceTo facilitate identification of high-pressure K-cymrite (KAlSi3O8·H2O) phase and its anhydrous form (kokchetavite) in natural rocks, we have synthesized both phases and have characterized them by micro-Raman and NMR spectroscopy. K-cymrite was synthesized at 5 GPa and 800 °C. Kokchetavite was obtained by dehydrating K-cymrite at ambient pressure and 550 °C. The 1H MAS and 1H-29Si CP MAS NMR spectra of K-cymrite are consistent with the reported crystal structure that contains H2O molecules and has a disordered Si-Al distribution. The Raman spectra obtained under ambient conditions for K-cymrite (and kokchetavite) contain major peaks at 114.0 (109.1), 380.2 (390.0) and 832.5 (835.8) cm-1. For K-cymrite, OH stretching vibration is also observed at 3541 cm-1 with a shoulder at 3623 cm-1. The Raman spectrum for kokchetavite is consistent with that previously reported for a natural sample found as inclusions in clinopyroxenes and garnets in a garnet-pyroxene rock. However, the data for K-cymrite are inconsistent with the Raman features of a previously reported “relict K-cymrite in K-feldspar” from an eclogite. Pressure- and temperature-dependencies of the Raman shifts for the strongest peak of both phases are also reported