Symmetry change of majorite solid solution in the system Mg3Al2Si3O12-MgSiO3

Akihiko Nakatsuka, Akira Yoshiasa, Yamanaka Takamitsu, Ohtaka Osamu, Katsura Tomoo, Eiji Ito

doi:10.2138/am-1999-7-816

Six single crystals of Mg 3 (Mg x Si x Al (sub 2-2x) )Si 3 O 12 with x = 0.05, 0.13, 0.24, 0.38, 0.52, and 0.64 (the majorite solid-solution) were synthesized at 20 GPa and 2000 degrees C with a "6-8" type uniaxial split-sphere apparatus. Single-crystal X-ray diffraction studies revealed discontinuities in compositional dependence of the molar volume, equivalent isotropic temperature factors (Be q ), and mean bond lengths between x = 0.24 and 0.38. Single crystals in the compositional range 0 I4 1 /acd --> I4 1 /a, with increasing MgSiO 3 component. The symmetry changes from Ia3d to I4 1 /acd cannot be explained by the cation ordering on the octahedral site. Strong electrostatic interaction between the dodecahedral (Mg (super 2+) ) and tetrahedral (Si (super 4+) ) cations was observed from atomic thermal motion and electron density distribution. Because one of the site symmetries of the two nonequivalent tetrahedral sites in I4 1 /acd structure loses the center of symmetry with the symmetry reduction from Ia3d to I4 1 /acd, the symmetry reduction may be caused by the electronic polarization of the cations due to the neighboring cation-cation interaction.

Symmetry change of majorite solid solution in the system Mg<sub>3</sub>Al<sub>2</sub>Si<sub>3</sub>O<sub>12</sub>-MgSiO<sub>3</sub>

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