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

Six single crystals of Mg-3(MgxSixAl2-2x)Si3O12 With x = 0.05, 0.13, 0.24, 0.38, 0.52, and 0.64 (the majorite solid-solution) were synthesized at 20 G Pa and 2000 degrees C with a "6-8" type uniaxial split-sphere apparatus. Si ngle-crystal X-ray diffraction studies revealed discontinuities in composit ional dependence of the molar volume, equivalent isotropic temperature fact ors (B-eq), and mean bond lengths between x = 0.24 and 0.38. Single crystal s in the compositional range 0 less than or equal to x less than or equal t o 0.24 show no birefringence, whereas those of x = 0.64 have a slight optic al anisotropy. Moreover, the cell symmetry for x = 0.64 obtained using sync hrotron X-ray radiation is tetragonal with a slight deviation from cubic. O n the basis of site splitting expected from compositional dependence of B-e q obtained by cubic refinement, the most probable space group in the range 0.38 less than or equal to x less than or equal to 0.64 is I4(1)/acd (tetra gonal), which is the maximal subgroup of the space group Ia (3) over bar d (cubic). Given that the previous reports that crystals with 0.8 less than o r equal to x less than or equal to 1.0 have the tetragonal space group I4(1 )/a, the majorite solid-solution in this system undergoes the series of sym metry changes, Ia (3) over bar d --> I4(1)/acd --> I4(1)/a, with increasing MgSiO3 component. The symmetry changes from Ia (3) over bar d to I4(1)/acd cannot be explained by the cation ordering on the octahedral site. Strong electrostatic interaction between the dodecahedral (Mg2+) and tetrahedral ( Si4+) cations was observed from atomic thermal motion and electron density distribution. Because one of the site symmetries of the two nonequivalent t etrahedral sites in I4(1)/acd structure loses the center of symmetry with t he symmetry reduction from Ia (3) over bar d to I4(1)/acd, the symmetry red uction may be caused by the electronic polarization of the cations due to t he neighboring cation-cation interaction.