High pressure single-crystal synthesis, structure and compressibility of the garnet Mn2+3Mn3+2[SiO4]3

Single crystals of the garnet Mn32+Mn23+[SiO4](3) and coesite were synthesised from MnO2-SiO2 oxide mixtures at 1000 degrees C and 9 GPa in a multianvil press. The crystal structure of the garnet [space group Ia3d, a=11.801(2) Angstrom] was refined at room temperature and 100 K from single-crystal X-ray data to R1=2.36% and R1=2.71%, respectively. In contrast to tetragonal Ca3Mn23+[GeO4](3) (space group I4(1)/a), the high-pressure garnet is cubic and does not display an ordered Jahn-Teller distortion of octahedral Mn3+. A disordered Jahn-Teller distortion either dynamic or static is evidenced by unusual high anisotropic displacement parameters. The room temperature structure is characterised by following bond lengths: Si-O=1.636(4) Angstrom (tetrahedron), Mn3+-O=1.995 (4) Angstrom (octahedron), Mn2+-O=2.280(5) and 2.409(4) Angstrom (dodecahedron). The cubic structure was preserved upon cooling to 100 K [a=11.788(2) Angstrom] and upon compressing up to 11.8 GPa in a diamond-anvil cell. Pressure variation of the unit cell parameter expressed by a third-order Birch-Murnaghan equation of state led to a bulk modulus K-0=151.6(8) GPa and its pressure derivatives K'=6.38(19). The peak positions of the Raman spectrum recorded for Mn32+Mn23+[SiO4](3) were assigned based on a calderite Mn32+Fe23+[SiO4](3) model extrapolated from andradite and grossular literature data.