Structure and twinning of tetragonal Ca3Mn2Ge3O12 garnet

Single crystals of tetragonal Ca3Mn2Ge3O12 garnet (space group symmetry I4( 1)/a, a = 12.3098(7) Angstrom, c = 12.3277(9) Angstrom) were characterized by X-ray diffraction and transmission electron microscopy. Their structure is topologically isosymmetric to tetragonal high-pressure garnets such as m ajorite, displaying the same two distinct types of macroscopic twin mechani sms. Twinning occurs as pseudomerohedral ferroelastic twin lamellac with pr eferred orientation of the twin-domain boundaries parallel to (101)(tet), w hereas merohedral ferrobielastic twin domains occur without any orientation al preference. The crystal structure was determined from single-crystal X-r ay diffraction data of a crystal Fragment completely free of pseudo-merohed ral twin domains. It shows two different Jahn-Teller distorted MnO6 octahed ra, with a different orientation of the axis of polyhedral elongation. The ordering scheme of those Jahn-Teller distorted octahedra follows in an alte rnating pattern the densest rod packing, and the cooperative effect of the electronically induced octahedral distortion was found to be responsible fo r the cubic-to-tetragonal symmetry breaking in Ca3Mn2Ge3O12 garnet. The ext ent of polyhedral distortion indicates a partially dynamic character of the Jahn-Teller effect. The distortion of the tetrahedral T2 sites controls th e cooperative effect of the lattice strain induced by the Jahn-Teller disto rtion of the Mn3+ octahedra and thus is responsible fur the overall lower s ymmetry.