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The garnet crystal structure is known for its high symmetry of both the whole atomic arrangement and of the individual cation sites and their coordination polyhedra. Recent calculations revealed that the garnet framework of tetrahedra and octahedra entirely lacks rigid-unit modes. This means that phase transitions in garnets cannot occur as a result of polyhedral rotation alone, but must involve polyhedral deformation as well. Because the electronic configuration of Mn³⁺ cations requires polyhedral distortion in order to stabilize its electronic structure as a result of the Jahn-Teller-effect, Mn³⁺ garnets are expected to display enhanced transition behaviour. We have studied the structure of tetragonal Ca₃Mn₂(GeO₄)₃ (=CMG), for which investigations of twinning and its microstructure have already been made (see Annual Report 1996), and the compressibility of cubic Mn²⁺₃Mn³⁺₂(SiO₄)₃ garnet.

Crystal structure studies of the CMG garnet have been performed independently at both varied pressure and temperature conditions by single-crystal X-ray diffraction. The crystal structure was found to be tetragonal (space group I4₁/a) between the lowest temperature of this study (-170°C) and 275°C. We found that the CMG single crystal undergoes a tetragonal-to-cubic phase transition at 275±5°C. As temperature decreases, the c/a ratio increases away from the value of 1.0 found in the cubic structure. A distinct change in the temperature dependence of the c axis was observed at temperatures below -100°C, at which point the c axis started to increase whereas the a axis continued to decrease with temperature. This is due to apparent partial dynamic disorder of the orientation of the direction of uniaxial elongation of the Mn³⁺O₆ octahedra at temperatures greater than ~ -100°C. This positional disorder, which may be static and statistical or dynamical in nature, increases with increasing temperature and is finally responsible for the phase transition at 275°C.

The effect of pressure on the CMG structure was found to reduce this apparent disorder as the uniaxial distortion of both polyhedra slightly increases with pressure. The compression of CMG (K_0,298= 133.0(6) GPa, K´ = 5.7(2)) is almost isotropic and the symmetry remains tetragonal over the investigated pressure range (0.0001 to 9.5 GPa). In contrast to CMG garnet, the Mn²⁺₃Mn³⁺₂(SiO₄)₃ garnet (K_0,298= 152.1(1.3), K´ = 6.3(4)) was found to remain cubic to the maximum pressure achieved (= 11.7 GPa) and not to undergo the expected pressure-induced cubic-to-tetragonal phase transition. This unusual stabilization of the Mn³⁺ in the high-symmetry environment (site symmetry:) might be due to the relative stiffness of the SiO₄ tetrahedron relative to the GeO₄ tetrahedron in CMG. Such is also suggested by the observation that the GeO₄ tetrahedra in CMG are more distorted as a result of the edge-sharing with the octahedra than are the SiO₄ tetrahedra in Mn²⁺₃Mn³⁺₂(SiO₄)₃ garnet.