High-pressure phase transformation in LiFeGe2O6 pyroxene

A synthetic pyroxene with composition LiFeGe2O6 and space group P2(1)/c at ambient conditions was investigated by single-crystal X-ray diffraction using a diamond anvil cell. The unit-cell parameters and crystal structure were determined at eight different pressures up to 8.7 GPa. Between 4.16 and 4.83 GPa, the sample shows a strongly first-order phase transition as indicated by a drastic drop in a, c, P, and unit-cell volume. The transition is marked by the disappearance of b-type reflections (h + k = odd) forbidden in a C-centered lattice. The volume bulk modulus of the P2(1)/c phase is estimated to be I 10 GPa as compared to 147 GPa of the C2/c one. The crystal structure evolution as a function of pressure is mainly influenced by the kinking of tetrahedral chains; the A and B non-symmetry equivalent chains of the P2(1)/c phase undergo strong deformations LIP to 4.16 GPa (A chain similar to 2%, B chain similar to 5.3%). At the transition, the two chains become symmetry equivalent and the single tetrahedral chain of the C2/c phase shows only minor deformations with pressure (similar to 1.9%) due to its already strong kinking (similar to 130 degrees). Such behavior is the main reason for the strong difference in compressibility between the low- and high-symmetry forms.