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High-pressure phase transformation in LiFeGe₂O₆ pyroxene

¹ Dipartimento di Geoscienze, Università di Padova, Via Giotto 1, I-35135 Padova, Italy
² Department of Materials Engineering and Physics, Division of Mineralogy, University of Salzburg, Hellbrunnerstr, 34, 24 A-5020 Salzburg, Austria

Correspondence: ^* E-mail: fabrizio.nestola{at}unipd.it

A synthetic pyroxene with composition LiFeGe₂O₆ and space group P2₁/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, β, 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 P2₁/c phase is estimated to be 110 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 P2₁/c phase undergo strong deformations up to 4.16 GPa (A chain ~2%, B chain ~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 (~1.9%) due to its already strong kinking (~130°). Such behavior is the main reason for the strong difference in compressibility between the low- and high-symmetry forms.

Key Words: Single-crystal structure analysis • germanates • high-pressure phase transformations • X-ray diffraction