Structural change associated with the incommensurate - normal phase transition in akermanite , Ca 2 MgSi 2 O 7 , at high pressure

The structural changes associated with the incommensurate (IC)-normal (N) phase transition in akermanite have been studied with high-pressure single-crystal X-ray diffraction up to 3.79 GPa. The IC phase, stable at room pressure, transforms to the N phase at ¥1.33 GPa. The structural transformation is marked by a small but discernable change in the slopes of all unitcell parameters as a function of pressure. It is reversible with an apparent hysteresis and is classified as a tricritical phase transition. The linear compressibility of the a and c axes are 0.00280 (10) and 0.00418 (6) GPa for the IC phase, and 0.00299 (11) and 0.00367 (8) GPa for the N phase, respectively. Weighted volume and pressure data, fitted to a second-order Birch-Murnaghan equation of state (K9;4.0), yield V05307.4 (1) Å and K05100 (3) GPa for the IC phase and V05307.6 (2) Å 3 and K0590 (2) GPa for the N phase. No significant discontinuities in Si – O, Mg – O and Ca – O distances were observed across the transition, except for the Ca – O 1 distance, which is more compressible in the IC phase than in the N phase. From room pressure to 3.79 GP the volume of the [SiO4] tetrahedron is unchanged (2.16 Å), whereas the volumes of the [MgO4] and [CaO8] polyhedra decrease from 3.61 to 3.55 (1) Å 3 and 32.8 to 30.9 (2) Å, respectively. Intensities of satellite reflections are found to vary linearly with the isotropic displacement parametr of Ca and the librational amplitude of the [SiO4] tetrahedron. At room pressure, there is a mismatch between the size of the Ca cations and the configuration of tetrahedral sheets, which appears to be responsible for the formation of the modulated structure; as pressure increases, the misfit is diminished through the relative rotation and distortion of [MgO4] and [SiO4] tetrahedra and the differential compression of individual Ca – O distances, concurrent with a displacement of Ca along the (110) mirror plane toward the O 1 atom. We regard the high-pressure normal structure as a result of the elimination of microdomains in the modulated structure.