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

Abstract

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 unit-cell 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−1 for the IC phase, and 0.00299(11) and 0.00367(8) GPa−1 for the N phase, respectively. Weighted volume and pressure data, fitted to a second-order Birch-Murnaghan equation of state (K′≡4.0), yield V0=307.4(1) A3 and K0=100(3) GPa for the IC phase and V0=307.6(2) A3 and K0=90(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–O1 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 A3), whereas the volumes of the [MgO4] and [CaO8] polyhedra decrease from 3.61 to 3.55(1) A3 and 32.8 to 30.9(2) A3, 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 O1 atom. We regard the high-pressure normal structure as a result of the elimination of microdomains in the modulated structure.