Elastic properties of the incommensurate phase of akermanite,Ca2MgSi2O7

The crystal structure of akermanite, Ca₂Mg-Si₂O₇, consists of mixed tetrahedral sheets formed by [MgO₄] tetrahedra and [Si₂O₇] groups interleaved along the c axis with Ca²⁺ ions in eight-fold coordination. Above 358 K, the structure is tetragonal , and below it is incommensurate with modulations parallel to [110] and . The elastic stiffness moduli, C^ij of the incommensurate phase at room temperature were measured from wave velocities in the 20–75 MHz carrier frequency range by the ultrasonic phase comparison method using optically clear synthetic single crystal plates (3×3×2 mm) oriented parallel to (100), (001), (110) and (101) planes. The C_ij values (GPa) are: C₁₁ 159.40, C₃₃ 149.43, C₄₄ 30.26, C₆₆ 58.10, C₁₂ 76.58 and C₁₃ 57.80. In (010) and (001) planes, the compressional modulus, V²(L) from the longitudinal wave, L is considerably larger than the shear moduli, V²(T₁, T₂) both from the in-plane and perpendicular-to-plane shear waves, T₁ and T₂. The relatively small values of the shear moduli indicate the ease of tetrahedral rotations in response to in-plane and perpendicular-to-plane shears and may provide preconditions for structural changes involving shear-type atomic movements.