Crystal Structure and Some Thermodynamic Properties of Ca7MgSi4O16-Bredigite

Abstract

Bredigite with the composition Ca7MgSi4O16 (Ca7MgSi4O16-Bre) has been synthesized by a solid-state reaction method at 1.2 GPa and 1373 K for 7 days, and its structure has been determined by single-crystal X-ray diffraction data. Following a relevant genealogy analysis in the literature, we have refined the structure into two space groups, Pnnm and Pnn2, and found that Ca7MgSi4O16-Bre belongs to the space group Pnnm, which can be essentially derived from the space group Pnn2 via an atomic coordinate transformation (with an average deviation of 0.039 Å only). Furthermore, some thermodynamic properties of the Ca7MgSi4O16-Bre have been obtained in this study. Using first-principles simulations based on density functional theory, the isothermal bulk modulus has been determined as 90.6(4) GPa with a pressure derivative of 5.7(1). Using density functional perturbation technique, the phonon dispersions and vibrational density of the states (VDoS) have been calculated. The VDoS has been combined with a quasi-harmonic approximation to compute the isobaric heat capacity (Cp) and standard vibrational entropy (S2980), yielding Cp = 8.22(2) × 102 − 3.76(6) × 103T−0.5 − 1.384(4) × 107T−2 + 1.61(8) × 109T−3 J mol−1 K−1 for the T range of 298-1000 K and S2980 = 534.1 (22) J mol−1 K−1.