A new Ca3MgSi2O8 compound and some of its thermodynamic properties

Abstract

A new calcium magnesium orthosilicate with the composition Ca3MgSi2O8 was synthesized by a solid-state reaction process at 1.2GPa and 1373K for 7 days. We refined the crystallographic structure of this new compound using single-crystal X-ray data, and obtained some of its thermodynamic properties by performing some first-principles simulations. Our single-crystal X-ray analysis has shown that this new compound is monoclinic with the space group C2/c, and its unit-cell parameters are a = 9.344(4) Å, b = 5.3308(3) Å, c = 13.290(6) Å, α = 90°, β = 92.072(7)°, γ = 90°, and V = 658.7(6) Å3. The compressibility of this new compound was studied with the CASTEP code using density functional theory and planewave pseudopotential technique, which led to an isothermal bulk modulus B0 of 99(2)GPa with a pressure derivative B0′ of 3.5(5). The phonon dispersions and vibrational density of the states (VDoS) of this new compound were calculated by using density functional perturbation theory. Subsequently, the VDoS was combined with a quasi-harmonic approximation to compute the isobaric heat capacity (Cp) and standard vibrational entropy (S2980), yielding Cp = 3.927(2) × 102 – 1.159(6) × 103T−0.5 – 1.054(4) × 107T−2 + 1.362(8) × 109T−3Jmol−1 K−1 for the T range of 298–1000K and S2980 = 270.5(60)Jmol−1K−1.