A new Ca3MgSi2O8 compound and some of its thermodynamic properties

Abstract A new calcium magnesium orthosilicate with the composition Ca 3 MgSi 2 O 8 was synthesized by a solid-state reaction process at 1.2 GPa and 1373 K 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 C 2 /c , and its unit-cell parameters are a = 9.344(4) A, b = 5.3308(3) A, c = 13.290(6) A, α = 90°, β = 92.072(7)°, γ = 90°, and V = 658.7(6) A 3 . 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 B 0 of 99(2) GPa with a pressure derivative B 0 ′ 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 ( C p ) and standard vibrational entropy ( S 298 0 ), yielding C p = 3.927(2) × 10 2 – 1.159(6) × 10 3 T −0.5 – 1.054(4) × 10 7 T −2 + 1.362(8) × 10 9 T −3  J mol −1 K −1 for the T range of 298–1000 K and S 298 0 = 270.5(60) J mol −1  K −1 .