Some thermodynamic properties of larnite (β-Ca₂SiO₄) constrained by high T/P experiment and/or theoretical simulation

Pure larnite (β-Ca₂SiO₄; Lrn) was synthesized at 6 GPa and 1473 K for 6 h by using a cubic press, its thermal expansivity was investigated up to 923 K by using an X-ray powder diffraction technique (ambient P), and its compressibility was investigated up to ~16 GPa by using a diamond-anvil cell coupled with synchrotron X-ray radiation (ambient T). Its volumetric thermal expansion coefficient (α_V) and isothermal bulk modulus (K_T) were constrained as α_V = 4.24(4)×10⁻⁵ K⁻¹ and K_T = 103(2) GPa [the first pressure derivative K_T′ obtained as 5.4(4)], respectively. Its compressibility was further studied with the CASTEP code using density functional theory and planewave pseudopotential technique. We obtained the K_T values as 123(3) GPa (LDA; high boundary) and 92(2) GPa (GGA; low boundary), with the values of the K_T′ as 4.4(9) and 4.9(5), respectively. The phonon dispersions and vibrational density of states (VDoS) of Lrn were simulated using density functional perturbation theory, and the VDoS was combined with a quasi-harmonic approximation to compute the isobaric heat capacity (C_P) and standard vibrational entropy (S⁰₂₉₈), yielding C_P = 212.1(1) – 9.69(5)×10²T^−0.5 – 4.1(3)×10⁶T⁻² + 5.20(7)×10⁸T⁻³ J/(mol·K) for the T range of ~298–1000 K and S⁰₂₉₈ = 129.8(13) J/(mol·K). The microscopic and macroscopic thermal Grüneisen parameters of Lrn at 298 K were calculated to be 0.75(6) and 1.80(4), respectively.