Some thermodynamic properties of larnite (β-Ca 2 SiO 4 ) constrained by high T / P experiment and/or theoretical simulation

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Abstract
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 (αV) and isothermal bulk modulus (KT) were constrained as αV = 4.24(4)×10−5 K−1 and KT = 103(2) GPa [the first pressure derivative KT′ 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 KT values as 123(3) GPa (LDA; high boundary) and 92(2) GPa (GGA; low boundary), with the values of the KT′ 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 (CP) and standard vibrational entropy (S0298), yielding CP = 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.

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