Crystal Structure and Some Thermodynamic Properties of Ca7MgSi4O16-Bredigite

Crystals

Zhigang Zhang¹  Xi Liu²  Xinjian Bao²  Mingyue He³ 

[1] Key Laboratory of Earth and Planetary Physics, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China;Key Laboratory of Orogenic Belts and Crustal Evolution, Ministry of Education of China, Beijing 100871, China;School of Gemmology, China University of Geosciences (Beijing), Beijing 100083, China;

关键词: bredigite;    single-crystal XRD;    structure refinement;    first-principle calculation;    compressibility;    heat capacity;   

DOI  :  10.3390/cryst11010014

来源: DOAJ

【 摘 要 】

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 (C_p) and standard vibrational entropy ($S_{298}^0$), yielding C_p = 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 $S_{298}^0$ = 534.1 (22) J mol⁻¹ K⁻¹.

【 授权许可】

Unknown