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Crystal Structure of BaCa(CO3)2 Alstonite Carbonate and Its Phase Stability upon Compression
ACS Earth and Space Chemistry ( IF 2.9 ) Pub Date : 2021-04-23 , DOI: 10.1021/acsearthspacechem.1c00032
Raquel Chuliá-Jordán 1 , David Santamaria-Perez 1 , Javier Ruiz-Fuertes 2 , Alberto Otero-de-la-Roza 3 , Catalin Popescu 4
Affiliation  

New single-crystal X-ray diffraction experiments and density functional theory (DFT) calculations reveal that the crystal chemistry of the CaO–BaO–CO2 system is more complex than previously thought. We characterized the BaCa(CO3)2 alstonite structure at ambient conditions, which differs from the recently reported crystal structure of this mineral in the stacking of the carbonate groups. This structural change entails the existence of different cation coordination environments. The structural behavior of alstonite at high pressures was studied using synchrotron powder X-ray diffraction data and ab initio calculations up to 19 and 50 GPa, respectively. According to the experiments, above 9 GPa, the alstonite structure distorts into a monoclinic C2 phase derived from the initial trigonal structure. This is consistent with the appearance of imaginary frequencies and geometry relaxation in DFT calculations. Moreover, calculations predict a second phase transition at 24 GPa, which would cause the increase in the coordination number of Ba atoms from 10 to 11 and 12. We determined the equation of state of alstonite (V0 = 1608(2) Å3, B0 = 60(3) GPa, B0 = 4.4(8) from experimental data) and analyzed the evolution of the polyhedral units under compression. The crystal chemistry of alstonite was compared to that of other carbonates and the relative stability of all known BaCa(CO3)2 polymorphs was investigated.

中文翻译:


BaCa(CO3)2 碳酸钙石的晶体结构及其压缩相稳定性



新的单晶X射线衍射实验和密度泛函理论(DFT)计算表明,CaO–BaO–CO 2系统的晶体化学比之前认为的更为复杂。我们在环境条件下表征了 BaCa(CO 3 ) 2镁石结构,该结构与最近报道的这种矿物的晶体结构在碳酸盐基团的堆积方面有所不同。这种结构变化需要不同阳离子配位环境的存在。使用同步加速器粉末 X 射线衍射数据和分别高达 19 GPa 和 50 GPa 的从头计算研究了镁橄榄石在高压下的结构行为。根据实验,在9 GPa以上,钠灰石结构扭曲成源自初始三角结构的单斜C 2 相。这与 DFT 计算中虚数频率和几何松弛的出现是一致的。此外,计算预测在 24 GPa 下会发生第二次相变,这将导致 Ba 原子的配位数从 10 增加到 11 和 12。我们确定了镁橄榄石的状态方程 ( V 0 = 1608(2) Å 3B 0 = 60(3) GPa, B0 = 4.4(8) 来自实验数据)并分析了多面体单元在压缩下的演化。将镁橄榄石的晶体化学性质与其他碳酸盐的晶体化学性质进行了比较,并研究了所有已知的 BaCa(CO 3 ) 2多晶型物的相对稳定性。
更新日期:2021-05-20
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