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首页> 外文期刊>Acta Crystallographica, Section B. Structural science, crystal engineering and materials >Evolution of the α-BaMg(CO_3)_2 low-temperature superstructure and the tricritical nature of its α-β phase transition
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Evolution of the α-BaMg(CO_3)_2 low-temperature superstructure and the tricritical nature of its α-β phase transition

机译：α-BAMG（CO_3）_2低温上层建筑的演变和其α-β相变的三分基础

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摘要

The crystal structure of the synthetic double carbonate norsethite [BaMg(CO_3)_2] has been reinvestigated using X-ray diffraction data within the temperature range 100-500 K using a high-sensitivity PILATUS pixel detector. The previously assumed positional shift of the crystallographically unique oxygen atom is confirmed. The shift is associated with a coupled rotation of symmetry-equivalent carbonate groups. It was possible to follow the shift using high-accuracy experiments under varying temperature conditions between 100 K and the critical transition temperature occurring at T_c = 363 ± 3 K. The transition of the α-form (space group R3c; below T_c), which represents a superstructure of the β-form (space group R3m, with c' = c/2; above T_c) was studied in detail. The tricritical order character of this displacive phase transition was verified by tracking the intensities of the recorded superstructure reflections (l = 2n + 1) from single-crystal diffraction and using high-precision lattice parameters obtained from powder diffraction in transmission geometry. Thermodynamic properties suggest both rotation of the CO_3 group and a coordination change of the BaO_(12) coordination polyhedra as the order parameters driving the temperature-dependent α-β phase transition. Nevertheless, a detailed structural analysis reveals the coordination change of the barium atoms to be the main driving force for the observed transformation.
机译：使用高灵敏度Pilatus Pixel检测器使用X射线衍射数据在100-500K内使用X射线衍射数据重新激活合成双碳酸酯的晶体结构。确认了先前假定的晶体独特氧原子的位置偏移。转变与对称等同物碳酸酯基团的偶联旋转相关。在T_C = 363±3K的临界转变温度下，使用高精度实验可以使用高精度实验进行换档。描述了β-形式的上部结构（空间组R3M，用C'= C / 2;上述T_C）进行了详细研究。通过跟踪从单晶衍射和使用从透射几何中的粉末衍射获得的高精度晶格参数来验证该移位相转变的三分比阶的特异性。热力学性质表明CO_3组的旋转以及BAO_（12）配位多面体的协调变化，作为驱动温度依赖性α-β相转变的顺序参数。然而，详细的结构分析显示了钡原子的协调变化是观察到的转化的主要驱动力。

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