Yamashita Keishiro, Komatsu Kazuki, Hattori Takanori, Machida Shinichi, Kagi Hiroyuki
Geochemical Research Center, Graduate School of Science, The University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan.
J-PARC Center, Japan Atomic Energy Agency, 2-4 Shirakata, Tokai, Naka, Ibaraki 319-1195, Japan.
Acta Crystallogr B Struct Sci Cryst Eng Mater. 2024 Dec 1;80(Pt 6):695-705. doi: 10.1107/S205252062400903X.
The odd hydration number has so far been missing in the water-rich magnesium chloride hydrate series (MgCl·nHO). In this study, magnesium chloride heptahydrate, MgCl·7HO (or MgCl·7DO), which forms at high pressures above 2 GPa and high temperatures above 300 K, has been identified. Its structure has been determined by a combination of in-situ single-crystal X-ray diffraction at 2.5 GPa and 298 K and powder neutron diffraction at 3.1 GPa and 300 K. The single-crystal specimen was grown by mixing alcohols to prevent nucleation of undesired crystalline phases. The results show orientational disorder of water molecules, which was also examined using density functional theory calculations. The disorder involves the reconnection of hydrogen bonds, which differs from those in water ice phases and known disordered salt hydrates. Shrinkage by compression occurs mainly in one direction. In the plane perpendicular to this most compressible direction, oxygen and chlorine atoms are in a hexagonal-like arrangement.
到目前为止，富含水的氯化镁水合物系列（MgCl₂·nH₂O）中一直缺少奇数水合数的情况。在本研究中，已鉴定出在高于2 GPa的高压和高于300 K的高温下形成的七水合氯化镁MgCl₂·7H₂O（或MgCl₂·7D₂O）。其结构是通过在2.5 GPa和298 K下的原位单晶X射线衍射以及在3.1 GPa和300 K下的粉末中子衍射相结合来确定的。通过混合醇类来生长单晶样品，以防止不需要的晶相成核。结果显示了水分子的取向无序，这也通过密度泛函理论计算进行了研究。这种无序涉及氢键的重新连接，这与水冰相和已知的无序盐水合物中的情况不同。压缩导致的收缩主要发生在一个方向上。在垂直于这个最可压缩方向的平面中，氧原子和氯原子呈六边形排列。
