ISIS Facility, STFC, Rutherford Appleton Laboratory, Harwell Oxford, Didcot, UK.
Phys Chem Chem Phys. 2012 Sep 7;14(33):11800-7. doi: 10.1039/c2cp23439d. Epub 2012 Jul 25.
The structure of the cubic polymorph of magnesium tetrahydroborate (γ-Mg(BH(4))(2)) has been determined in space group Ia3d from a structural database of the isoelectronic compound SiO(2); this has been corroborated by DFT calculations. The structure is found to concur with that recently determined by Filinchuk et al. (Y. Filinchuk, B. Richter, T. R. Jensen, V. Dmitriev, D. Chernyshov and H. Hagemann, Angew. Chem. Int. Ed., 2011, DOI: 10.1002/anie.201100675). The phase transformations and subsequent decomposition of γ-Mg(BH(4))(2) on heating have been ascertained from variable-temperature synchrotron X-ray diffraction data combined with thermogravimetric and mass spectrometry measurements. At ~160 °C, conversion to a disordered variant of the β-Mg(BH(4))(2) phase (denoted as β') is observed along with a further unidentified polymorph. There is evidence of amorphous phases during decomposition but there is no direct crystallographic indication of the existence of Mg(B(12)H(12)) or other intermediate Mg-B-H compounds. MgH(2) and finally Mg are observed in the X-ray diffraction data after decomposition.
四氢硼酸镁（γ-Mg(BH(4))(2))的立方多晶型结构已通过与等电子化合物 SiO(2)的结构数据库相关联的 Ia3d 空间群从结构数据库中确定；这已通过 DFT 计算得到证实。该结构与 Filinchuk 等人最近确定的结构相吻合（Y. Filinchuk、B. Richter、T. R. Jensen、V. Dmitriev、D. Chernyshov 和 H. Hagemann，Angew. Chem. Int. Ed.，2011，DOI：10.1002/anie.201100675）。通过与热重和质谱测量相结合的变温同步辐射 X 射线衍射数据，确定了 γ-Mg(BH(4))(2)在加热时的相变和随后的分解。在~160°C 时，观察到向无序变体β-Mg(BH(4))(2)相（表示为β'）的转变，同时还观察到另一种未识别的多晶型。在分解过程中有非晶相的证据，但没有 Mg(B(12)H(12))或其他中间 Mg-B-H 化合物存在的直接晶体学迹象。在分解后，在 X 射线衍射数据中观察到 MgH(2)和最终的 Mg。
