The structure of LiMg(AlD 4) 3

Abstract

The structure of LiMg(AlD 4) 3 was investigated using synchrotron radiation powder X-ray diffraction, powder neutron diffraction, and density functional theory atomic simulations. The experimental crystallographic data were refined with the monoclinic P2 1/ c space group and unit cell dimensions: a = 8.37113(16) Å, b = 8.73910(17) Å, c = 14.3012(3) Å and β = 124.8308(8) Å. The atomic simulations verified the increased stability of this structure with respect to a wide range of other possible candidate space group structures. The P2 1/ c structure consists of isolated AlD 4 tetrahedra, connected separately through the four corner D atoms to two Li and two Mg atoms. Each Li and Mg atom is octahedrally coordinated to the corner D atoms of six AlD 4 tetrahedra, so that the structure consists of a corner-sharing network of alternating AlD 4 tetrahedra and LiD 6 or MgD 6 octahedra. Using soft constraints in the refinement, the Al–D distances are 1.62(1) Å, the Li–D distances vary from 1.87(3) to 2.09(3) Å and the Mg–D distances vary from 1.861(14) to 1.919(11) Å. The shortest D–D distance of 2.525(19) Å was found within the Al-tetrahedra. The shortest interpolyhedral D–D distance is 2.780(16) Å. The structure can be described as a distorted hexagonal closed packed geometry of AlD 4 tetrahedra, with Li and Mg occupying 2/3 of the interstitial octahedral sites. We observed that LiMg(AlD 4) 3 decomposes to LiMgAlD 6 at 130 °C.