Crystal structure of rimkorolgite, Ba[Mg5(H2O)7(PO4)4](H2O), and its comparison with bakhchisaraitsevite

Abstract

The crystal structure of rimkorolgite, ideally Ba[Mg 5 (H 2 O) 7 (PO 4 ) 4 ](H 2 O), (monoclinic, P 2 1 / c, a = 8.3354(9), b = 12.8304(13), c = 18.313(2) A, β = 90.025(2)°, V = 1958.5(4) A 3 , Z = 4) has been solved by direct methods and refined to R 1 = 0.052 using X-ray diffraction data collected from a crystal twinned on (001). There are five symmetrically independent Mg 2+ cations that are each octahedrally coordinated by four O atoms and two H 2 O groups. One symmetrically independent Ba 2+ cation is coordinated by eight O atoms and two H 2 O groups. The Mgϕ 6 octahedra (ϕ = O, H 2 O) and PO 4 tetrahedra form sheets parallel to (001). Their main elements are zigzag chains of the Mgϕ 6 edge-sharing octahedra. The chains are linked via common vertices to form an octahedral sheet in which Mg atoms are located at the vertices of the 6 3 hexagonal net. The PO 4 tetrahedra are above and below hexagonal rings of Mg octahedra and are linked to them by sharing common O vertices. The Ba atoms and H 2 O(1) and H 2 O(22) groups are located between the sheets providing their linkage into three-dimensional structure. The structure of rimkorolgite is closely related to that of bakhchisaraitsevite, Na 2 Mg 5 (PO 4 ) 4 7H 2 O. Both structures are based on the octahedral-tetrahedral sheets of the same type. In bakhchisaraitsevite, the sheets are linked into three-dimensional framework by edge-sharing between the Mgϕ 6 octahedra from two adjacent sheets, whereas in rimkorolgite, there is no linkage between adjacent sheets. The structure of rimkorolgite can be considered as bakhchisaraitsevite-like framework interrupted by the presence of large Ba 2+ cations.