The crystal structure of Na2MgGd2(Si4O12)F-2 and its relationship with leucophanite

The structure of a newly synthesized rare-earth fluorosilicate, Na2MgGd2(Si 4O12)F-2, was determined using single-crystal X-ray diffraction. It crystal lizes in the monoclinic system, P2(1)/c, with unit-cell parameters a 5.178( 3), b 7.510(2), c 14.381(4) Angstrom, beta 90.22(2)degrees, V 559.1(2) Angs trom(3) and Z = 2. The structure consists of SiO4 tetrahedra in [010] zigza g chains connected by MgO4F2 octahedra. The stacking of the resulting Mg(S4 O12)F-2 sheets defines voids in which Gd3+ and Na+ are found. Similar silic ate chains, connected by BeO3F tetrahedra, exist in leucophanite and result in Be-2(Si4O12)F-2 sheets.