PREPARATION AND STRUCTURE OF LANB5O14

Single crystals of LaNb5O14 could be prepared by chemical transport reactions (T2 --> T1; T2 = 1050-degrees-C; T1 = 950-degrees-C) using chlorine as transport agent. LaNb5O14 crystallizes in the orthorhombic space group Pbcm with cell dimensions a = 3.8749(2) angstrom; b = 12.4407(6) angstrom and c = 20.2051(9) angstrom; Z = 4; R = 6.28%, R(w) = 3.74%. The structure consists of two types of Nb-O-polyhedra. Especially remarkable are chains of edge-sharing pentagonal NbO7-bipyramids, which are interconnected by corner-sharing NbO6-octahedra. Tunnels running in a-direction are created by this framework of NbO6- and NbO7-polyhedra. Lanthanum atoms are located in these tunnels at levels inbetween the niobium atoms. The relationship to O-LaTa3O9 and M-CeTa3O9 type structures will be discussed.