Crystal structure of isolueshite and its synthetic compositional analogue

The crystal structure of isolueshite, a recently described member of the pe rovskite group from the Khibina complex (Russia), was refined by single-cry stal methods in the space group Pm (3) over bar m [a = 3.909(1) Angstrom] t o R = 0.031 and wR = 0.061. The previously proposed cubic symmetry of isolu eshite was confirmed. In contrast to the known perovskite-group minerals cr ystallizing with the cubic symmetry, isolueshite shows a disordered arrange ment of oxygen atoms in a Pm (3) over bar m-type cell. In isolueshite, the oxygen atoms are disordered from their "ideal" site at 3c (1/2, 0, 1/2) ove r four 12h sites (x, 0, 1/2) with a refined x of 0.579(5). The refinement w as based on the structural formula (Na(0.75)Lao(0.19)Ca(0.06))(Sigma 1.00)( Nb0.50Ti0.50)(Sigma 1.00)O-3.00 closely corresponding to the empirical form ula of the mineral. The presence of hydroxyl groups in the structure of iso lueshite was confirmed by infrared spectroscopy (absorption lines at 3450 c m(-1) and 1080 cm(-1)). Structural formulae of the mineral accounting for t he presence of (OH)(1-) groups are given. A synthetic compound of the compo sition (Na0.75La0.25) (Nb0.50Ti0.50)O-3 was prepared using the ceramic tech nique (final heating at 1200 degrees C), and shown not to crystallize with cubic symmetry. The crystal structure of this compound is best refined (Rie tveld method) in orthorhombic symmetry [Cmcm,a = 7.7841(9), b = 7.8045(2), c = 7.7831(9) Angstrom]. This structure is derived from the ideal perovskit e lattice (Pm (3) over bar m) by rotation of the (Nb,Ti)O-6 octahedra about two tetrad axes of the cubic subcell (tilt system a(0)b(+)c(-)). Our data and, in particular, the presence of oxygen disorder (incongruent octahedral tilting) in the structure of isolueshite, suggest that stabilization of th e cubic symmetry of this mineral may be controlled by thermodynamic or kine tic factors (crystallization temperature, pressure or cooling rate), rather than by the isomorphic substitutions invoking La and Ti. Hence, isolueshit e should probably be considered a quenched or "frozen" polymorph of NaNbO3.