CRYSTAL STRUCTURE OF ISOLUESHITE AND ITS SYNTHETIC COMPOSITIONAL ANALOGUE

Abstract

The crystal structure of isolueshite, a recently described member of the perovskite group from the Khibina complex (Russia), was refined by single-crystal methods in the space group Pm3m [a = 3.909(1) Å] to R = 0.031 and wR = 0.061. The previously proposed cubic symmetry of isolueshite was confirmed. In contrast to the known perovskite-group minerals crystallizing with the cubic symmetry, isolueshite shows a disordered arrangement of oxygen atoms in a Pm3m-type cell. In isolueshite, the oxygen atoms are disordered from their 'ideal' site at 3c ( 1/2 , 0, 1/2 ) over four 12h sites (x, 0, 1/2 ) with refined x of 0.579(5). The refinement was based on the structural formula (Na0.75La0.19Ca0.06)(Σ1.00)(Nb0.50Ti0.50)(Σ1.00)O3.00 closely corresponding to the empirical formula of the mineral. The presence of hydroxyl groups in the structure of isolueshite was confirmed by infrared spectroscopy (absorption lines at 3450 cm-1 and 1080 cm-1). Structural formulae of the mineral accounting for the presence of (OH)1- groups are given. A synthetic compound of the composition (Na0.75La0.25) (Nb0.50Ti0.50)O3 was prepared using the ceramic technique (final heating at 1200°C), and shown not to crystallize with cubic symmetry. The crystal structure of this compound is best refined (Rietveld method) in orthorhombic symmetry [Cmcm, a = 7.7841(9), b = 7.8045(2), c = 7.7831(9) Å]. This structure is derived from the ideal perovskite lattice (Pm3m) by rotation of the (Nb,Ti)O6 octahedra about two tetrad axes of the cubic subcell (tilt system a0b+c-). Our data and, in particular, the presence of oxygen disorder (incongruent octahedral tilting) in the structure of isolueshite, suggest that stabilization of the cubic symmetry of this mineral may be controlled by thermodynamic or kinetic factors (crystallization temperature, pressure or cooling rate), rather than by the isomorphic substitutions involving La and Ti. Hence, isolueshite should probably be considered a quenched or 'frozen' polymorph of NaNbO3.