The crystal structure of loparite: a new acentric variety

Abstract

The crystal structure of a new structural variety of loparite (Na0.56Ce0.21La0.14Ca0.06Sr0.03Nd0.02Pr0.01)Σ=1.03(Ti0.83Nb0.15)Σ=0.98O3 from the Khibiny alkaline massif, Kola peninsula, Russia, was solved by direct methods and refined to R 1 = 0.029 for 492 unique observed reflections with I > 2σ(I). The mineral is orthorhombic, Ima2, a = 5.5129(2), b = 5.5129(2) and c = 7.7874(5) A. Similarly to other perovskite-group minerals with the general formula ABO3, the crystal structure of loparite is based upon a three-dimensional framework of distorted corner-sharing BO6. The A cations are coordinated by 12 oxygen atoms and are situated in distorted cuboctahedral cavities. In contrast to the ideal perovskite-type structure (\( Pm\overset{-}{3} m \)), the unit cell is doubled along the c axis and the a and b axes are rotated in the ab plane at 45o. The BO6 octahedron displays distortion characteristic for the d 0 transition metal cations with the out-of-center shift of the B site. The symmetry reduction is also attributable to the distortion of the BO6 octahedra which are tilted and rotated with respect to the c axis. The occurrence of a new acentric variety of loparite can be explained by the pecularities of its chemical composition characterized by the increased content of Ti compared to the previously studied samples.