CRYSTAL STRUCTURE OF LOPARITE(Ce)

Zubkova N. V.¹, Arakcheeva A. V.², Pushcharovsky D. Yu.¹

¹Geology Department, Moscow State University, 119899 Moscow, Russia, E-mail: dmitp@dgeo.phys.msu.su
²Institute of Metallurgy RAN, Leninsky av., 49, 117334 Moscow, Russia

Keywords: loparite, perovskite.

Crystal structure of loparite(Ce), (Ti,Nb)(Na,Ce,Ca,Sr)O₃, had not been refined up today in spite of the fact that it was discovered in 1890. According to its chemical composition and XRD powder diagram it was considered as a member of perovskite group with general formula ABO₃. We have investigated the sample of loparite(Ce) from Lovozero alkaline massive with the crystall dimensions ~0.1x0.1x0.3 mm. Electron microprobe analyses of the crystals led to the following chemical formula: (Ti_0.867Nb_0.132)(Na_0.480Ca_0.117Ce_0.192La_0.082Nd_0.043Th_0.008Sr_0.054Pr_0.014Eu_0.001Sm_0.008)O₃. Double perovskite parameters of the cubic unit cell of loparite(Ce) were determined on a Syntex P-1 diffractometer using 15 reflections collected in the interval 2<15^o (graphite-monochromated MoK-radiation, = 0.71073), a = 7.767(1). The structure was refined up to R=4.1% [1] in space group Pn3m.

The analysis of interatomic distances in polyhedrons, coefficients of occupancy and values of isotropic displacement parameter for two independent B- positions different occupation of cations was determined for them. 4(b) position is occupied only by Ti and 4(c) position - by Ti,Nb in the following ratio: Ti_0.73 + Nb_0.27. Analyzes of the same parameters for A- cations showed that in that case in two independent positions only atoms of Na and Ca are ordered and other cations stay equal in both of them: preliminary Ca- polyhedron occupies 2(a) position and preliminary Na- polyhedron - 6(d) position. The structure consists of framework of alternating corner-linked larger (Ti,Nb)O₆ octahedra (Ti,Nb - O = 1.96(1) A) and smaller TiO₆ octahedra (Ti - O = 1.94(1)A ) similar to that of ABO₃ perovskites, but with B- cations ordered onto two distinct octahedral sites. Preliminary Ca- polyhedron is not distorted smaller cuboctahedron (A - O = 2.50(1)A) and preliminary Na- polyhedron is larger tetrahedrally distorted cuboctahedron (A - O varies from 2.75(1)A up to 2.99(1)A). This cation ordering takes place in the structure of loparite(Ce) because of the possibility of O atoms to displace forming two tilted B- octahedra of different size and two cuboctahedra of different size and value of distortion.

The crystal structure of loparite(Ce). Ti-octahedra (white) and (Ti, Nb)- octahedra (marked with lines) form the framework. the cicrles represent "Na" (white) and "Ca" (black) positions.

So, because of the described ordering of A- and B- cations we have the perovskite-like structure of loparite(Ce) with double parameter of cubic perovskite unit cell and general formula A'A"₃B'₂B"₂O₁₂. It should be noted that double perovskite parameters for loparite were met for the first time as well as the described type of caution ordering for the perovskite - like structures. Thus, the refined structure of loparite(Ce) enlarges the list of known yet possible deformations of the important perovskite structural type.

1. Andrianov V. I. Sov. Phys. Crystallog, 32(1987) 130-132.