Crystal structure of loparite

The crystal structure of the cubic modification of the natural mineral lopa rite has been studied for the first time by the methods of the X-ray diffra ction analysis (lambda MoKalpha radiation, 105 independent reflections with I > 3 sigma(I), R = 0.041 in the anisotropic approximation). The structure belongs to the perovskite type (ABO(3)) with the double period of the cubi c unit cell, a = 7.767(1) Angstrom (sp. gr. Pn3m; Z = 2 for the composition (Ca,Na,Ce)(Na,Ce)(3)(Ti,Nb)(2)Ti2O12. Period doubling is explained by orde ring of cations both in the A and the B positions. (C) 2000 MAIK "Nauka/Int erperiodica".