The crystal structure of lewisite, (Ca,Sb3+,Fe3+,Al,Na,Mn,square)(2)(Sb5+,Ti)(2)O-6(OH)

The crystal structure of lewisite, ideally (Ca,Sb3+,Fe3+,Al,Na,Mn,square)(2 )(Sb5+,Ti)(2)O-6(OH), cubic, space group Fd3m, a = 10.311(7) Angstrom, V = 1096.23 Angstrom(3), Z = 8, d(calc.) = 4.73, d(obs.) = 4.95 g/cm(3), F(000) = 1531, has been solved by single-crystal X-ray diffraction analysis and r efined to R = 0.015 for 218 unique reflections with \F\ > 4 sigma(F). The p yrochlore-like structure contains distorted A cubes and B octahedra. A comp arison among lewisite and several other pyrochlore-like structures with B = Sb5+ shows that increase of the unit cell parameter depends on the size of the A-cations. XRD study of lewisite did not reveal the intergrowth of two sublattices, previously anticipated in this structure. A comparison of two models with a static distribution of Sb3+ (splitting of the A site) and a dynamic distribution (anharmonic thermal displacements of cations in the A site) led us to prefer the latter. A structural study of romeite is now nec essary to establish the identity of lewisite and romeite. (C) 2000 Elsevier Science S.A. All rights reserved.