A NEW REFINEMENT OF THE CRYSTAL-STRUCTURE OF MNALF5 - NEW STRUCTURAL CORRELATIONS WITH MNCRF5 AND CR2F5 USING ORTHOGONAL SUBCELL TWINNING

A new refinement of the crystal structure of MnAlF5, needed for further powder neutron diffraction studies, shows that it is centric (S.G. Cmcm, a = 3.5837(4) Å, b = 9.854(1) Å, c = 9.537(1) Å, Z = 4). The structure was refined from 720 independent reflections (R 0.023, Rw = 0.022). Its crystal chemistry is basically the same as previously described, but a new comparison with MnCrF5 and Cr2F5 is proposed. It first shows that the latter two structures represent two different monoclinic distortions of the same structural type, despite large differences in their lattice parameters. This leads to a description of MnAlF5 as a twinning of two subcells of this structural type and to an introduction of the concept of subcell orthogonal twinning. © 1993 Academic Press, Inc.