Antiphase périodique orientationnelle et transformation de phase dans le fluosilicate de fer

FeSiF₆.6H₂O above 240 K is described following the structural model previously proposed for MgSiF₆.6H₂O above 300 K: periodic antiphase built on the ordered monoclinic cell of the low-temperature form. In the present case a progressive ordering is observed and the antiphase boundaries are no longer planar but exhibit steps which lead to a mean boundary canted with respect to the c hexagonal axis (space group P3). Above 240 K a phase transformation occurs with a threefold twinning and a significant lattice deformation. The monoclinic cell (P2₁/c) is described.