Simple models of crystallographic shear planes were proposed for the interpretation of the structural relationships between the structures of various oxides and fluorides. The first model was related to the K2NiF4 network. The shear planes were parallel to the (110) plane; with a displacement vector that was an integer multiple of R = ¼[001]. Octahedral sites were created, by this shear operation, which shared faces with those of the K2NiF4 slabs. The composition of the phases was governed by the periodicity of the shear planes, and the structure was determined by the sequence of displacement vectors. This model of extended planar defects permitted a correlation to be made between the structures of Ba4(Ti,Pt)3O10-type, which were also found in Cs4Cu3F10 and Cs6Ni3F15. The second model was based upon the hexagonal perovskite network; especially the 9R polytype. The shear planes were parallel to the (¯111) plane, and the displacement vector was directed along the [001] direction. The modulus corresponded to twice the distance between 2 successive close-packed [AX3] layers. The sequence of structures, BaRuO3 (9R)  Ba7Ir6O19 or Cs7Cu6F19  Cs4Cu3F10, was explained by changing the periodicity of the shear planes. All of the structures resulted from the stacking of defective close-packed layers that were formed by hexagonal or orthorhombic [AX3] arrays. The O vacancies were arranged in rows in the latter case. The simplest case corresponded to the K2NiF4 structure, and there was a localization of the excess O.

C.Dussarrat, F.Grasset, J.Darriet: European Journal of Solid State and Inorganic Chemistry, 1995, 32[6], 557-76