CRYSTALLOGRAPHIC SHEARS IN HEXAGONAL PEROVSKITES AND K2NIF4-TYPE STRUCTURE - STRUCTURAL RELATIONSHIPS BETWEEN BARUO3-BA7IR6O19(CS7CU6F19)-CS4CU3F10 AND RELATED-COMPOUNDS

Simple novel models of crystallographic shear planes are proposed to i nterpret the structural relationships between various structures of ox ides and fluorides. The first model is relative to the K2NiF4 network. The shear planes are parallel to the (110) plane with a displacement vector which is an integer multiple of R approximate to 1/4 [001]. By this shear operation, octahedral sites are created which share faces w ith those of the K2NiF4 slabs. The composition of the phases is govern ed by the periodicity of the shear planes and the structure by the seq uence of the displacement vectors. This model of extended planar defec ts allows to correlate the structures of Ba-4(Ti,Pt)O-10 type also fou nd in fluorides, Cs4Cu3F10 and Cs6Ni5F15. The second model is based on the hexagonal perovskite network and especially on the 9R polytype. T he shear planes are parallel to the ((1) over bar 11) plane and the di splacement vector is along the [001] direction. The modulus correspond s to two times the inter-distance between two successive close-packed [AX(3)] layers. The sequence of the structures BaRuO3 (9R) --> Ba7Ir6O 19 (Cs7Cu6F19) --> Cs4Cu3F10 is explained by changing the periodicity of the shear planes. All the structures result from the stacking of de fective close-packed layers formed by hexagonal [AX(3)] (as in the per ovskite) and orthorhombic [AX(3)] arrays. The oxygen vacancies are arr anged in rows in the last one. The simplest case corresponds to the K2 NiF4 Structure and the localization of the excess of oxygen as observe d in the high Tc superconducting A(2)CuO(4+x) phases is discussed.