Structural correlations of the lanthanide halides and related compounds. Derivatives of the anti-nickel-arsenide structure

An examination of structural data for lanthanide halides and related compounds has shown that a substantial number of different structure types are conveniently described as layered structures derived from anti-NiAs by removal or shear of cation layers and distortion of the residual layers. The structural correlations of various MX, MX_1.5, MX₂, MXY, MX₃, and MX₂Y compositions (M = cation, X and Y = anions) are described by the presentation of a subgroup-supergroup diagram relating their space groups and by comparison of their structural projections. A close relationship between the CsCl- and NiAs-type structures is observed. The occurrence of displacive and order-disorder phase transitions, the formation ternary derivatives by ion accommodation processes and the possible formation of intermediate halides, M₂X_2n+1, by coherent intergrowth of MX₂ and MX₃ structures are discussed. The effects of radius ratio and cation coordination number on the stabilities of halide structures and on the formation of complex MX₂ layers derived from hexagonal-closest-packed metal arrays are examined.