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, MX1.5, 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, M2X2 n+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.