Crystal-chemical aspects of the structural similarity of rare earth polychalcogenides LnX2?x (x=0–0.25)

The structures of rare earth dichalcogenides are investigated. The symmetry and unit cell parameters are considered, and their relationship with those of the PbFCl and anti-Cu₂Sb structural prototypes is analyzed. The interatomic distances in the Ln³⁺, X²⁻, and (X₂)²⁻ layers and the compatibility factors are examined. It is discussed how the degree of polymerization of the chalcogen ions in the (X₂)²⁻ layers and the compatibility of the latter with the Ln³⁺ and X²⁻ layers affect the symmetry and the twinning and polymorhism abilities of the compounds. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 6, pp. 1140–1170, November–December, 1996. Translated by L. Smolina