Crystal-chemical aspects of the structural similarity of rare earth polychalcogenides LnX(2-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 Ln(3+), X2-, and (X-2)(2-) layers and the compatibility factors are examined. It is discussed how the degree of polymerization of the chalcogen ions in the (X-2)(2-) layers and the compatibility of the latter with the Ln(3+) and X2- layers affect the symmetry and the twinning and polymorhism abilities of the compounds.