Contribution to the crystal chemistry of rare earth chalcogenides. I. The compounds with layer structures LnX(2)

A number of lanthanide compounds LnX(2) (Ln = Sc, Y, La-Lu; X = S, Se, Te), LnY(2) (Y = P, As, Sb), and LnZ(2), (Z = Si, Ge) are known to crystallize with a structure derivable from only one aristotype, the ZrSSi type. This s tructure type contains square layers of silicon atoms (in general: square l ayers of the element of medium electronegativity). The structural deviation s observed within the different compounds are usually small and principally affect this square layer. The space groups of the derivative structures un der consideration are subgroups of that of the aristotype and the group-sub group relations are outlined. Distortions of the square layers caused by el ectronic reasons can mostly be understood by applying simple electron count ing rules. A number of compounds with chalcogen deficiency LnX(2-delta) is noted, too, in which the occurrence of vacancies in the square layers often leads to even larger superstructures.