PECULIARITIES OF CRYSTAL-STRUCTURES OF RARE-EARTH-METAL SULFIDES WITHTHE PERIOD 4 ANGSTROM

The crystal structures of rare-earth metal sesquisulfides and some rel ated compounds (Tm5S7, Tm8S11, Tm15S22) Of orthorhombic or monoclinic symmetry with the mirror plane perpendicular to the translation of 3.7 Angstrom - 4.1 Angstrom have been studied using an analysis of close- packed atomic nets. The cation and the anion matrices have been shown to possess a similar construction for all compounds. A system of paral lel mirror planes (similar to 2 Angstrom apart) causes all atoms to be located only in these planes; in fact, this means that a crystal stru cture becomes pseudo two-dimensional. It was found that in each struct ure there were sets of cation close-packed crystallographic planes per pendicular to the mirror planes; when intersecting they form a node ne t close to the ideal 3(6) net. Because of the proximity of the neighbo uring mirror planes the cation node in one mirror plane corresponds wi th a vacancy in the adjacent mirror plane. Hence the cation matrices c an be represented by a single-layer hexagonal structure whose basal pl anes split into the two mirror planes with half the cations moving ont o each of the mirror planes (the AA' type according to our classificat ion). The anion matrices have a similar construction but they are less regular and on a smaller scale. Each structure type has an inherent l aw governing the distribution of atoms and vacancies in the net and th e proper conditions for the compatibility between the cation and anion matrices that determine the unit cell dimensions. So, the system of t he nearest mirror planes is the main factor which regulates the atomic arrangement, that is, keeps to the minimum the number of arbitrarily changed coordinates or, more properly, degrees of a freedom of the str ucture.