Mirror plane as a regulator of the crystal structure of sulfides. rare-earth element sesquisulfides

A CAS-PAN analysis (isolation of close-packed atomic planes) of three most widespread structural types of rare-earth sesquisulfides — monoclinic Ho2S3 and orthorhombic Gd2S3 and Tm2S3 — showed that mirror plane systems separated by 2 å in sulfide structures often contain trigonal nets of both cations and anions filled according to the “splitting” principle. The mirror plane forces all atoms into one plane and thus actually deprives them of one of the three degrees of freedom, stabilizing the one-layer hexagonal packing in which the basal plane is split into two mutually complementary planes (AA’ type).