THE WAIST-CONTACT-RESTRICTION RULE USED TO CHECK TERNARY STRUCTURES BUILT FROM CENTERED TRIGONAL PRISMS

The atom distribution on the prism centers in ternary rare earth (R)-transition metal (T)-silicides or germanides (M) can be rationalized by means of a waist-contact-restriction rule. The rule states that the centers of the rare-earth prisms are occupied by T and M atoms in such a way that the T atoms have neither T-R nor T-T waist contacts. Applications of this waist-contact-restriction rule are discussed. It is possible to predict whether the composition of a compound with supposedly random occupation of prism-center sites is correct or if it must be changed to obtain perfect order of the T and M atoms on these sites.