ON THE COORDINATION OF THCR2SI2 (BAAL4)-TYPE COMPOUNDS WITHIN THE FIELD OF FREE PARAMETERS

Nearly 600 compounds crystallizing in the ThCr2Si2 type of structure h ave been critically screened and sorted according to the free paramete rs c/a and z, where cia is the lattice parameter ratio of the tetragon al unit cell and z is the structural parameter of the Si position. The considerable scatter of these parameters well outside the errors of m easurement is accompanied by an essential change in the coordination g eometry. Using the concept of isodiastematics, several families of rep resentatives can be formed exhibiting similar values of the free param eters. As the analysis shows, non-nearest-neighbour interactions appar ently control the stability of these compounds within the c/a-z field. Moreover, standard data of reference books have to be modified as fol lows. (i) Coordination numbers based on the maximum gap criterion vary considerably within the free-parameter field for Cr sites (8-16) and Si sites (4-9) whereas they remain constant for the Th site (22). (ii) The ThCr2Si2 type belongs to the three-environment types as introduce d by Villars et al. Through variation of the free parameters the deter mining coordination polyhedra change, e.g. from Th 16(2.2.0) 4(4.2.0) 2(0.4), Cr 4(3.1.0) 4(4.1.0) 4(2.2.0), Si 4(2.1.0) 1(4.0) at c/a = 2.3 095, z = 0.3750 to Th 16(2.2.0) 4(4.2.0) 2(0.4), Cr 4(1.2.0) 4(2.2.0), Si 4(0) at c/a = 3.464, z = 0.3333. (iii) The preceding analysis may be interpreted as a geometrical classification of the contribution of the pair interaction of atoms. The present results may serve as guidan ce for considering stability on the basis of energy calculations.