On the coordination of ThCr2Si2 (BaAl4)-type compounds within the field of free parameters

Nearly 600 compounds crystallizing in the ThCr2Si2 type of structure have been critically screened and sorted according to the free parameters c/a and z, where cia is the lattice parameter ratio of the tetragonal unit cell and z is the structural parameter of the Si position. The considerable scatter of these parameters well outside the errors of measurement is accompanied by an essential change in the coordination geometry. Using the concept of isodiastematics, several families of representatives can be formed exhibiting similar values of the free parameters. As the analysis shows, non-nearest-neighbour interactions apparently control the stability of these compounds within the c/a-z field. Moreover, standard data of reference books have to be modified as follows. (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 introduced by Villars et al. Through variation of the free parameters the determining 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 = 2.3095, 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 guidance for considering stability on the basis of energy calculations.