Preferential site occupation in the quasi-ternary channel compounds TlCr5S8-ySey (1 <= y <= 7) introducing anisotropic variations of interatomic interactions: A systematical study of their crystal structures

The results of single crystal X-ray investigations of the quasi-ternary cha nnel compounds TlCr5S8-ySey (1 less than or equal to y less than or equal t o 7) show a linear increase of the unit cell volume with increasing Se cont ent as expected for a solid solution. The successive occupation of the four distinct chalcogene sites by Se, however occurs in a nonlinear way. During the substitution the two sites X1 and X2 are strongly preferred by the Se atoms. For site X3 no significant preference is observed and site X4 is cle arly less favoured by the Se atoms. This unusual behaviour can be interpret ed on the basis of the different polarizabilities of the constituting atoms . The occupation of X1 and X2 by Se gives the stongest covalent bonding int eractions, especially between TI and Se thus being the driving force for th e high preference of Se for these two sites. The preferential occupation of the four crystallographically independent chalcogene sites leads to an une xpected and spectacular dependence of the unit cell parameters and of the i nteratomic distances as function of the Se content. The a axis shows a nega tive deviation from linearity, whereas the c axis exhibits a positive one. The monoclinic angle beta passes a maximum at y = 5, and finally drops to a value comparable with that obtained for the pure sulfide. A detailed analy sis demonstrates that the series can be divided into the two different regi ons 0 less than or equal to y less than or equal to 4 and 5 less than or eq ual to y less than or equal to 8, both characterized by a linear variation of the lattice parameters as well as of the interatomic distances.