REGULAR VERSUS ALTERNATING (FES4)(N) CHAINS - MAGNETISM IN KFES2 AND CSFES2

We report crystal magnetic susceptibility results of two S = 1/2 one-d imensional Heisenberg antiferromagnets, KFeS2 and CsFeS2. Both compoun ds consist of (FeS4)(n) chains with an average Fe-Fe distance of 2.7 A ngstrom. In KFeS2, all intrachain Fe-Fe distances are identical. Its m agnetic susceptibility is typical of a regular antiferromagnetic chain with spin-spin exchange parameter J = -440.7 K. In CsFeS2, however, t he Fe-Fe distances alternate between 2.61 and 2.82 Angstrom. This is r eflected in its magnetic susceptibility, which could be fitted with J = -640 K, and the degree of alternation, alpha = 0.3. These compounds form a unique pair, and allow for a convenient experimental comparison of the magnetic properties of regular versus alternating Heisenberg c hains.