Spin dimer analysis of the three-dimensional antiferromagnetic ordering in the quaternary manganese sulfides Ba Ln2MnS 5 ( Ln=La, Ce, Pr)

Abstract

The quaternary manganese sulfides Ba Ln 2MnS 5 ( Ln=La, Ce, Pr) consist of (MnS 4) 6− anions separated with short S⋯S distances slightly longer than the van der Waals distance. Nevertheless, these sulfides are known to undergo a three-dimensional (3D) antiferromagnetic ordering at a reasonably high temperature (i.e., T N=58.5, 62.0 and 64.5 K for Ln=La, Ce and Pr, respectively). The origin of this observation was probed by studying the Mn–S⋯S–Mn super–superexchange interactions of Ba Ln 2MnS 5 on the basis of spin dimer analysis. The non-bonding S⋯S contacts in the vicinity of the van der Waals distance are found essential in determining the strengths of the Mn–S⋯S–Mn super–superexchange interactions. The antiferromagnetic spin exchange between adjacent (MnS 4) 6− anions along the c-direction ( J 2) is calculated to be stronger than that in the ab-plane ( J 1) by a factor of ∼10, so that the strongly interacting spin units of Ba Ln 2MnS 5 ( Ln=La, Ce, Pr) are 1D chains made up of the exchange paths J 2. The relative strengths of the spin exchange interactions for the J 1 and J 2 paths are consistent with the finding that the Néel temperatures of Ba Ln 2MnS 5 are reasonably high, and they increase in the order BaLa 2MnS 5<BaCe 2MnS 5< BaPr 2MnS 5.