Syntheses and structures of AgLnS 2 (LnSmYb) compounds

Abstract

The ternary compounds AgLnS 2 (LnSmYb) have been synthesized and characterized. It has been found, in accordance with a previous study, that all compounds, except AgYbS 3, are monoclinic with a ≈ b ≈ 7.7 Å, c ≈12 Å and 90° < β ⪕ 91°. AgYbs 2 is a body-centred tetragonal with a′= a √2 . Single crystals of AgLnS 2 were grown for LnGd, Ho and Yb by means of vapour transport. The structures of the monoclinic compounds AgLnS 2 (LnGd, Ho), space group P2 1, were determined using single crystal X-ray data. Silver atoms are situated in strongly distorted S 6-octahedrons with two short, three intermediate and one long AgS distances. Rare-earth metal atoms are situated in slightly distorted S 6-octahedrons with LnS distances in good agreement with literature data for Ln 3+ in six fold coordination by sulphur. The structure is a superstructure of the NaCl-type LnS structure, with half of the cations having been replaced by silver. Contrary to the powder data, the single crystal data showed that AgYbS 2 is cubic. It has the NaCl structure as YbS, but with silver and ytterbium atoms randomly distributed on the cation sites. The disordered phase is metastable at room temperature; the ordered tetragonal phase could be obtained by annealing for 5 days at 1000 K. The magnetic susceptibility measurement for AgGdS 2 showed Curie-Weiss behaviour above T≈35K corresponding to μ eff = 7.38 μ b (in agreement with Gd 3+) and Θ c = −32.0 K.