Orbital Contribution to Paramagnetism and Noninnocent Thiophosphate Anions in Layered Li₂MP₂S₆ Where M = Fe and Co

Transition-metal thiophosphates and selenophosphates are layered systems with the potential for displaying two-dimensional (2D) magnetic phenomena. We present the crystal structures and magnetic properties of two lithium transition-metal thiophosphates, Li_1.56Co_0.71P₂S₆ and Li_2.26Fe_0.94P₂S₆. The previously unreported Li_1.56Co_0.71P₂S₆ crystallizes in the trigonal space group <i>P</i>31<i>m</i> with lattice parameters <i>a</i> = 6.0193(6) Å and <i>c</i> = 6.5675(9) Å. The CoS₆ octahedra are arranged in a honeycomb lattice and form 2D layers separated by lithium cations. The previously solved Li_2.26Fe_0.94P₂S₆ is isostructural to Li_1.56Co_0.71P₂S₆ but displays site mixing between the Li⁺ and Fe²⁺ cations within the thiophosphate layer. Unusually, Li_1.56Co_0.71P₂S₆ appears to have P₂S₆^3– and not P₂S₆^4– anions. We therefore term it a “noninnocent” anion because of the ambiguous nature of its oxidation state. Combined neutron diffraction and magnetization measurements reveal that both Li_1.56Co_0.71P₂S₆ and Li_2.26Fe_0.94P₂S₆ display magnetic anisotropy as well as no long-range magnetic order down to 5 K. In the iron thiophosphate, susceptibility indicates an effective moment of 5.44(3) μ_B, which may be best described by an <i>S</i> + <i>L</i> model, where <i>S</i> = 2 and <i>L</i> = 2, or close to the free ion limit. In the cobalt thiophosphate, we found the effective moment to be 4.35(2) μ_B, which would point to an <i>S</i> = ³/₂ and <i>L</i> = 1 model due to octahedral crystal-field splitting.