Layered Oxychalcogenides

Much of the work in the group focuses on layered oxide chalcogenide and oxide pnictide solids. Oxide and chalcogenide (S, Se, Te) or pnictide (P, As, Sb, Bi) anions differ in both size and chemical requirements, so mixed anion compounds can often be described as oxide layers separated by distinct chalcogenide or pnictide-containing slabs. The physical and chemical properties of these materials are related to the crystal structures and the chemical properties of the elements. The properties exhibited by oxide chalcogenides include semiconductor properties, for example, in LaOCuCh (Ch = chalcogenide) and derivatives, unusual magnetic properties exhibited by the class Sr₂MO₂Cu_2-δS₂ (M = Mn, Co, Ni), and redox properties exhibited by the materials Sr₂MnO₂Cu_2m-0.5S_m+1 (m = 1 - 3) and Sr₄Mn₃O_7.5Cu₂Ch₂ (Ch = S, Se). Oxide pnictides have received less recent attention, but the discovery of high-temperature superconductivity (> 40 K) in derivatives of the layered iron oxide arsenide LaOFeAs initiated a persistent wave of interest in these layered multi-anion compounds.

The oxide and chalcogenide or pnictide layers in these layered compounds may be considered quite electronically distinct. The degree to which this is true depends on, for example, the size of the electropositive cation A²⁺ in the A₂MO₂Cu₂S₂ structure, which dictates the M - S distance and hence the extent to which the transition metal M is engaged in bonding interactions with the chalcogenide anion. One other clear element of flexibility in these structures concerns the variable occupancy of the metal ion (normally Cu⁺ or Ag⁺) in the tetrahedral sites in the chalcogenide layer. We have shown that the members of the series Sr₂MnO₂Cu_2m-δS_m+1 (m = 1 -3 ) all have a very substantial copper deficiency (δ ∼ 0.5), and a range of copper deficiencies have been reported for CeOCu_1-δS (0 < δ < 0.25). The implication of copper deficiency is that the transition metal or lanthanide is oxidized, and we have shown using Mn K-edge XANES measurements and magnetometry backed up by bond-valence calculations that the Mn oxidation state in Sr₂MnO₂Cu_2m-δS_m+1 (m = 1 - 3; δ ∼ 0.5) is 2.5+, which has important implications for the magnetic properties. Recently we have shown that extreme oxidation of A₂MO₂Cu₂S₂ can result in full removal of the Cu ions from the sulfide layer with anion oxidation occurring to yield disulfide [S₂]^2- ions, and we are currently exploring further this anion-redox chemistry.