Single crystals of the stoichiometric iron calcium oxysulfide CaOFeS have been grown by a solid-state reaction. Structural analysis of CaOFeS at room temperature by combining single-crystal X-ray diffraction data and transmission electron microscopy leads to a stoichiometric hexagonal noncentrosymmetric P63mc layered structure isostructural to CaOZnS. It is built from alternating layers made of FeOS3 tetrahedra sharing sulfur apexes and stacked with Ca(2+) planes. All Fe-O bonds are parallel to the stacking axis; this breaks the centrosymmetry, leading to a polar structure. The dielectric measurements reveal the existence of a magnetodielectric effect near 33 K in good agreement with the Neel temperature, as evidenced near 35 K by specific heat measurements reported by a different group.

The development of effective and eco-friendly cooling technology demands an investigation of new magnetocaloric materials. Compounds containing gadolinium are one of the best candidates due to the large spin-only magnetic moment of Gd 3+ ion. This work reports on the magnetocaloric properties of the AGdS2 family (A = Li, Na, K, Rb) in relation to crystal chemistry of these compounds. These sulfides crystallize in two different structure-types: NaCl (A = Li) and -NaFeO2 (A = Na, K, Rb). Although one would expect the larger magnetocaloric effect to be associated to LiGdS2 due to its higher magnetic/non-magnetic mass ratio, our study demonstrates that the NaGdS2 member leads to the best properties among the investigated series. The change of structure from the 3D NaCl structure of LiGdS2 to the layered -NaFeO2 structure of NaGdS2 drastically improves the magnetocaloric properties.Hence, thanks to its structural features associated to negligible exchange interactions, NaGdS2 exhibits a magnetic entropy change up to 54 J kg -1 K -1 at 2.5 K for µ0H = 5 T, which is comparable to the top ranked inorganic Gd-based materials operating in the cryogenic temperature range. These magnetocaloric figures of merit provide evidence that Gd-based sulfides are promising materials for magnetic refrigeration and, more broadly, this highlights the potential of sulfides in that field.

Both layered multiple-anion compounds and homologous series are of interest for their electronic properties, including the ability to tune the properties by changing the nature or number of the layers. Here we expand, using both computational and experimental techniques, a recently reported three-anion material, Bi4O4Cu1.7Se2.7Cl0.3, to the homologous series Bi2+2n O2+2n Cu2−δSe2+n–δXδ (X = Cl, Br), composed of parent blocks that are well-studied thermoelectric materials. All of the materials show exceptionally low thermal conductivity (0.2 W/mK and lower) parallel to the axis of pressing of the pellets, as well as narrow band gaps (as low as 0.28 eV). Changing the number of layers affects the band gap, thermal conductivity, carrier type, and presence of a phase transition. Furthermore, the way in which the different numbers of layers are accessed, by tuning the compensating Cu vacancy concentration and halide substitution, represents a novel route to homologous series. This homologous series shows tunable properties, and the route explored here could be used to build new homologous series out of known structural blocks.