Structural chemistry and metamagnetism of an homologous series of layered manganese oxysulfides

An homologous series of layered oxysulfides Sr2MnO2Cu2m-delta Sm+1 with metamagnetic properties is described. Sr2MnO2Cu2-delta S2 ( m) 1), Sr2MnO2Cu4-delta S3 ( m) 2) and Sr2MnO2Cu6-delta S4 ( m) 3), consist of MnO2 sheets separated from antifluorite-type copper sulfide layers of variable thickness by Sr2+ ions. All three compounds show substantial and similar copper deficiencies (delta approximate to 0.5) in the copper sulfide layers, and single-crystal X- ray and powder neutron diffraction measurements show that the copper ions in the m) 2 and m) 3 compounds are crystallographically disordered, consistent with the possibility of high two-dimensional copper ion mobility. Magnetic susceptibility measurements show high- temperature Curie-Weiss behavior with magnetic moments consistent with high spin manganese ions which have been oxidized to the ( 2+delta)(+) state in order to maintain a full Cu-3d/S-3p valence band, and the compounds are correspondingly p-type semiconductors with resistivities around 25 Omega cm at 295 K. Positive Weiss temperatures indicate net ferromagnetic interactions between moments. Accordingly, magnetic susceptibility measurements and low-temperature powder neutron diffraction measurements show that the moments within a MnO2 sheet couple ferromagnetically and that weaker antiferromagnetic coupling between sheets leads to A-type antiferromagnets in zero applied magnetic field. Sr2MnO2Cu5.5S4 and Sr2MnO2Cu3.5S3 are metamagnets which may be driven into the fully ordered ferromagnetic state below 25 K by the application of fields of 0.06 and 1.3 T respectively. The relationships between the compositions, structures, and physical properties of these compounds, and the prospects for chemical control of the properties, are discussed.