Structural Chemistry and Metamagnetism of an Homologous Series of Layered Manganese Oxysulfides

An homologous series of layered oxysulfides Sr2MnO2Cu2m-δSm+1 with metamagnetic properties
is described. Sr2MnO2Cu2-δS2 (m = 1), Sr2MnO2Cu4-δS3 (m = 2) and Sr2MnO2Cu6-δS4 (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 (δ ≈ 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+δ)+ 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 Ω 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.