Structural Chemistry and Metamagnetism of an Homologous Series of Layered Manganese Oxysulfides
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Abstract
An homologous series of layered oxysulfides Sr2MnO2Cu(2m-delta)S(m+1) with metamagnetic properties is described. Sr2MnO2Cu(2-delta)S2 (m = 1), Sr2MnO2Cu(4-delta)S3 (m = 2) and Sr2MnO2Cu(6-delta)S4 (m = 3), consist of MnO2 sheets separated from antifluorite-type copper sulfide layers of variable thickness by Sr(2+) ions. All three compounds show substantial and similar copper deficiencies (delta approximately equal 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 MnO(2) sheet couple ferromagnetically and that weaker antiferromagnetic coupling between sheets leads to A-type antiferromagnets in zero applied magnetic field. Sr2MnO2Cu(5.5)S4 and Sr2MnO2Cu(3.5)S3 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.