Synthesis, structure, and properties of randomly mixed and layer-ordered SrMn1-xGaO3-δ perovskites

We report the synthesis of SrMn1-xGaxO3-delta perovskite compounds and describe the dependence of their phase stability and structural and physical properties over extended cation and oxygen composition ranges. Using special synthesis techniques, we have extended the solubility limit of Ga3+ in the cubic perovskite phase to xapproximate to0.33. Higher Ga concentrations lead to mixed phases until a single-phase ordered double-perovskite structure is obtained at x = 0.5, i.e., Sr2MnGaO6-delta. In the cubic perovskite phase the maximum oxygen content is 3 - x/2, which corresponds to 100% Mn4+. All maximally oxygenated solid solution compounds are found to order antiferromagnetically, with the transition temperature linearly decreasing as Ga content increases. Reducing the oxygen content introduces frustration into the magnetic system and a spin-glass state is observed for SrMn0.7Ga0.3O2.5 below 30 K. The brownmillerite phase at low oxygen content, Sr2MnGaO5, is found to have Icmm crystallographic symmetry. At 12 K its magnetic structure is found to order in the magnetic symmetry corresponding to a G-type antiferromagnetic structure of Mn3+ ions. At higher oxygen content, Sr2MnGaO5.5 is found to have Cmmm crystallographic symmetry with disordered oxygen vacancies. At 12 K two competing long-range magnetic structures are found for the Mn4+ sublattice having C(I)m'm'm symmetry (G-type), and C(p)m'm'm symmetry (C-type), together with a G-type short-range magnetic correlations. (C) 2003 Elsevier Inc. All rights reserved.