Structure and Properties of Sr1−xCaxMn0.5Ru0.5O3 Perovskites: Using Chemical Pressure to Control Mn/Ru Mixed Valency
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Abstract
The structure and properties of Sr1−xCaxRu0.5Mn0.5O3 compositions have been investigated. Both bond distances and X-ray absorption measurements reveal Mn3+ + Ru5+ ↔ Mn4+ + Ru4+ mixed valency across the entire series. Despite a complete lack of Ru/Mn chemical order, all samples magnetically order between 220 and 300 K. The characteristics and type of magnetic order are sensitive to the occupation and ordering of the Mn eg orbitals, which can be manipulated by changes in chemical pressure, via the Sr/Ca ratio. Sr-rich samples are tetragonally distorted by a cooperative Jahn−Teller distortion (CJTD) that leads to an elongation of the c-axis as well as antiphase rotations of the octahedra about the c-axis (a0a0c− tilt system). The CJTD results from orbital ordering involving occupied dz2 orbitals on Mn3+, which stabilize C-type antiferromagnetic order. For Sr-rich samples, the various oxidation states contribute in approximately equal proportions (i.e., Ru+4.5 and Mn+3.5). Substituting Ca2+ for Sr2+ triggers additional rotations of the octahedra (a−b+a− tilt system) that result in orthorhombic symmetry for Sr1−xCaxMn0.5Ru0.5O3 samples with x ≥ 0.3. The crossover to orthorhombic symmetry is accompanied by the loss of orbital order and the emergence of an itinerant electron ferrimagnetic state. X-ray absorption near edge structure (XANES) measurements show that as the Ca2+ content increases there is a shift in the valence degeneracy toward Mn4+ + Ru4+. This helps to explain the absence of orbital ordering in Ca-rich samples, as well as the crossover from antiferromagnetism to ferrimagnetism. Neutron diffraction and ac susceptibility measurements show that CaRu0.5Mn0.5O3 undergoes magnetic phase separation into a disordered ferrimagnetic state (TC ≈ 230 K) and a G-type antiferromagnetic state (TN ≈ 95 K).
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