Structure and Properties of Sr_(1-x)Ca_x:Mn_(0.5)Ru_(0.5)O3 Perovskites: Using Chemical Pressure to Control Mn/Ru Mixed Valency

摘要

The structure and properties of Sr_(1-x)Ca_xRu_(0.5)Mn_(0.5)O3 compositions have been investigated. Both bond distances and X-ray absorption measurements reveal Mn~(3+) + Ru~(5+) Mn~(4+) + Ru~(4+) 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 e_g 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 (a~0a~0c~- tilt system). The CJTD results from orbital ordering involving occupied d_(z~2) orbitals on Mn~(3+), 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 Ca~(2+) for Sr~(2+) triggers additional rotations of the octahedra (a~-b~+a~- tilt system) that result in orthorhombic symmetry for Sr_(1-x)Ca_xMn_(0.5)Ru_(0.5)O3 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 Ca~(2+) content increases there is a shift in the valence degeneracy toward Mn~(4+) + Ru~(4+). 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 CaRu_(0.5)Mn_(0.5)O3 undergoes magnetic phase separation into a disordered ferrimagnetic state (T_C ≈ 230 K) and a G-type antiferromagnetic state (T_N ≈ 95 K).