Strain effects in perovskite manganites

Colossal magnetoresistance (CMR) – the dramatic reduction of electrical resistivity in a magnetic field – and charge ordering (CO) in rare earth manganites of the type Ln1−xAxMnO3 [Ln: rare earth, A: divalent cation] are manifestations of the intricate relation between orbital, spin, charge and lattice degrees of freedom. Recent studies indicate that the inhomogeneous state of mixed-valence manganites – evidenced by the presence of texture and multiple phase coexistence – is important for the CMR property of manganites. Theoretical models that explain the multiphase coexistence are based on quenched disorder or strain. Here we show that lattice strain due to the Jahn–Teller (JT) distortions of MnO6 octahedra and their tilt rotations are not sufficient to provide a unique structure–property relation. We present evidence that the science of manganites should take into account shear distortions of the MnO6 octahedra as well. Pressure evolution of the lattice strain of Nd0.5Ca0.5MnO3 shows a minimum around 7 GPa, with the same lattice strain above and below this pressure achieved by shear- and JT-type distortions, respectively. In general, a particular lattice strain of manganites can be achieved by different combinations of JT-type and shear-type distortions of the MnO6 octahedra, together with their tilts, which suggest a plausible description of the inhomogeneous state in manganites as one where phases with differently distorted states having the same lattice strain are preserved.