We report a high-pressure study of orthorhombic rare-earth manganites AMnO3 using Raman scattering (forA = Pr, Nd, Sm, Eu, Tb, and Dy) and synchrotron x-ray diffraction (XRD), for A = Pr, Sm, Eu, and Dy.In all cases, a phase transition was evidenced by the disappearance of the Raman signal at a critical pressurethat depends on the A cation. For the compounds with A = Pr, Sm, and Dy, XRD confirms the presence ofa corresponding structural transition to a noncubic phase, so that the disappearance of the Raman spectrumcan be interpreted as an insulator-to-metal transition. We analyze the compression mechanisms at work in thedifferent manganites via the pressure dependence of the lattice parameters, the shear strain in the ac plane, andthe Raman bands associated with out-of-phase MnO6 rotations and in-plane O2 symmetric stretching modes.Our data show a crossover across the rare-earth series between two different kinds of behavior. For the smallerA cations considered in this study (Dy and Tb), the compression is nearly isotropic in the ac plane, with onlysmall evolutions of the tilt angles and cooperative Jahn-Teller distortion. As the radius of the A cation increases,the pressure-induced reduction of Jahn-Teller distortion becomes more pronounced and increasingly significantas a compression mechanism, while the pressure-induced tilting of octahedra chains becomes conversely lesspronounced. We finally discuss our results in light of the notion of chemical pressure and show that the analogywith hydrostatic pressure works quite well for manganites with the smaller A cations considered in this paper butcan be misleading with large A cations.
展开▼
