Dynamic and structural properties of orthorhombic rare-earth manganites under high pressure

Abstract

We report a high-pressure study of orthorhombic rare-earth manganites AMnO 3 using Raman scattering (for A = 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 pressure that depends on the A cation. For the compounds with A = Pr, Sm, and Dy, XRD confirms the presence of a corresponding structural transition to a noncubic phase, so that the disappearance of the Raman spectrum can be interpreted as an insulator-to-metal transition. We analyze the compression mechanisms at work in the different manganites via the pressure dependence of the lattice parameters, the shear strain in the ac plane, and the Raman bands associated with out-of-phase MnO 6 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 smaller A cations considered in this study (Dy and Tb), the compression is nearly isotropic in the ac plane, with only small 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 significant as a compression mechanism, while the pressure-induced tilting of octahedra chains becomes conversely less pronounced. We finally discuss our results in light of the notion of chemical pressure and show that the analogy with hydrostatic pressure works quite well for manganites with the smaller A cations considered in this paper but can be misleading with large A cations.

Spin-phonon coupling and high-pressure phase transitions ofRMnO3(R=Caand Pr): An inelastic neutron scattering and first-principles study

“…Section (d) deals with the first principle DFT calculation of the phonon spectra taking into account the magnetic structure. Then in section (e) we apply the same ab-initio calculation to understand certain ambiguity of structural and insulator to metal transitions at high pressure as reported in the literature [13]. ), the latter one represents mostly the contribution from the neutron scattering on phonons.…”

“…The main emphasis of this study is to investigate the pressure dependence of the likely Jahn-Teller distortion and existence of insulator to metal transition at high pressure. Recently, Mota et al [13] reported a high pressure study of orthorhombic rare-earth manganites using a combination of synchrotron x ray diffraction and Raman scattering technique. The authors observed change in the diffraction patterns of PrMnO 3 and disappearance of the Raman spectrum with pressure.…”

“…The study of perovskite manganite RMnO 3 (R= Ca, La, Pr etc) has been of great relevance [6][7][8][9][10][11][12][13][14][15][16][17]. Among these, CaMnO 3 has received much interest in the past decade due to its structural [18], physical [19], magnetic [20], and thermo-electrical properties [21,22].…”

“…Recently, Mota et al [13] reported a high pressure study in which they observed changes in the x-ray diffraction patterns of PrMnO 3 and disappearance of the Raman spectrum at 45 GPa. The authors [13] suspected that the changes might be related to the structural phase transition from orthorhombic to tetragonal structure.…”

“…The authors [13] suspected that the changes might be related to the structural phase transition from orthorhombic to tetragonal structure. However, the disappearance of Raman peaks could also be due to insulator to metal transition.…”