Impact of high-pressure evolution of elementary distortions on the phase-transition of RFeO_3

11 Sept 2019, 11:30
20m

Speaker

Rui Vilarinho (IFIMUP, Departament of Physics and Astronomy, Faculty of Sciences, University of Porto )

Description

Rare-earth orthoferrites (RFeO3) have been intensively studied in last years, as they exhibit a variety of interesting physical properties, with a large number of works having addressed the underlying microscopic mechanisms [1]. The rare-earth size drives the cooperative rotations of the FeO6 octahedra, which is known to linearly scale with the octahedra tilt angles [1]. Their physics has been studied with hydrostatic pressure as external parameter. Though empirical rules regarding the pressure dependence of octahedral tilts and rare-earth displacements in 3:3 perovskites have been proposed [2], a systematic and detailed experimental study of the pressure evolution of the lattice distortions of a representative set of RFeO3 is still missing. Moreover, contradictory theoretical results have been discussed in literature, such as the prediction that the octahedral tilts and rare-earth displacements should decrease with pressure, via their trilinear coupling [2]. This decrease should yield a structural phase transition into a higher-symmetric structure at some critical pressure PC, which, in fact, is not the case of RFeO3, presenting a pressure-driven isostructural transition [3].
Our work reports on the evolution of the octahedral tilts and mean Fe-O bond lengths of RFeO3 (R = Nd, Sm, Eu, Gd, Tb and Dy) with applied hydrostatic pressure by Raman scattering and synchrotron XRD up to 55 GPa. We have found out that the octahedra tilts decreases with pressure for R = Nd-Sm, whereas it increases for R = Tb-Lu, affecting the displacement of the rare-earths due to trilinear coupling, and compression rate of the FeO6 octahedra [3]. EuFeO3 stands at the borderline, with nearly pressure-independent tilt angles. The surprising crossover between the two opposite pressure behaviors is discussed in relation with the general rules proposed from different theoretical approaches. The similarity of the pressure-induced isostructural insulator-to-metal phase transition, observed in the whole series, point out that the tilts play a minor role in its driving mechanisms. A clear relationship between octahedral compressibility and critical pressure is ascertained with respect to a critical volume of the FeO6 octahedra.

[1] E. Bousquet and A. Cano, J. Phys.: Cond. Matt. 28, 123001 (2016)
[2] H. J. Xiang, J. Íñiguez, J. Kreisel and L. Bellaiche, Phys. Rev. B 96, 054102 (2017)
[3] R. Vilarinho, P. Bouvier, M. Guennou, I. Peral, et al., Phys. Rev. B 99, 064109 (2019)

Summary

This work reports on the evolution of the octahedral tilts and mean Fe-O bond lengths of RFeO3 (R = Nd, Sm, Eu, Gd, Tb and Dy) with applied hydrostatic pressure by Raman scattering and synchrotron XRD up to 55 GPa, and their impact on the high-pressure structural insulator-to-metal phase transition.

Topic 9. Materials under high pressure

Primary authors

Rui Vilarinho (IFIMUP, Departament of Physics and Astronomy, Faculty of Sciences, University of Porto ) Dr Mael Guennou (Luxembourg Institute of Science and Technology, rue du Brill, Belvaux, Luxembourg) Dr Pierre Bouvier (Université Grenoble-Alpes, CNRS, Institut Néel, 38000 Grenoble, France) Dr Mads Weber (Luxembourg Institute of Science and Technology, rue du Brill, Belvaux, Luxembourg) Dr Inmaculada Peral (Luxembourg Institute of Science and Technology, rue du Brill, Belvaux, Luxembourg) Prof. Pedro Tavares (Centro de Química, Universidade de Trás-os-Montes e Alto Douro, Vila Real, Portugal) Dr Gaston Garbarino (European Synchrotron Radiation Facility, 38043 Grenoble, France ) Dr Mohamed Mezouar (European Synchrotron Radiation Facility, 38043 Grenoble, France ) Prof. Jens Kreisel (Luxembourg Institute of Science and Technology, rue du Brill, Belvaux, Luxembourg) Prof. Abílio Almeida (IFIMUP, Departament of Physics and Astronomy, Faculty of Sciences, University of Porto ) Prof. Joaquim Agostinho Moreira (IFIMUP, Departament of Physics and Astronomy, Faculty of Sciences, University of Porto )

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