Crossover in the pressure evolution of elementary distortions inRFeO3perovskites and its impact on their phase transition

Abstract

This work reports on the pressure dependence of the octahedra tilts and mean Fe-O bond lengths in RFeO3 (R=Nd, Sm, Eu, Gd, Tb and Dy), determined through synchrotron X-ray diffraction and Raman scattering, and their role on the pressure induced phase transition displayed by all of these compounds. For larger rare-earth cations (Nd-Sm), both anti-and in-phase octahedra tilting decrease as pressure increases, whereas the reverse behavior is observed for smaller ones (Gd-Dy). EuFeO3 stands at the borderline, as the tilts are pressure independent. For the compounds where the tilts increase with pressure, the FeO6 octahedra are compressed at lower rates than for those ones exhibiting opposite pressure tilt dependence. The crossover between the two opposite pressure behaviors is discussed and faced with the rules grounded on the current 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 octahedra compressibility and critical pressure is ascertained.

Investigating the anisotropic compression and high-pressure phase symmetry of orthorhombic RFeO3 vs RMnO3

“…This part of the work will be mainly based on the experimental results reported by R. Vilarinho et al for the RFeO3 [6] and by D. Mota et al for the RMnO3 [5], as they embraced the high-pressure behaviour for a large set of rare-earth ions in these systems. These works were based on the synchrotron x-ray diffraction and Raman scattering techniques.…”

“…The transition is reversible being both the Raman spectrum and the crystallographic structure recovered upon pressure decreasing, with a significant hysteresis ranging between 3 to 6 GPa depending on the rare-earth cation. The value for the critical pressure PC, obtained from the synchrotron x-ray diffraction and Raman scattering data, are depicted in Figure 1 [5,6]. In both cases, as the rare-earth cation size decreases, the critical pressure increases.…”

“…A general scenario can be summarized, wherein both types of compounds undergo compression before a structural phase transition occurs at PC in the 35 to 50 GPa range, depending on the rare-earth ion. This phase transition, usually of the first-order, with a volume collapse of the order of 5%, is accompanied with an insulator-to-metal transition, which has been explained by a high-spin to low-spin state change of the Fe 3+ and Mn 3+ cations by Mössbauer spectroscopy [5,6,[9][10][11][12].…”

“…Hydrostatic pressure allows modifying the interatomic distances and, thus, tuning the interactions to a much larger extent that any other external parameters. We have amply used this advantage in the past for the understanding of model perovskites [4][5][6].…”

“…In order to disclose the effect of the structural distortions referred to above on the physical properties of RFeO3, RMnO3, their high-pressure behaviour has been studied to some extend by x-ray diffraction, Mössbauer spectroscopy, and resistivity measurements [ 5,6,[9][10][11][12][13][14]. A general scenario can be summarized, wherein both types of compounds undergo compression before a structural phase transition occurs at PC in the 35 to 50 GPa range, depending on the rare-earth ion.…”