Electronic, Magnetic and Structural Properties of the RFeO 3 Antiferromagnetic-Perovskites at Very High Pressures
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Electronic, Magnetic and Structural Properties of the RFeO3 Antiferromagnetic-Perovskites at Very High Pressures Moshe P. Pasternak, W. M. Xu, G. Kh. Rozenberg, and R. D. Taylor1 School of Physics and Astronomy, Tel Aviv University, Tel Aviv, ISRAEL, 1 MST-10, Los Alamos National Laboratory, Los Alamos, NM 87545, U.S.A.
ABSTRACT At ambient pressure the orthorhombic perovskites R-orthoferrites (R ≡ Lu, Eu, Y, Pr, and La) exhibit very large optical gaps. These large- gap Mott insulators in which the 3d5 high-spin ferric ions carry large local moments and magnetically order at TN > 600 K, undergo a sluggish structural first-order phase transition in the 30-50 GPa range, with the exception of the LuFeO3 which undergoes an isostructural volume reduction resulting from a high to low-spin crossover. High-pressure methods to 170 GPa using Mössbauer spectroscopy, resistance, and synchrotronbased XRD in diamond anvil cells were applied. Following the quasi-isostructural volume reduction (3-5%) the new phase the magnetic-ordering temperature is drastically reduced, to ~ 100 K, the direct and super-exchange interactions are drastically weakened, and the charge-transfer gap is substantially reduced. The high-pressure (HP) phases of the La and Pr oxides, at their inception, are composed of high- and low-spin Fe3+ magnetic sublattices, the abundance of the latter increasing with pressure but HP phases of the Eu, Y, and Lu oxides consist solely of low-spin Fe3+. Resistance and Mössbauer studies in La and Pr orthoferrites reveal the onset of a metallic state with moments starting at P > 120 GPa. Based on the magnetic and electrical data of the latter species, a Mott phase diagram was established. INTRODUCTION The rare-earth orthoferrites - RFeO3 - crystallize in a distorted orthorhombic structure, a derivative of cubic perovskite, with space group Pbnm. The unit cell contains four equivalent Fe3+ ions situated at octahedral centers formed by six nearest-neighbor oxygens. The tilting of the octahedral axes with respect to the c-axis is minimal, of the order of 10 mrads.[1]. The common apex of two adjacent octahedral is the intervening R3+ cation that provides the superexchange interaction path between two iron atoms. Thus, each iron ion is coupled by superexchange to six iron nearest neighbors resulting in relatively high Néel temperatures (TN) ranging from 620 to 740 K for Lu and La, respectively. The ferric ions are antiferromagnetically coupled. The main motivation for this systematic study is the unique opportunity to vary the R-cation size, keeping practically the same ambient-pressure crystallographic structure and same nature of the R3+ - O chemical bond thus allowing to examine the effect of the R3+ ionic radii, which range from 86 pm (Lu3+) to 103.2 pm (La3+), on the evolution and properties of the ensuing new high-pressure phases. Recently a detailed experimental study of the pressure-induced Mott-Hubbard phase diagram of the large La- and Pr-orthoferrites has been published [2]. Those results and previously, D2.7.1 Downlo
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