Mixed Valence in YBaFe₂O₅

Abstract

YBaFe(2)O(5) has been synthesized by heating a nanoscale citrate precursor in a carefully controlled reducing environment. Successful synthesis of a single-phase sample can only be achieved in a narrow window of oxygen partial pressures and temperatures. YBaFe(2)O(5) adopts an oxygen-deficient perovskite-type structure, which contains double layers of corner sharing FeO(5) square pyramids separated by Y(3+) ions. At T(N) congruent with 430 K, tetragonal (P4/mmm) and paramagnetic YBaFe(2)O(5) orders antiferromagnetically (AFM) experiencing a slight orthorhombic distortion (Pmmm). Around this temperature, it can be characterized as a class-III mixed valence (MV) compound, where all iron atoms exist as equivalent MV Fe(2.5+) ions. The magnetic structure is characterized by AFM Fe-O-Fe superexchange coupling within the double layers and a ferromagnetic Fe-Fe direct-exchange coupling between neighboring double layers. Upon cooling below approximately 335 K, a premonitory charge ordering (2Fe(2.5+) --> Fe(2.5+delta) + Fe(2.5)(-delta)) into a class-II MV phase takes place. This transition is detected by differential scanning calorimetry, but powder diffraction techniques fail to detect any volume change or a long-range structural order. At approximately 308 K, a complete charge ordering (2Fe(2.5+) --> Fe(2+) + Fe(3+)) into a class-I MV compound takes place. This charge localization triggers a number of changes in the crystal, magnetic, and electronic structure of YBaFe(2)O(5). The magnetic structure rearranges to a G-type AFM structure, where both the Fe-O-Fe superexchange and the Fe-Fe direct-exchange couplings are antiferromagnetic. The crystal structure rearranges (Pmma) to accommodate alternating chains of Fe(2+) and Fe(3+) running along b and an unexpectedly large cooperative Jahn-Teller distortion about the high-spin Fe(2+) ions. This order of charges does not fulfill the Anderson condition, and it rather corresponds to an ordering of doubly occupied Fe(2+) d(xz) orbitals. Comparisons with YBaMn(2)O(5) and YBaCo(2)O(5) are made to highlight the impact of changing the d-electron count.

Orbital occupancy evolution across spin- and charge-ordering transitions in YBaFe 2 O 5

“…Its valence-mixed phase crystallizes as a double-cell perovskite of Pmmm symmetry, with Y and Ba atoms ordered into layers [11,12]. The Y layer, free of oxygen atoms, leaves the two Fe 2.5+ in squarepyramidal coordinations facing each other as a mirror image.…”

“…The Y layer, free of oxygen atoms, leaves the two Fe 2.5+ in squarepyramidal coordinations facing each other as a mirror image. These two high-spin iron atoms couple ferromagnetically (FM) in an overall antiferromagnetic (AFM) order of spins along a slightly contracted b (as opposed to a) of the perovskite cell [11,13]. In the charge-ordered YBaFe 2 O 5 , also these two Fe atoms couple AFM in a magnetic structure of the Wollan-Koehler [14] G-type.…”

“…In the charge-ordered YBaFe 2 O 5 , also these two Fe atoms couple AFM in a magnetic structure of the Wollan-Koehler [14] G-type. The orthorhombic distortion is increased profoundly by ordering the doubly occupied orbitals d xz in the infinite -Fe 2+ -O-Fe 2+ -chains along b of the Pmma structure formed upon further doubling the unit cell [11]. Powder-diffraction methods "see" that the two phases coexist as they convert to each other, and this defines their Verwey-type phase transition as discontinuous.…”

“…According to Boltzmann's formula and Stirling approximation, the mixing entropy of two valences is R R −2 ( ln + ln ) = 2 ln2 per YBaFe 2 O 5 . Differential scanning calorimetry (DSC) detects about 3/4 of it and reveals that it is acquired in two steps [11,17]. Almost all mixing entropy is supplied at the first step; seen by powder diffraction as a loss of the Fe 2+ -Fe 3+ order [11] at the transition temperature denoted T V .…”

“…Differential scanning calorimetry (DSC) detects about 3/4 of it and reveals that it is acquired in two steps [11,17]. Almost all mixing entropy is supplied at the first step; seen by powder diffraction as a loss of the Fe 2+ -Fe 3+ order [11] at the transition temperature denoted T V . The entropy of the second step, at T p , is much smaller and no discontinuous structural change is detected by powder diffraction; only a smooth decrease of the tiny orthorhombic distortion that vanishes at the Néel temperature T N [13,18].…”