Verwey transition in mixed-valenceTbBaFe2O5: Two attempts to order charges

Abstract

Electronic, magnetic, and structural phase transitions in nearly stoichiometric TbBaFe 2 O 5ϩw (0.00Ͻw Ͻ0.05) have been investigated. At high temperatures this compound is a paramagnetic, mixed-valence (Fe 2.5ϩ ) conductor with identical square-pyramidal coordinations at all iron atoms. Upon cooling below T N ϭ450 K, an antiferromagnetic ͑AFM͒ spin order appears, accompanied by a magnetostrictive orthorhombic distortion. At lower temperatures the increasing distortion sets the frame for a first attempt to order charges. Mössbauer spectroscopy shows that one squeezed and one expanded square pyramid appear with different orientations of their magnetic and electric field tensors, each centered by its own mixed-valence iron state, one Fe 2.5ϩ⑀ , the other Fe 2.5Ϫ⑀ . The lattice retains its distortion, but a small, structurally homogeneous, and continuous increase in volume is experienced. At somewhat lower temperature (T V ) a discontinuous increase of the orthorhombic distortion occurs, marking the second attempt to order charges, now with the classical symptoms of the Verwey transition: a large change in volume, entropy, and electrical conductivity. Below T V , a normal Fe 3ϩ high-spin state in a symmetrical square-pyramidal coordination appears, whereas Fe 2ϩ is distorted. The long-range order of this arrangement is solved from high-resolution powder neutron diffraction data. Rietveld refinements show that the charge-ordered spins have AFM interactions in all three directions (G type͒ whereas in the mixed-valence state a ferromagnetic ͑FM͒ interaction appears between the iron atoms facing each other across the Tb layer. This FM interaction is suggested to be essential for the appearance of the mixed-valence state via the double-exchange sharing of the Fe 2ϩ -originated electron. This also allows for the total ordered spin moment being unchanged at the Verwey transition, following one single Brillouin curve. Analogous cases are pointed out where the Verwey transition proceeds in a similar manner, also at the molecular level.

Oxygen and cation ordered perovskite, Ba2Y2Mn4O11

“…BaYCo 2 O 5.0 (DP) [4]) and simultaneous valence-separation and charge-ordering transition (e.g. BaREFe 2 O 5.0 (DP) [5,6]). …”

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

“…Upon cooling through T V , these ab-plane domain walls disappear as all FM couplings become AFM [13]. As their Fe atoms are ready to join the charge order, the total concentration of the planar-defect components decreases to about 2/3 below T V .…”

“…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].…”

“…12), typical also for other rare-earth variants of YBaFe 2 O 5 [13,[24][25][26], where the large dipolar field of the minority-spin electron in d xz nearly cancels the Fermi-contact field. This same electron makes the EFG large (Fig.…”

“…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.…”