Combined Experimental and Computational Study of Oxide Ion Conduction Dynamics in Sr₂Fe₂O₅ Brownmillerite

Abstract

We report a detailed study of the dynamics of oxide ionic conduction in brownmillerite-type Sr 2 Fe 2 O 5 , including lattice anisotropy, based on neutron scattering studies of a large (partially twinned) single crystal in combination with ab initio molecular dynamics simulations. Single-crystal diffraction reveals supercell peaks due to long-range ordering among chains of corner-sharing FeO 4 tetrahedra, which disappears on heating above 540 °C due to confined local rotations of tetrahedra. Our simulations show that these rotations are essentially isotropic, but are a precondition for the anisotropic motion that moves oxide ions into the tetrahedral layers from the octahedral layers, which we observe experimentally as a Lorentzian broadening of the quasielastic neutron scattering spectrum. This continual but incoherent movement of oxide ions in turn creates conduction pathways and activates long-range diffusion at the interface between layers, which appears to be largely isotropic in two dimensions, in contrast with previously proposed mechanisms that suggest diffusion occurs preferentially along the c axis.

Crystal structure and electronic properties of bulk and thin film brownmillerite oxides

“…33 Additionally, more recent neutron diffraction experiments on single crystals grown by the floating zone method have indicated that P bcm is indeed the preferred structure type for Sr 2 Fe 2 O 5 . 18,19 Furthermore, all of these recent experiments are supported by the aforementioned results of our ab initio calculations; the small energy difference between the P bcm and I2bm structures, however, indicate that intergrowths of the phases is not out of the question. In contrast to the ambiguity of Sr 2 Fe 2 O 5 , the structure of Ca 2 Fe 2 O 5 is well known to form in the P nma phase, 17 again in agreement with our theoretical results.…”

A new defect chemistry model for Nb-doped SrCoO2.5+δ: The role of oxygen interstitials and delocalized-to-localized electron holes

“…where, N A is the Avogadro constant, 6.02×10 23 (mol -1 ); a is the lattice constant of a pseudo-cubic SCN (with 10mol%Nb doping) is 3.90 Å 35 . According to prior studies, Co-3d orbital and O-2p orbital tend to hybridize strongly in SCO systems 21,32 , producing a broad conduction band. The mean-free-path between hole scattering event is short to give a weak temperature dependence of mobility =e/m* 36 , where e is the charge of the charge carrier;  and m* are collision time and effective mass of the charge-carrier, respectively.…”

“…Furthermore, these ordered oxygen vacancies have also been considered as good candidates for anion intercalation such as O 2and Fwith a full occupation of the vacant sites to yield a simple perovskite structure [29][30][31] . Aukett et al further confirmed by Lorentzian broadening of the quasielastic neutron scattering spectrum that induced rotations of BO 4 by higher temperatures are a precondition for moving oxide-ion into tetrahedral layers from the octahedral layers 32,33 , creating oxideion conduction pathways and activating long-range diffusion at the interface between layers; the latter is thought to be the driving force for oxygen vacancies disorder and isotropic oxygen diffusion. was given in their original paper why ( ) was chosen as a point defect.…”

Atomic occupancy mechanism in brownmillerite Ca2FeAlO5 from a thermodynamic perspective

“…For example, brownmillerite SrCoO 2.5 (SCO) can undergo polymorphic phase transition under electric field control and has resistive switching behavior. 5,[7][8][9][10] Brownmillerite SrFeO x (SFO) with similar structure of SCO has been extensively studied due to its promising application in the fields of electrodes of fuel cell, magnetic storage, redox reaction catalyst, [11][12][13][14][15] etc. In addition, partial substitutions of the B specie of ABO 2.5 produce complex brownmillerite oxides such as Ca 2 FeAlO 5 , Ca 2 FeCoO 5 , etc.…”