A Vacancy-Disordered, Oxygen-Deficient Perovskite with Long-Range Magnetic Ordering: Local and Average Structures and Magnetic Properties of Sr₂Fe_1.5Cr_0.5O₅

Abstract

The local and average crystal structures and magnetic properties of the oxygen-deficient perovskite Sr(2)Fe(1.5)Cr(0.5)O(5+y) were studied using powder X-ray and neutron diffraction, neutron-pair distribution function analysis, and electron energy-loss spectroscopy. This material crystallizes in the cubic Pm3m space group, with a = 3.94491(14) Å. The oxygen vacancies are distributed randomly throughout the perovskite-type structure, and the average coordination number of the Fe(Cr) sites is 5. Refinement of the neutron diffraction data indicates y ∼ 0.05. This is in discordance with an earlier report on a material with the same nominal composition and cell constant. Electron energy-loss Cr L(2,3)-edge spectroscopy shows that Cr(3+) is present, which is also contrary to previous speculation. Neutron-pair distribution function studies show that a brownmillerite-like model involving ordered vacancies and alternating octahedral and tetrahedral coordination at the metal sites, gives a better description of the local structure out to ∼5 Å. A remarkable phenomenon determined by neutron diffraction in Sr(2)Fe(1.5)Cr(0.5)O(5) is the occurrence of a long-range G-type antiferromagnetic ordering with T(c) ≈ 565 K because cubic oxygen-deficient perovskites with B-site disorder usually do not undergo transitions to magnetically ordered states. The observation of long-range antiferromagnetic order and the T(c) value are in accordance with previous Mössbauer spectroscopic studies.

Oxygen deficient perovskites: effect of structure on electrical conductivity, magnetism and electrocatalytic activity.

“…As a result, the average structure looks similar to that of a typical perovskite 70,88 ( Figure 6.2), but with partial occupancy on the oxygen positions (Table 6.1). This is in sharp contrast to the structure of CaSrFe2O6-δ (δ=1), 4 ( Figure 6.2) which is known to contain an ordered array of defects, that appear in alternating layers, converting the octahedral geometry into tetrahedra in those layers.…”

“…The vacant sites that are created as a result of oxygen-deficiency can spread in the structure arbitrarily, forming a disordered system. 11,70 An example is Sr2FeMnO5, 11 where oxide vacancies are distributed randomly, forming a disordered cubic structure. 11 However, it is also possible to form oxygen-deficient perovskites, in which vacancies have an ordered distribution.…”

“…72 The lack of structural order usually leads to the absence of magnetic order. 72 However, there are exceptions, such as Sr2Fe1.5Cr0.5O5, 76 where the vacancies are spread randomly in the structure, forming a disordered system, but the magnetic order is still present, and a long-range antiferromagnetic order is observed. 76 In this article, we describe the magnetic structure of CaSrFeCoO5, 77 determined using neutron diffraction experiments.…”

“…72 However, there are exceptions, such as Sr2Fe1.5Cr0.5O5, 76 where the vacancies are spread randomly in the structure, forming a disordered system, but the magnetic order is still present, and a long-range antiferromagnetic order is observed. 76 In this article, we describe the magnetic structure of CaSrFeCoO5, 77 determined using neutron diffraction experiments. The magnetic order has been discussed in the context of materials with similar compositions, to highlight the magnetic properties that are unique to this compound.…”

“…The crystal structure of oxygen-deficient perovskites also depends on the arrangement of defects that are generated due to oxygendeficiency. 4,[84][85][86][87] The defects can be distributed in a disordered 76,[88][89] or ordered fashion. 4, 12, 73-75, 84-87, 90 One of the common structures, resulting from the ordering of defects, is brownmillerite-type structure.…”