Abstract
Here, we present a comprehensive study that encompasses changes within the crystal and magnetic structure in the brownmillerite-type phase Ca2Fe2O5 induced by the substitution of Fe3+ with Ga3+. 61 synthetic single-crystal samples of Ca2Fe2−xGaxO5 0.00 ≤ x ≤ 1.328 have been investigated by single-crystal X-ray diffraction at 25 °C. We find that pure Ca2Fe2O5 and samples up to x ~ 1.0 have space group Pnma, Z = 4, whereas samples with x > 1.0 show I2mb symmetry, Z = 4. The Raman spectroscopic measurements exhibit that the change from Pnma to I2mb space group symmetry is reflected by a significant shift of two Raman modes below 150 cm−1. These Raman modes are obviously linked to changes in the Ca–O bond lengths at the phase transition. 57Fe Mössbauer spectroscopy was used to characterize the cation distribution and magnetic structure as a function of composition and temperature. Thereby, the strong preference of Ga3+ for the tetrahedral site is verified, as an independent method besides XRD. At room-temperature, Ca2Fe2−xGaxO5 solid solution compounds with 0 ≤ x ≤ 1.0 are antiferromagnetic ordered, as revealed by the appearance of magnetically split sextets in the Mössbauer spectra; samples with higher Ga3+ contents are paramagnetic. Over and above, the substitution of Fe3+ by Ga3+ results in the appearance of sharp, additional magnetic hyperfine split sextets, which can be attributed to cluster configurations within the individual tetrahedral chains. The temperature-dependent (20–720 K) Mössbauer study reveals a transition from the magnetically ordered to the paramagnetic state at a temperature of about 710 K for the Ca2Fe2O5 end-member.
Highlights
Brownmillerite-type compounds with the general formula A2B’xB2−xO5, where A = alkaline earth metals (e.g. Ca, Sr, Ba) and B’/B = group III or transition-metal atoms (e.g. Al, Ga, Cr, Mn, Co, Fe, and In), are among the most frequently studied oxygen-deficient perovskites. (Abakumov et al 2004; Anderson et al 1993; Parsons et al 2009; Tilley 2016)
The interest in Brownmillerite compounds has risen considerably, as these are handled as promising candidates for oxygen ion conductors, which operate at moderate temperatures for the use as electrodes in solid oxygen fuel cells (SOFC), ceramic membranes for oxygen
The Bragg peaks 111, or 131, appearing between 23–24° and 29–30° 2θ, can be used to detect the phase transition from Pnma to a body-centered cell. These h + k + l = 2n + 1 reflections are forbidden in the body-centered cell of the I2mb space group
Summary
At room temperature, Ca2Fe2 −xGaxO5 solid solution compounds with 0 ≤ x ≤ 1.0 are antiferromagnetic ordered, as revealed by the appearance of magnetically split sextets in the Mössbauer spectra, samples with higher Ga3+ contents are paramagnetic. The size of internal magnetic field of the different components at the tetrahedral site decreases with increasing Ga3+ content.
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