The Effect of Oxygen Stoichiometry on Phase Relations and Structure in the System La 1- xSr xFeO 3-δ (0 ≤ x ≤ 1, 0 ≤ δ ≤ 0.5)

Abstract

An extensive range of compounds in the La 1- x Sr x FeO 3-δ (0 ≤ x < 1, 0 < δ ≤ 0.5) system have been synthesized. When δ = 0 the system is composed of three crystallographically different regions: orthorhombic for 0 ≤ x ≤ 0.2, rhombohedral for 0.4 ≤ x ≤ 0.7, and cubic for 0.8 ≤ x ≤ 1.0. Products were characterized by thermogravimetrie analysis and Mössbauer spectroscopy yielding information on oxygen stoichiometry and iron oxidation state. At 4.2 K, Mössbauer spectra are composed of magnetic sextet components arising from different charge states of iron; in the orthorhombic and rhombohedral phases the charge states Fe 3+ and Fe 5+ coexist, and Fe 4+ and Fe 3+ are present in the cubic region. Room temperature Mössbauer spectra show that samples in the orthorhombic region are still magnetically ordered and the coexistence of Fe 3+ and Fe 5+ remains, while between x = 0.4 and x = 1.0 the samples are paramagnetic; in the rhombohedral region the spectra show the presence of Fe 4+ and Fe 3+, and in the cubic region both Fe 4+ and an intermediate valence state are assigned. High-resolution powder neutron diffraction data were collected on samples annealed under high-pressure oxygen, and full structure refinements were carried out using the Rietveld method. Accurate bond length data obtained for all phases show that the iron coordination geometry becomes more regular with increasing Sr, and hence Fe 4+, content. A phase diagram for the full range of lanthanum/strontium and oxygen stoichiometry is proposed.