Structural behavior and thermoelectric properties of the brownmillerite system Ca2(ZnxFe2−x)O5

Pressure- and temperature-dependent Raman spectra of Ca2Fe2O5 oxygen defect perovskite

The Raman spectra of Ca₂Fe₂O₅ were investigated up to 21.8 GPa at room temperature and up to 1073 K at ambient pressure, respectively. A phase transition begins around 13.6 GPa and it is reversible after decompression. No temperature-induced phase transition was observed due to the quality of Raman spectra at temperatures above 773 K. The effects of pressure and temperature on the Raman vibration were quantitatively analyzed. All the observed Raman active vibrations of Ca₂Fe₂O₅ show positive linear pressure dependences and negative temperature dependences with different slopes. Combined with previous experimental results, the isothermal and isobaric mode Grüneisen parameters of Ca₂Fe₂O₅ were estimated, and the intrinsic anharmonicity was discussed.

Unraveling the Role of Structural Order in the Transformation of Electrical Conductivity in Ca2FeCoO6-δ, CaSrFeCoO6-δ, and Sr2FeCoO6-δ

The ability to control the electrical conductivity of solid-state oxides using structural parameters has been demonstrated. A correlation has been established between the electrical conductivity and structural order in a series of oxygen-deficient perovskites using X-ray and neutron diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and electrical conductivity studies at a wide temperature range, 25-800 °C. The crystal structure of CaSrFeCoO_6-δ has been determined, and its stark contrast to Ca₂FeCoO_6-δ and Sr₂FeCoO_6-δ has been demonstrated. The Fe/Co distribution over tetrahedral and octahedral sites has been determined using neutron diffraction. There is a systematic increase in the structural order in progression from Sr₂FeCoO_6-δ (δ = 0.5) to CaSrFeCoO_6-δ (δ = 0.8) and Ca₂FeCoO_6-δ (δ = 0.9) . The oxygen contents of these materials were determined using iodometric titration and TGA. At room temperature, there is an inverse correlation between the electrical conductivity and structural order. The ordered Ca₂ and CaSr compounds are semiconductors, while the disordered Sr₂ compund shows metallic behavior. The metallic nature of the Sr₂ material persists up to 1073 K (800 °C), while the Ca₂ and CaSr compounds undergo a semiconductor-to-metal transition above 500 and 300 °C, respectively, highlighting another important impact of the structural order. At high temperature, the CaSr compound has the highest conductivity compared to the Ca₂ and Sr₂ materials. There appears to be an optimum degree of structural order that leads to the highest conductivity at high temperature. Another consequence of the structural order is the observation of mixed ionic-electronic conductivity in CaSr and Ca₂ compounds, as is evident from the hysteresis in the conductivity data obtained during heating and cooling cycles. The average ionic radius required for each structural transition was determined through the synthesis of 21 different materials by systematic variation of the Ca/Sr ratio. In addition, SEM and XPS were employed to gain insight into the crystallite morphology and oxidation states of transition metals, revealing an interesting redox process between Fe and Co.

Weak ferromagnetic ordering in brownmillerite Ca2Fe2O5 and its effect on electric field gradients

Electric field gradients in weakly ferromagnetic CaFe₂O₅ remarkably change below T_N, because of local electric polarizations induced by Dzyalosinskii-Moriya interactions.