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

The Ca2(ZnxFe2−x)O5 series was synthesized and characterized to determine the influence of zinc dopant on the brownmillerite structure for thermoelectric applications. All single-phase compounds exhibited Pnma symmetry at room temperature up to the solubility limit at x=0.10. High-temperature X-ray powder diffraction was used to show that the nature of the Pnma–Imma(0 0 γ)s00 transition in Ca2Fe2O5 is modified by the presence of zinc. While the Zn-free composition transitions to an incommensurate phase, the Zn-containing phases transition instead to a commensurate phase, Imma(0 0 γ)s00 with γ=1/2. Both the Néel temperature and the onset temperature of the Pnma–Imma(0 0 γ)s00 phase transition decreased with increasing zinc concentration. Rietveld analysis of the in situ diffraction pattern for the x=0 sample at 1300 °C demonstrates that the structure contains statistically disordered chain orientations as described by space group Imma. Thermoelectric properties were analyzed in air from 100 to 800 °C. The positive Seebeck coefficient revealed hole-type conduction for all compositions. Doped samples exhibited electrical conductivities up to 3.4 S/cm and thermal conductivity of 1.5 W/mK. Transport analysis revealed thermally activated mobility consistent with polaron conduction behavior for all compositions.

Graphical AbstractPreferential substitution by zinc for iron at the tetrahedral sites of brownmillerite dicalcium ferrite modifies the high temperature Imma(0 0 γ)s00 phase from incommensurate to commensurate.Figure optionsDownload as PowerPoint slideHighlights
► Preferential tetrahedral occupancy by zinc in brownmillerite dicalcium ferrite.
► Ca2Fe2O5 exhibits two additional phase transitions above 1000°C: a commensurately modulated Imma(0 0 γ)s00 phase and a statistically disordered Imma phase.
► Thermal hysteresis in phase transition increased with increasing zinc.
► Electrical conductivity up to 3.4 S/cm at 800 °C.