Published online by Cambridge University Press: 30 August 2019
Enthalpy increments, $\Delta _{298}^T{H^0}$, for highly nonstoichiometric SrFeO3−δ (δ = 0.18–0.41) were obtained between 373 and 1273 K in air using drop calorimetry. The analysis of the
$\Delta _{298}^T{H^0}\left( T \right)$ dependence at lower temperatures allowed evaluating the enthalpy of tetragonal to cubic
${{I4} / {mmm}} \,{\tf="TeX CM Bold Maths Symbols"\char33}\, Pm\bar{3}m$ phase transition at 560 K, 1.57 kJ/mol, and the Maier–Kelley function for
$\Delta _{298}^T{H^0}\left( T \right)$ of tetragonal SrFeO3−δ (space group
${{I4} / {mmm}}$). Combined investigation of oxygen nonstoichiometry
$\bolddelta \left( T \right)$ dependence, measured by thermogravimetry, and higher-temperature
$\Delta _{298}^T{H^0}\left( T \right)$ of cubic SrFeO3−δ (space group
$Pm\bar{3}m$) yielded the temperature-dependent reduction (oxygen release) enthalpy,
$\Delta H_{{\rm{red}}}^{\rm{0}}$. Calorimetrically-determined
$\Delta H_{{\rm{red}}}^{\rm{0}}$ of SrFeO3−δ increases from 65 ± 7 kJ/mol O at 873–973 K to 84 ± 7 kJ/mol O at 1073–1273 K, which may indicate that the short-range vacancy ordering in SrFeO3−δ is hampered at higher temperatures.
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