Oxygen defect formation and migration in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:msub> <mml:mi>Sr</mml:mi> <mml:mn>2</mml:mn> </mml:msub> <mml:msub> <mml:mi>FeO</mml:mi> <mml:mrow> <mml:mn>4</mml:mn> <mml:mo>−</mml:m

<a:math xmlns:a="http://www.w3.org/1998/Math/MathML"> <a:mrow> <a:msub> <a:mi>Sr</a:mi> <a:mn>2</a:mn> </a:msub> <a:msub> <a:mi>FeO</a:mi> <a:mrow> <a:mn>4</a:mn> <a:mo>−</a:mo> <a:mi>δ</a:mi> </a:mrow> </a:msub> </a:mrow> </a:math> is a first-order Ruddlesden-Popper perovsike-type material, prospective as a positrode (O-electrode) material for solid oxide and protonic ceramic electrochemical cells. By means of <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"> <b:mrow> <b:mi>DFT</b:mi> <b:mo>+</b:mo> <b:msub> <b:mi>U</b:mi> <b:mi>eff</b:mi> </b:msub> </b:mrow> </b:math> , oxygen vacancies ( <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"> <c:msubsup> <c:mi mathvariant="normal">V</c:mi> <c:mrow> <c:mi mathvariant="normal">O</c:mi> </c:mrow> <c:mrow> <c:mo>•</c:mo> <c:mo>•</c:mo> </c:mrow> </c:msubsup> </c:math> ) and interstitials ( <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"> <f:msubsup> <f:mi mathvariant="normal">O</f:mi> <f:mi>i</f:mi> <f:msup> <f:mrow/> <f:mrow> <f:mo>″</f:mo> </f:mrow> </f:msup> </f:msubsup> </f:math> ) have been modelled in a broad oxygen stoichiometry range of <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"> <h:mrow> <h:mi>δ</h:mi> <h:mo>=</h:mo> <h:mn>0.125</h:mn> </h:mrow> </h:math> to 0.5. For each δ value, all symmetry inequivalent configurations were identified, yielding the <i:math xmlns:i="http://www.w3.org/1998/Math/MathML"> <i:msubsup> <i:mi mathvariant="normal">V</i:mi> <i:mrow> <i:mi mathvariant="normal">O</i:mi> </i:mrow> <i:mrow> <i:mo>•</i:mo> <i:mo>•</i:mo> </i:mrow> </i:msubsup> </i:math> formation energy in the range of 2.0–2.4 eV, almost independent of δ (unlike for simple perovskites). We assign the individual Fe charge states in <l:math xmlns:l="http://www.w3.org/1998/Math/MathML"> <l:mrow> <l:msub> <l:mi>Sr</l:mi> <l:mn>2</l:mn> </l:msub> <l:msub> <l:mi>FeO</l:mi> <l:mrow> <l:mn>4</l:mn> <l:mo>−</l:mo> <l:mi>δ</l:mi> </l:mrow> </l:msub> </l:mrow> </l:math> (“”) based on magnetic moment and average Fe-O distance. Interestingly, <m:math xmlns:m="http://www.w3.org/1998/Math/MathML"> <m:msubsup> <m:mi mathvariant="normal">V</m:mi> <m:mrow> <m:mi mathvariant="normal">O</m:mi> </m:mrow> <m:mrow> <m:mo>•</m:mo> <m:mo>•</m:mo> </m:mrow> </m:msubsup> </m:math> and <p:math xmlns:p="http://www.w3.org/1998/Math/MathML"> <p:mrow> <p:mi mathvariant="normal">F</p:mi> <p:msup> <p:mrow> <p:mi mathvariant="normal">e</p:mi> </p:mrow> <p:mrow> <p:mn>3</p:mn> <p:mo>+</p:mo> </p:mrow> </p:msup> </p:mrow> </p:math> energetically prefer not to be nearest neighbors. Regarding oxygen interstitial species, a <s:math xmlns:s="http://www.w3.org/1998/Math/MathML"> <s:mi>Sr</s:mi> <s:mtext>−</s:mtext> <s:msubsup> <s:mi mathvariant="normal">O</s:mi> <s:mn>2</s:mn> <s:mrow> <s:mn>2</s:mn> <s:mo>−</s:mo> </s:mrow> </s:msubsup> <s:mtext>−</s:mtext> <s:mi>Fe</s:mi> </s:math> defect is the most stable form. Oxygen interstitial transport mechanism occurs predominantly within the SrO layer with the intermediate formation of a single oxide <u:math xmlns:u="http://www.w3.org/1998/Math/MathML"> <u:msubsup> <u:mi mathvariant="normal">O</u:mi> <u:mi>i</u:mi> <u:mrow> <u:mn>2</u:mn> <u:mo>−</u:mo> </u:mrow> </u:msubsup> </u:math> and an average barrier of 0.5 eV. Oxygen vacancy migration is preferred along a zig-zag path within the <w:math xmlns:w="http://www.w3.org/1998/Math/MathML"> <w:mrow> <w:mi>Fe</w:mi> <w:msub> <w:mi mathvariant="normal">O</w:mi> <w:mn>2</w:mn> </w:msub> </w:mrow> </w:math> layer with an effective barrier of 0.6 eV. The comprehensive analysis of <y:math xmlns:y="http://www.w3.org/1998/Math/MathML"> <y:mrow> <y:msub> <y:mi>Sr</y:mi> <y:mn>2</y:mn> </y:msub> <y:msub> <y:mi>FeO</y:mi> <y:mrow> <y:mn>4</y:mn> <y:mo>−</y:mo> <y:mi>δ</y:mi> </y:mrow> </y:msub> </y:mrow> </y:math> —serving as prototype also for higher Ruddlesden-Popper phases—including oxidation state map and oxygen defect migration, paves the way for coming investigations of the protonated materials.