| Summary: | Despite chalcopyrite (CuFeS<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula>) being one of the oldest known copper ores, it exhibits various properties that are still the subject of debate. For example, the relative concentrations of the ionic states of Fe and Cu in CuFeS<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula> can vary significantly between different studies. The presence of a plasmon-like resonance in the visible absorption spectrum of CuFeS<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula> nanocrystals has driven a renewed interest in this material over recent years. The successful synthesis of CuAl<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mrow><mn>1</mn><mo>−</mo><mi>p</mi></mrow></msub></semantics></math></inline-formula>Fe<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mi>p</mi></msub></semantics></math></inline-formula>S<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula> nanocrystals that exhibit a similar optical resonance has recently been demonstrated in the literature. In this study, we use density functional theory to investigate Fe substitution in CuAlS<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula> and find that the formation energy of neutral <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msup><mrow><mo>[</mo><msub><mi>Fe</mi><mi>Cu</mi></msub><mo>]</mo></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msup><mo>+</mo><msup><mrow><mo>[</mo><msub><mi>Cu</mi><mi>Al</mi></msub><mo>]</mo></mrow><mrow><mn>2</mn><mo>−</mo></mrow></msup></mrow></semantics></math></inline-formula> defect complexes is comparable to <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow><mo>[</mo><msub><mi>Fe</mi><mi>Al</mi></msub><mo>]</mo></mrow><mn>0</mn></msup></semantics></math></inline-formula> antisites when <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>p</mi><mo>≥</mo><mn>0.5</mn></mrow></semantics></math></inline-formula>. Analysis of electron density and density of states reveals that charge transfer within these defect complexes leads to the formation of local Cu<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msup></semantics></math></inline-formula>/Fe<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msup><mrow></mrow><mrow><mn>2</mn><mo>+</mo></mrow></msup></semantics></math></inline-formula> ionic states that have previously been associated with the optical resonance in the visible absorption of CuFeS<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula>. Finally, we comment on the nature of the optical resonance in CuAl<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mrow><mn>1</mn><mo>−</mo><mi>p</mi></mrow></msub></semantics></math></inline-formula>Fe<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mi>p</mi></msub></semantics></math></inline-formula>S<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msub><mrow></mrow><mn>2</mn></msub></semantics></math></inline-formula> in light of our results and discuss the potential for tuning the optical properties of similar systems.
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