Electronic structure of KCa<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow/><mml:mn>2</mml:mn></mml:msub></mml:math>Nb<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow/><mml:mn>3</mml:mn></mml:msub></mml:math>O<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mrow/><mml:mn>10</mml:mn></mml:msub></mml:math>as envisaged by density functional theory and valence electron energy loss spectroscopy

KCa${}_{2}$Nb${}_{3}$O${}_{10}$ is a layered Dion--Jacobson-type perovskite important for a number of applications such as photocatalysis and as a building block for heteronanostructures. Despite this, some of its central electronic properties such as the band gap and dielectric function are not well understood. In this report we have attempted to determine the band gap and understand the electronic structure of KCa${}_{2}$Nb${}_{3}$O${}_{10}$ using density functional theory. Simultaneously, the band gap and loss function have been determined experimentally using valence electron energy loss spectroscopy. The theoretical results indicate that KCa${}_{2}$Nb${}_{3}$O${}_{10}$ is a direct band gap semiconductor with a sparse density of states close to the onset of the conduction band. The calculated band gap value of 3.1 eV is in excellent agreement with the 3.2\ifmmode\pm\else\textpm\fi{}0.1 eV measured experimentally. The loss functions computed and experimentally determined show good agreement up to 20 eV, but the theoretical peak positions at higher energy do not agree with the experimental electron energy loss spectrum. These transitions originate from K-3$p$, Ca-3$p$, and Nb-4$p$ semicore states and their positions are not well described by Kohn-Sham eigenvalues. After a scissors shift of transitions due to these states by about 2.5 eV to higher energies we obtain good agreement with the experimental loss function and can thus explain the origin of all the features seen in the experimental electron energy loss spectrum.