Ab initio X-ray Near-Edge Spectroscopy of Sodium-Based Multi-Alkali Antimonides

Abstract

Multi-alkali antimonides (MAAs) are promising materials for vacuum electron sources. While sodium-based MAAs have demonstrated superior characteristics for ultrabright electron sources, their synthesis remains challenging, often resulting in mixed stoichiometries and polycrystalline domains. To address this complexity and guide the characterization of experimentally grown photocathodes, we present a comprehensive theoretical study of the X-ray near-edge spectroscopy (XANES) of four ternary MAAs: cubic Na$_2$KSb and hexagonal NaK$_2$Sb, representing the experimentally known phase of each stoichiometry, as well as hexagonal Na$_2$KSb and cubic NaK$_2$Sb, two computationally predicted polymorphs. Employing state-of-the-art ab initio methods based on all-electron density-functional theory and the solution of the Bethe-Salpeter equation (BSE), we compute and analyze the XANES at the sodium and potassium K-edges, potassium L$_{2,3}$-edge, and antimony K and L$_2$-edges. Our analysis reveals distinct spectral fingerprints for the experimentally known phases, cubic Na$_2$KSb and hexagonal NaK$_2$Sb, particularly at the sodium K-edge and potassium L$_{2,3}$-edge, providing useful indications for their identification in complex samples. We further investigate the role of excitonic effects by comparing BSE spectra and their counterparts obtained in the independent-particle approximation, highlighting their significant influence on the near-edge features, especially for shallower core levels. Our findings offer a useful theoretical benchmark for the experimental characterization and diagnostics of sodium-based MAA photocathodes, complementing experiments on resolved phases and providing the spectral fingerprints of computationally predicted phases that could emerge in polycrystalline samples.