BSE insights into excitons in optical properties of K ₂ AgX (X=As, Sb, Bi) Zintl crystals

The present study deals with the optical properties of ternary Zintl crystals of K 2 AgX (X=As, Sb, Bi), focusing on the electron-hole coupling, i.e., excitonic effects in a many-body procedure. K 2 AgX are direct band gap semiconductors with the band gap values of 1.12, 1.09, and 0.72 eV for X=As, Sb, and Bi, respectively. The valence bands of the compounds are formed by the hybridization of Ag d states with X p states, and the conduction bands arise from the hybridization of Ag and X p states, making them sensitive to chemical substitution. Excitonic effects play a crucial role in the optical spectra of these ternary Zintl compounds. The main excitonic features originate from direct electronic transitions between the highest valence bands (hybridized Ag d and X p states) and lowest conduction bands (hybridized Ag and X p states) at the Γ point where the band gap occurs. Along the series from K 2 AgAs to K 2 AgSb and K 2 AgBi, the excitonic effects and the corresponding exciton binding energies exhibit a nonmonotonic trend, increasing from As to Sb and decreasing toward Bi. The positive differences of the singlet-triplet binding energies show that the spin-singlet excitons are more stable than the spin-triplet ones. Excitonic effects reform the loss function obtained by single-particle method, and the plasmonic excitations appear at different energies depending on the crystallographic direction.