Demonstrating polaron mediated photoluminescence in vanadates by a combined experimental and computational study

Abstract

Photoluminescence in vanadate compounds has traditionally been attributed to charge-transfer transitions within isolated [VO4]3− centres, with excitation and emission described using molecular orbitals. In previous work, we proposed an alternative mechanism: photoluminescence arises from conventional interband excitation, followed by electron polaron-mediated formation of self-trapped excitons. In this study, we provide further evidence for this model through spectroscopic measurements and ab initio calculations. Using density functional theory calculations on a series of alkali vanadates (MVO3, M = Li, Na, K, Rb, Cs), we show that photoexcited electrons spontaneously localise on V5+ ions, leading to self-trapped exciton formation via hole localisation on neighbouring oxygen atoms. The calculated energies for band gaps and self-trapped excitons closely match experimental values obtained from diffuse reflectance and luminescence spectroscopy. Importantly, temperature- and time-resolved luminescence measurements reveal that quenching predominantly occurs before the formation of the luminescent state, challenging earlier models that assumed quenching to occur from the final emitting state. To explain this behaviour, we note that the width of the conduction band states is expected to govern the rate of electron trapping, and we indeed find a correlation between calculated bandwidths and measured quantum efficiencies. This suggests that non-radiative relaxation of free carriers at defects, prior to self-trapping, is the dominant quenching mechanism. Consequently, the electron self-trapping rate, the self-trapped exciton formation rate, and the defect concentration are expected to critically determine the luminescent efficiency of vanadate phosphors.