Pressure-induced Co2+photoluminescence quenching in MgAl2O4

Abstract

This work investigates the electronic structure and photoluminescence (PL) of Co${}^{2+}$-doped MgAl${}_{2}$O${}_{4}$ and their pressure dependence by time-resolved spectroscopy. The variations of the visible absorption band and its associated emission at 663 nm (\ensuremath{\tau} = 130 ns at ambient conditions) with pressure/temperature can be explained on the basis of a configurational energy model. It provides an interpretation for both the electronic structure and the excited-state phenomena yielding photoluminescence emission and the subsequent quenching. We show that there is an excited-state crossover (ESCO) ${[}^{4}{T}_{1}(P)\ensuremath{\leftrightarrow}{}^{2}\phantom{\rule{-0.16em}{0ex}}E(G)]$ at ambient pressure, which is responsible for the evolution of the emission spectrum from a broadband emission between 300 K and 100 K to a narrow-line emission at lower temperatures. Contrary to expectations from the Tanabe-Sugano diagram, instead of enhancing ESCO phenomena, pressure reduces PL and even suppresses it (PL quenching) above 6 GPa. We explain such variations in terms of pressure-induced nonradiative relaxation to lower excited states: ${}^{2}\phantom{\rule{-0.16em}{0ex}}E(G)\ensuremath{\rightarrow}{}^{4}$${T}_{1}$($F$). The variation of PL intensity and its associated lifetime with pressure supports the proposed interpretation.