Photoluminescence of MgAl₂O₄:Co²⁺ through time-resolved spectroscopy under pressure

Abstract

This work investigates the Co²⁺(T_d) electronic structure and photoluminescence (PL) in MgAl₂O₄:Co²⁺ and their variation with pressure by time-resolved spectroscopy. The absorption spectra and the pressure and temperature dependences of the emission band at 663 nm (τ=120 ns, at ambient conditions) can be explained on the basis of a configurational energy model, which provides an interpretation of the electronic structure and the excited-state phenomena. We show that there is an excited-state crossover [⁴T ₁(P)↔²E (G)] at ambient pressure, which is responsible for the evolution of the emission spectrum from a broadband emission between 300 and 100 K to a narrow-peak emission at 6 K. The pressure, instead of enhancing excited-state crossover phenomena, reduces PL and suppresses it (PL quenching) above 6 GPa. We explain such variations in terms of pressure-induced non-radiative crossover relaxation to lower excited states: ²E (G)→⁴T ₁(F). The variation of PL intensity and its associated lifetime with pressure supports the proposed interpretation.

Keywords:

• Co²⁺ T_d
• MgAl₂O₄
• excited-state crossover
• time-resolved spectroscopy

Acknowledgements

Financial support from the Spanish MEC (Project No. MAT2005-00099) and MALTA INGENIO-CONSOLIDER 2010 (Ref. CDS2007-0045) are acknowledged. Dr. Lucie Nataf thanks a Postdoctoral fellowship from the University of Cantabria.