On the influence of calcium substitution to the optical properties of Cr3+ doped SrSc2O4

Cr3+ doped (Sr,Ca)Sc2O4 samples were synthesized and investigated in view of their potential application as luminescent converters for high power broad band near infrared (NIR) sources. The synthesis of the Cr3+ activated MSc2O4 (M=Ca, Sr) samples was performed by solid state reactions. Obtained powders show broad band near infrared emission between 700 and 1000 nm under excitation in the blue spectral range. The emission is assigned to the intraconfigurational (3d3) 4T2→4A2 transition on Cr3+. As the calcium concentration in SrSc2O4 was increased, a blue shift of the emission spectra was observed. In order to gain insight in the luminescence properties, the crystal field strength, Racah parameters and phonon coupling parameters were examined. Additionally, the influence of temperature in the range between 4 and 500 K on the photoluminescence of the CaSc2O4:Cr3+ and SrSc2O4:Cr3+ was investigated. At 4 K a μs decay time typical of Cr3+ broad band d-d emission was observed. Between 70 and 350 K the decay time and emission intensity decrease due to thermal quenching. The large Stokes shift of the Cr3+ emission (~3000 cm-1) combined with the low energy position of the 4T2 excited state causes non-radiative relaxation to the ground state, already below room temperature. The low thermal quenching temperature of broad band NIR emission is a general problem and the present results provide insight in factors determining thermal quenching.