Electron paramagnetic resonance and photoluminescence properties of α-Al2O3:Cr3+ phosphors

Abstract

The red emitting Cr3+ activated α-Al2O3 powder phosphor has been prepared by easy combustion reactions from mixed metal nitrate reactants and urea with ignition temperatures of 500 °C. The as-synthesized powder was characterized by X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared techniques. The X-ray diffraction pattern reveals that the phosphor crystallized in the hexagonal α-Al2O3 phase directly from the combustion reaction. The EPR spectrum exhibits an intense resonance signal with effective g value at g=3.33 along with a few weak resonance signals with effective g values at g=13.7, 2.34, 1.95, 1.49, and 1.26. The spin concentration (N) and its paramagnetic susceptibility (χ) have been evaluated. The excitation spectrum consists of two broad intense bands at 415 nm and 555 nm and are assigned to 4A2g (F)→4T1g (F) and 4A2g (F)→4T2g (F) transitions, respectively. The intense fluorescence peak around 691 nm is attributed to 2E g →4A2g transition of Cr3+ ion. By correlating EPR and optical data, the crystal field splitting parameter (Dq), Racah inter-electronic repulsion parameter (B) have been evaluated and discussed. The EPR and optical studies reveal that Cr3+ ions are occupying in Al3+ sites in octahedral coordination.