Photoluminescence of Cr-Doped CaTiO3

At 4 and 77 K, photoluminescence from nominally undoped and 0.0025% Cr-doped CaTi${\mathrm{O}}_3$ shows identical spectra in the infrared with an intensity ratio of 1:2. The emission is attributed to the ${}_{}{}^2{}_{}{}^{}E\to {}_{}{}^4{}_{}{}^{}A_2^{}{}_{}{}^{}$ transition of the ${\mathrm{Cr}}^{3+}$ ion at a ${\mathrm{Ti}}^{4+}$ site. At 4 K it consists of a single no-phonon line at 1.744 eV of half-width 1.5 meV and a complex vibronic spectrum which is anomalously intense. Data at 77 K show a 2-meV splitting of the ${}_{}{}^2{}_{}{}^{}E$ state. The absorption spectrum of CaTi${\mathrm{O}}_3$:Cr is dominated by a "background absorption" by unknown defects. From the coincidence of the excitation spectrum with the absorption spectrum, we conclude that the excitation is dominated by absorption by these unknown defects which transfer energy into ${\mathrm{Cr}}^{3+}$. In undoped CaTi${\mathrm{O}}_3$ a single-exponential decay of the emission is reported, with decay time 700 μsec. In CaTi${\mathrm{O}}_3$:0.0025% Cr, on the other hand, two exponentials with decay times 700 μsec and 11 msec were observed. The former is assigned to ${\mathrm{Cr}}^{3+}$, the latter to the time of energy transfer from defects responsible for the background absorption to the ${\mathrm{Cr}}^{3+}$. We also report a visible emission consisting of a broad structureless band centered at 2.7 eV, with half-width 0.7 eV. Presumably, it is recombination radiation caused by the recombination of a free charge carrier with a trapped charge at an unknown defect. Its excitation spectrum gives a band gap for CaTi${\mathrm{O}}_3$ of 3.70 eV at 4 K.