Thermal ionization and thermally activated crossover quenching processes for5d−4fluminescence inY3Al

We investigated thermally activated ionization and thermally activated crossover as the two possibilities of quenching of $5d$ luminescence in $\mathrm{P}{\mathrm{r}}^{3+}$-doped ${\mathrm{Y}}_{3}\mathrm{A}{\mathrm{l}}_{5\ensuremath{-}x}\mathrm{G}{\mathrm{a}}_{x}{\mathrm{O}}_{12}$. Varying the Ga content $x$ gives the control over the relative energy level location of the $5d$ and $4{f}^{2}:^{3}P_{J}$ states of $\mathrm{P}{\mathrm{r}}^{3+}$ and the host conduction band (CB). Temperature-dependent luminescence lifetime measurements show that the $5d$ luminescence quenching temperature ${T}_{50%}$ increases up to $x=2$ and decreases with further increasing Ga content. This peculiar behavior is explained by a unique transition between the two quenching mechanisms which have an opposite dependence of thermal quenching on Ga content. For low Ga content, thermally activated crossover from the $4f5d$ state to the $4{f}^{2}(^{3}P_{J})$ states is the operative quenching mechanism. With increasing Ga content, the activation energy for thermally activated crossover becomes larger, as derived from the configuration coordinate diagram, while from the vacuum referred binding energy diagram the activation energy of thermal ionization becomes smaller. Based on these results, we demonstrated that the thermal quenching of $\mathrm{P}{\mathrm{r}}^{3+}:5{d}_{1}\ensuremath{-}4f$ luminescence in ${\mathrm{Y}}_{3}\mathrm{A}{\mathrm{l}}_{5\ensuremath{-}x}\mathrm{G}{\mathrm{a}}_{x}{\mathrm{O}}_{12}$ with $x=0,\phantom{\rule{0.28em}{0ex}}1,\phantom{\rule{0.28em}{0ex}}2$ is a thermally activated crossover while for $x=3,\phantom{\rule{0.28em}{0ex}}4,\phantom{\rule{0.28em}{0ex}}5$ it results from the thermal ionization.