${\mathrm{Y}}_2{\mathrm{O}}_3$ codoped with $\mathrm{B}{\mathrm{i}}^{3+}$ and $\mathrm{Y}{\mathrm{b}}^{3+}$ is considered as an efficient downconversion material combining strong broadband absorption of $\mathrm{B}{\mathrm{i}}^{3+}$ with photon splitting by cooperative energy transfer from $\mathrm{B}{\mathrm{i}}^{3+}$ to two $\mathrm{Y}{\mathrm{b}}^{3+}$ neighbors. However, evidence for photon splitting is lacking. Here we investigate the $\mathrm{B}{\mathrm{i}}^{3+}\text{−}\mathrm{to}\text{−}\mathrm{Y}{\mathrm{b}}^{3+}$ energy-transfer mechanism. For cooperative energy transfer the $\mathrm{Y}{\mathrm{b}}^{3+}$-concentration-dependent luminescence decay will show clear characteristics of cooperative dipole-dipole transfer. Analysis of $\mathrm{Y}{\mathrm{b}}^{3+}$-concentration and temperature-dependent decay curves however demonstrates that the energy-transfer mechanism is not cooperative but single step, probably through a $\mathrm{B}{\mathrm{i}}^{4+}\text{−}\mathrm{Y}{\mathrm{b}}^{2+}$ charge-transfer state. The temperature dependence of the $\mathrm{B}{\mathrm{i}}^{3+}\text{−}\mathrm{to}\text{−}\mathrm{Y}{\mathrm{b}}^{3+}$ energy-transfer efficiency is unusual as it decreases with temperature, unlike commonly observed thermally activated energy transfer. This is a signature of energy transfer via exchange interaction. The present results provide evidence for the absence of photon splitting in ${\mathrm{Y}}_2{\mathrm{O}}_3:\mathrm{B}{\mathrm{i}}^{3+},\mathrm{Y}{\mathrm{b}}^{3+}$ and form a convincing demonstration of exchange interaction mediated energy transfer.

• Received 29 June 2018
• Revised 27 September 2018

DOI:https://doi.org/10.1103/PhysRevB.98.134308