As increasing Yb³⁺ co-doped concentrations in 6 mol% Er³⁺ doped PbTiO₃, BaTiO₃, and SrTiO₃ polycrystalline powder samples, we have observed the room-temperature green upconversion (UC) emission at 550 nm being quenched by the simultaneously enhanced red UC emission at 660 nm under the 980-nm laser excitation. For codoping Yb³⁺ ions up to 6 mol% in PbTiO₃ and only Er³⁺ doped PbTiO₃ samples, which still possess relatively large tetragonality, the green UC emission is still much stronger than red one. In these cases, both the UC emissions are dominated by the two-photon process. But, as further increasing the Yb³⁺ ion concentration, the crystal structures tend to become cubic phase with enhancing red UC emission and almost diminishing in green emission. Since the pure BaTiO₃ crystal exhibits weaker tetragonality than PbTiO₃, the stronger red emission and weaker green one were expected at the lower codoped Yb³⁺ concentration in BaTiO₃ system than in PbTiO₃ one.  The observed quench of green radiation accompanied with enhancement of red radiation should be due to the efficient energy back-transfer (EBT) process as reported by Chen, et al. by raising Yb³⁺ concentration [1]. The efficient EBT process requires not only level match of ⁴S_3/2→⁴I_13/2 in Er³⁺ and ²F_7/2→²F_5/2 in Yb³⁺ but also resonant match of the laser photon energy with the transitions of ⁴I_15/2→⁴I_11/2 and ⁴I_13/2→⁴F_9/2 in Er³⁺ as well as ²F_7/2→²F_5/2 in Yb³⁺. As a result, declining tetragonality results in the centrosymmetric crystal field for high Yb³⁺-ion concentration to achieve the level matches that is difficult to be fulfilled with asymmetric crystal field in ferroelectric phase.

[1] G. Chen, G. Somesfalean, Y. Liu, Z. Zhang, Q. Sun, and F. Wang, Phys. Rev. B 75, 195204 (2007).

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