Improved transparency and conferred luminescence property via Er³⁺/Ho³⁺ co-doped in KNN-based ceramics

Transparent luminescent ceramics have broad application prospects in fields such as optical switches, optical communication, and optical storage. 0.94K_0.5Na_0.5NbO₃–0.06Ca(Bi_0.5Nb_0.5)O₃:0.4wt%Er³⁺/xwt%Ho³⁺ (abbreviated as KNN-0.06CBN:Er³⁺/xHo³⁺, x = 0.1, 0.2, 0.3, 0.4, 0.5) transparent fluorescent ceramics were prepared by a solid-phase sintering method, in which the Er/Ho co-doped with a second component Ca(Bi_0.5Nb_0.5)O₃ are introduced to regulate the phase structure, grain size, and bandgap width of KNN-based ceramics for enhancing both the transparency and luminescence performance. It was found that the change in Ho content has a significant impact on the ceramic phase structure, grain size, and bandgap width while maintaining appropriate content levels of Ca(Bi_0.5Nb_0.5)O₃ and Er. The KNN-0.06CBN: Er³⁺/xwt%Ho³⁺ ceramics with different Ho content both exhibit pseudo cubic phase, but an increase in the content of the rare earth element Ho³⁺ will lead to the ceramic grain size decreases firstly and then increases, at the same time the value of optical bandgap increases first and then decreases. When the Ho content x is 0.1, KNN-0.06CBN: Er³⁺/xHo³⁺ ceramics achieve excellent up-conversion luminescence performance with two weak green light emissions at 531, 549 nm, respectively, and a strong red emission at 660 nm, while ensuring high transmittance of 59% at 780 nm in the visible region and 69% at 1100 nm in the near infrared region, benefiting by the smaller grain size and higher E_g value.