Ambient‐Pressure Stabilization of β‐GdB3O6 by Doping with Bi3+ and Color‐Tunable Emissions by Co‐Doping with Tb3+ and Eu3+: The First Photoluminescence Study of a High‐Pressure Polymorph

Rare‐earth borates are good candidates for optical applications. To date, however, the high‐pressure/high‐temperature technique has produced a large number of novel borates with optical properties that have rarely been investigated due to the severe problem of substantial defects. We targeted the high‐pressure polymorph of β‐GdB₃O₆ and synthesized three solid solutions of β‐Gd_0.75−x Bi_0.25Tb_x B₃O₆ (0≤x ≤0.75), β‐Gd_0.75−y Bi_0.25Eu_y B₃O₆ (0≤y ≤0.75), and β‐Gd_0.50−z Bi_0.25Tb_0.25Eu_z B₃O₆ (0≤z ≤0.05) by using typical solid‐state reactions at 820 °C. Here, the function of Bi³⁺ is to stabilize the high‐pressure phase by lowering the synthetic temperature and being the sensitizer to promote the green and red emissions of Tb³⁺ and Eu³⁺. The multiple energy transfer paths were investigated by using lifetime decay experiments and photoluminescent spectra, and both efficiency and mechanism were determined. Eventually, color‐tunable and white emissions were achieved by rational doping of Bi³⁺, Tb³⁺, and Eu³⁺ into β‐GdB₃O₆, that is, the CIE chromaticity coordinate for β‐Gd_0.44Bi_0.25Tb_0.30Eu_0.01B₃O₆ is (0.318, 0.365) with a correlated color temperature of 6101 K.