Ambient-Pressure Stabilization of β-GdB3 O6 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-GdB3 O6 and synthesized three solid solutions of β-Gd0.75-x Bi0.25 Tbx B3 O6 (0≤x≤0.75), β-Gd0.75-y Bi0.25 Euy B3 O6 (0≤y≤0.75), and β-Gd0.50-z Bi0.25 Tb0.25 Euz B3 O6 (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-GdB3 O6 , that is, the CIE chromaticity coordinate for β-Gd0.44 Bi0.25 Tb0.30 Eu0.01 B3 O6 is (0.318, 0.365) with a correlated color temperature of 6101 K.