Syntheses and luminescence of La1−xEux[B8O11(OH)5] and β-La1−xEuxB5O9 (0 ≤ x ≤ 0.135)

Abstract

Luminescence studies on rare earth borates have been active for a long time. Here we performed a careful synthesis to prepare pure samples of La_1−xEu_x[B₈O₁₁(OH)₅] (0 ≤ x ≤ 0.135) using the boric acid flux method, and after evaluating its thermal behavior by thermogravimetric analysis and powder X-ray diffraction, an appropriate thermal treatment was applied to prepare anhydrous β-La_1−xEu_xB₅O₉ (810 °C, 10 h), thus allowing for the study of their Eu³⁺-luminescence in a relatively wide range of Eu³⁺-concentrations. The comparatively strong f–f transition of La_1−xEu_x[B₈O₁₁(OH)₅] in the excitation spectra suggests its potential as a red-emitting phosphor for white-light emitting diodes, which can be efficiently excited by ultraviolet-light emitting diode chips and show the strongest red emission at 622 nm. In contrast, β-La_1−xEu_xB₅O₉ shows a very intense O²⁻ → Eu³⁺ CT absorption, and give strong red emissions with red/orange ratios ∼2.4. Both La_1−xEu_x[B₈O₁₁(OH)₅] and β-La_1−xEu_xB₅O₉ show no concentration quenching up to the maximal x = 0.135, which is as-expected due to their large spatial distances between rare earth cations. It is the first time to study the luminescent performance when La[B₈O₁₁(OH)₅] and β-LaB₅O₉ were used as hosts. In addition, La_0.93Eu_0.07[B₈O₁₁(OH)₅] was selected as a representative to study the luminescence evolution during thermal treatments.