Meta-stable rare-earth or bismuth borates, key components of solid-state laser sources, were originally obtained under high-temperature and high-pressure conditions. Progress toward their synthesis has been made in 2012 [1],  when δ-Bi_1-xREB₃O₆ (RE = La, Ce, Pr, Nd) was formed at ambient pressure. These borates showed a strong second harmonic generation, which means that they could effectively expand the laser frequency range.

Rihong Cong, Chongqing University, China, and colleagues have shown that β-Gd_1-xBi_xB₃O₆ also easily forms at ambient pressure. The thermal stability of these borates could be manipulated by mutual doping between rare earth and bismuth ions, which the team found via a systematic photoluminescence study on the high-pressure phase.

This study leads to better understanding of the synthetic chemistry of borates. In addtion, it helps to explore the optical performance of high-pressure polymorphs of borates. The rational co-doping of β-Gd_1-xBi_xB₃O₆ with Tb³⁺ and Eu³⁺ creates color-tunable and bright emissions.

• Ambient-pressure stabilization of β-GdB₃O₆ by doping Bi³⁺ and color-tunable emissions by co-doping Tb³⁺ and Eu³⁺: the first photoluminescence study on the high-pressure polymorph,
  Tao Yang, Xiaorui Sun, Pengfei Jiang, Wenliang Gao, Xianju Zhou, Rihong Cong,
  Chem. Asian J. 2017.
  DOI: 10.1002/asia.201700405

Reference

• [1] Rare earth induced formation of δ-BiB₃O₆ at ambient pressure with strong second harmonic generation, 
  R. Cong, T. Yang, Z. Lin, L. Bai, J. Ju, F. Liao, Y. Wang, J. Lin,
  J. Mater. Chem. 2012, 22, 17934–17941.
  DOI: 10.1039/C2JM32744A