Crystal growth, structure, infrared spectroscopy, and luminescent properties of rare-earth gallium borates RGa3(BO3)4, R = Nd, Sm–Er, Y

• Different techniques were applied to analyze polytype structures in RGa3(BO3)4.
• Concentrated RGa3(BO3)4 (R = Ho and Eu) exhibit long luminescence decay times.
• RGa3(BO3)4 (R = Ho and Eu) are promising for minilasers.
• A broad band gap of RGa3(BO3)4 is favorable for blue and UV lasers.

Crystals of the rare-earth gallium borates RGa3(BO3)4, where R = Nd, Sm–Er, or Y, were grown by the flux method. The crystal structures of RGa3(BO3)4 (R = Eu, Ho) were studied on the basis of single-crystal X-ray diffraction measurements. The hexagonal unit-cell parameters are a = 9.4657(1) Å, c = 7.4667(1) Å and a = 9.4394(2) Å, c = 7.4322(1) Å for EuGa3(BO3)4 and HoGa3(BO3)4, respectively, space group R32. Structure model was determined by “charge flipping” method and refined to R = 1.93% [EuGa3(BO3)4] and R = 1.89% [HoGa3(BO3)4] in anisotropic approximation. All grown gallium borates were investigated by infrared (IR) spectroscopy technique in a middle and far IR region. IR spectra of rare-earth gallium borates correspond to a pure rhombohedral (R32) polytype structure. Small inclusions of a monoclinic phase were detected only in Eu and Nd compounds. Luminescence of Eu and Ho gallium borates was studied at room temperature. The measured decay times for the most intensive emission lines of EuGa3(BO3)4 (∼614 nm) and HoGa3(BO3)4 (434 nm) are 940 μs and 140 μs, respectively. The scheme of crystal-field energy levels of Eu3+ in EuGa3(BO3)4 was built on the basis of the temperature-dependent optical transmission measurements combined with the luminescence data. The measured UV absorption edge for RGa3(BO3)4 is at about 300 nm.