Synthesis, crystal structures and photoluminescence properties of new oxyborates, Mg5NbO3(BO3)3 and Mg5TaO3(BO3)3, with novel warwickite-type superstructures

Single crystals of new oxyborates, Mg5NbO3(BO3)3 and Mg5TaO3(BO3)3, were prepared at 1370 °C in air using B2O3 as a flux. They were colorless and transparent with block shapes. X-ray diffraction analysis of the single crystals revealed Mg5NbO3(BO3)3 and Mg5TaO3(BO3)3 to be isostructural. The X-ray diffraction reflections were indexed to the orthorhombic Pnma (No. 62) system with a=9.3682(3) Å, b=9.4344(2) Å, c=9.3379(3) Å and Z=4 for Mg5NbO3(BO3)3 and a=9.3702(3) Å, b=9.4415(3) Å, c=9.3301(2) Å and Z=4 for Mg5TaO3(BO3)3. The crystal structures of Mg5NbO3(BO3)3 and Mg5TaO3(BO3)3 are novel warwickite-type superstructures having ordered arrangements of Mg and Nb/Ta atoms. Polycrystals of Mg5NbO3(BO3)3 prepared by solid state reaction at 1200 °C in air showed broad blue-to-green emission with a peak wavelength of 470 nm under 270 nm ultraviolet excitation at room temperature.