Optical floating zone method growth and optical properties of corundum Mg4Nb2O9 single crystal

Mg4Nb2O9 single crystals were prepared by the optical floating zone method. While the as-grown were boules polycrystalline and cracked with dimensions of ∅ 5-8 mm×L 91 mm, they contained single crystal regions as large as ∅ 4 mm×L 15 mm. The X-ray diffraction analysis showed that the crystals had the corundum structure, grew along the a-axis and cleaved parallel to the c plane. Polarized light microscopy, in a cross-transmission configuration measurement, showed that the crystal was free of low-angle grain boundaries and bubbles. The optical interband transitions were measured from the absorption spectrum of the crystal along the c-axes at room temperature in the range of 200-800 nm. Based on the theory of band-to-band transitions, the direct band gap was determined for the first time to be Eg=5.09 eV. The photoluminescence spectra exhibited a broad and strong blue emission band centered at 450 nm.