Growth, structural and thermophysical properties of TbNbO4 crystals

A new TbNbO4 single-crystal with a monoclinic structure has been grown successfully using the Czochralski method for the first time. The material belongs to the space group C2/c, with unit-cell parameters of a = 7.173(3) angstrom, b = 11.186(5) angstrom, c = 5.156(2) angstrom and beta = 131.082(3)degrees. The thermal properties of the as-grown TbNbO4 crystal, including specific heat, thermal expansion coefficient, thermal diffusion coefficient, and thermal conductivity, were measured systematically. The TbNbO4 crystal exhibits strong anisotropic thermal expansion; a negative thermal expansion along the c direction is presented, and a zero thermal expansion coefficient in some directions can be obtained. The emission and absorption spectra were also measured. The strongest emission peak in the green spectral domain of TbNbO4 corresponding to the D-5(4) -> F-7(5) transition of Tb3+ is located at 549 nm. In the range of 350-1500 nm, the as-grown TbNbO4 crystal has low absorption. X-ray photoelectron spectroscopy (XPS) was used to study the valence of the elements in the TbNbO4 crystal; the result shows that there are two different valence states of Tb in the as-grown crystal.