Optical properties of undoped NdTaO4, ErTaO4 and YbTaO4 ceramics

The optical properties of undoped polycrystalline ceramics NdTaO4 (NTO), ErTaO4 (ETO) and YbTaO4 (YTO) were investigated for the first time to our knowledge. The ceramics were prepared by solid-state reaction and exhibited two different crystalline structures, according to x-ray diffraction: M-fergusonite type (I2/a, #15), for NTO, and M′-fergusonite type (P2/a, #13), for ETO and YTO. The results from photoluminescence (PL) absorption and emission showed that the optical properties of these compounds have some variation in accordance with the rare earth ion present on the TaO4 host matrix. In these ceramics, the intraconfigurational f-f transitions were observed. The specific emissions in the infrared (IR) regions were assigned to all ceramics, as well as emission in the visible range for ETO. Under near-IR excitation (at 980 nm), the ETO ceramic is characterized by intense purplish pink up-conversion luminescence with coordinates x=0.460 and y=0.271 in the chromaticity diagram (CIE), and about 47% of color purity. Under visible excitation at 532 nm, clear evidences of the down-conversion process in ETO through the cross-relaxation have been found. Fluorescence lifetimes for dominated IR-emission were determined as 81.5, 122.1 and 131.2 μs for NTO, YTO and ETO, respectively. The broad full width at half-maximum (FWHM) and optical results indicate that the polycrystalline ceramics NTO, ETO and YTO investigated here are promising luminescent inorganic materials for solid state lasers.