Spectroscopic properties, concentration quenching and Yb3+ site occupations in vacancied scheelite-type molybdates

Abstract A series of micro-crystalline Yb 3+ -doped vacancied Cd 1−3 x Yb 2 x □ x MoO 4 solid solutions has been prepared by a high-temperature solid state reaction method. The structural studies performed by X-ray powder diffraction measurements have shown that the samples are monophasic and crystallize in the tetragonal scheelite-type structure (the space group I4 1 /a , with point symmetry close to D 2d ) when the x parameter is greater than 0 and does not reach 0.1430 (33.36 mol% of Yb 3+ ions). The substitution of divalent Cd 2+ by trivalent Yb 3+ cations leads to the formation of cationic vacancies in the framework (which are denoted in the chemical formula as □), due to the charge compensation: 3Cd 2+ →2Yb 3+ +□ vacancy. Direct excitation of Yb 3+ by means of 2 F 7/2 → 2 F 5/2 absorption at 940–980 nm leads to reversed 2 F 5/2 → 2 F 7/2 transitions giving Yb 3+ emission in the range of 970–1130 nm. The intense and broad emission lines of Yb 3+ ions, which are also used as a structural probe at 77 K have been observed. The existence of more than one component of the 0-phonon line at 975 nm and 976.6 nm indicate two Yb 3+ distribution sites, which is in agreement with results obtained for the Nd 3+ ion. Basing on the absorption and emission spectra the Yb 3+ electronic energy levels have been proposed. The effect of dopant concentration had an influence on luminescent properties but had no influence on the powder morphology. Yb 3+ concentration dependences of the 2 F 5/2 experimental decay time were analyzed in order to attempt the understanding of the concentration quenching mechanism and estimate the main parameters useful for a theoretical approach of laser potential.