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Single crystals of K5LaLi2F10:Er3+ and K5LaLi2F10 were synthesized and the structural determination was made by x-ray diffraction analysis. The structure is orthorhombic with space group Pnma. The unit cell parameters are: a = 20.775, b = 7.822 and c = 6.963 Å. The infrared transmission and Raman scattering spectra of oriented single crystals were measured and discussed in terms of factor group analysis and x-ray structure. A vibrational assignment is proposed. Preliminary optical properties of Er3+ ions in K5LaLi2F10 crystals have been investigated using absorption, luminescence and lifetime measurements in the 5-300 K temperature range. The low temperature absorption and luminescence spectra of Er3+ have allowed us to identify the Stark levels of low-lying multiplets. The luminescence observed at 5 K has been attributed to the 4S3/24I15/2 (18 300 cm-1), 4F9/24I15/2 (15 000 cm-1), 4S3/24I13/2 (11800 cm-1) and 4I11/24I15/2 (10000 cm-1) transitions. The lifetimes of selected excited states were measured at 5 and 300 K temperature. The obtained data gave the first insight into the luminescent properties of Er3+ ions in the fluorine surroundings in K5LaLi2F10 crystals.

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