The crystal structure of monoclinic KLu(WO 4 ) 2 (KLuW) crystals was determined at room temperature by using single-crystal X-ray diffraction data. The unit-cell parameters were a = 10.576 (7), b = 10.214 (7), c = 7.487 (2) Å , = 130.68 (4) , with Z = 4, in space group C2/c. The unit-cell parameters of KLu 1Àx Yb x (WO 4 ) 2 were determined in relation to Yb concentration. Vickers micro-indentations were used to study the microhardness of KLuW. The linear thermal expansion tensor was determined and the principal axis with maximum thermal expansion ( 0 33 = 16.72 Â 10 À6 K À1 ), X 0 3 , was located 13.51 from the c axis. The room-temperature optical tensor was studied in the near-infrared (NIR) and visible range. The principal optical axis with maximum refractive index (n g = 2.113), N g , was located 18.5 from the c axis at 632.8 nm. Undoped and ytterbium-doped KLuW crystals were grown by the TSSG (top-seeded-solution growth) slow-cooling method. The crystals show {110}, { " 1 111}, {010} and {310} faces that basically constitue the habit of the KLuW crystals.

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Epitaxial monoclinic double tungstate laser crystals were grown with high crystalline quality. Based on these Yb-doped composites, laser operation was demonstrated. Continuous-wave laser emission of a Yb:KYW∕KYW crystal was achieved at 1030nm. The 25-μm-thin Yb:KYW layer was pumped at wavelengths near 980nm by a Ti:sapphire laser. A maximum output power of 40mW was obtained at room temperature.

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Epitaxial layers of KLu 1-x Yb x (WO 4 ) 2 /KLu(WO 4 ) 2 with different Yb concentrations were grown by liquid-phase epitaxy. The quality of the epitaxial layers principally depends on the Yb concentration and the crystal face. In solutions with up to 50 mol % of substitution of Lu 2 O 3 by Yb 2 O 3 , defect-free layers can be grown and the highest quality face is (010). The layers grown on the (-111) face tend to have a higher density of morphologies. The optical spectroscopy of Yb 3+ in thin layers coincides with that obtained in doped bulk crystals.