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This contribution is devoted to the new ytterbium-doped alkali metal lutetium double phosphates and their structural and spectroscopic characterisations. The absorption, emission and excitation spectra have been measured at 4, 10 and 293 K in the VUV to IR range. The results have been compared with the analogous yttrium phosphates. The luminescence spectra of the double phosphates have been recorded using various excitation lines, including the synchrotron radiation. The low temperature absorption and emission spectra in the region of the 2 F 7/2 ! 2 F 5/2 transition together with the IR spectra have been used to assign the electronic and vibronic components. Detailed analysis of the Yb 3+ -doped sodium lutetium double phosphates has been performed to determine the energy levels in these hosts and to compare their optical behaviour with analogous sodium yttrium double phosphates. The emission and excitation spectra have been investigated using synchrotron radiation and the origin of the emission has been analysed proving that the emission occurs from the charge transfer state.

Abstract.A new synthesis method of Y 3 TaO 7 is reported. Pure phase powders were prepared by modified Pechini technique. Precursors obtained through that route were heated at the temperature about 1000ºC with or without use of Li 2 SO 4 flux. That implicates in different morphology of specimens and also their spectroscopic properties. Eu ions were used as a spectroscopic probe to prove inhomogeneous broadening of excitation and emission spectra. Samples with Er and Yb doping are also presented. Photoluminescence and up-conversion emissions, under 378 nm and 980 nm excitation respectively, strongly depend on the method of powders synthesis.

Abstract. Powder samples of the ternary phosphates A 3 Lu(PO 4 ) 2 :Yb 3ϩ (A ϭ Na, Rb) have been synthesised by solid state reactions. The spectroscopic properties of Yb 3ϩ in double phosphate host lattices were investigated using absorption, emission and excitation spectra at 4, 10 and 293 K in the VUV to IR range. The results obtained for both types of phosphates are compared. Together with the fundamental 2 F 5/2 Ǟ 2 F 7/2 emission in the IR range, a broad band emission with two peaks in the UV region Compounds containing Yb 3ϩ ions have potential applications as laser devices because of their simple electronic structure. There is no concentration quenching and no excited state absorption reducing the effective laser crossrelaxation process. The Yb 3ϩ ion exhibits intense and broad absorption bands in the IR region which is suitable for a diode-pumped laser. It was reported that ytterbium-doped lasers have longer emission lifetimes than the Nd 3ϩ -doped materials. All these properties of Yb 3ϩ indicate that the ytterbium-doped materials seem to be more promising than the neodymium-doped systems, already employed in laser materials. The fundamental luminescence of the Yb 3ϩ ion originating from the 2 F 5/2 Ǟ 2 F 7/2 transitions between the manifold of Stark-splitted levels occur in the IR range around 1000 nm.While searching for interesting laser host materials for solid state lasers, alkali rare-earth double phosphates of the M 3 RE(PO 4 ) 2 type were studied by several authors [1Ϫ10]. These compounds have their host absorption edge at a rather short wavelength which makes them suitable as host lattices for various luminescent materials.In addition to the intraconfigurational 4f-4f transitions, a broad emission band (called charge transfer band, CT) were observed in Yb 3ϩ -activated Na 3 Lu(PO 4 ) 2 . The second emission is due to a chargeϪtransfer transition type. The influence of the alkali metal cation site of the host lattice on the luminescence properties was investigated. The tentative energy level scheme of the ground and excited 2 F J (Jϭ7/2, 5/2) levels is described.

Synthesis of cubic Y 3 NbO 7 :Er using Li 2 SO 4 flux is reported and spectroscopic properties of the resultant powders are presented. The dopant content has varied in the range of 0.1-15 at.%. Absorption, excitation and emission spectra are characterized by relatively broad structures related to the Er 3+ ion. The broadening results from some inhomogeneity of the metal surrounding. For diluted samples both photoluminescence and up-converted emission are dominated by a green luminescent band around 550 nm. With an increasing concentration of the dopant a red band located around 665 nm gets systematic enhancement and in up-conversion it finally dominates the spectrum. For low concentrations the up-conversion occurs through excited state absorption mechanism while for higher Er contents energy transfer mechanism dominates.