NaBi(WO 4 ) 2 (NBW), NaBi(MoO 4 ) 2 (NBMo) and LiBi(MoO 4 ) 2 (LBMo) single crystals grown by the Czochralski technique have been doped up to a praseodymium concentration of [Pr] ≈ 1 × 10 20 cm −3 in the crystal. 10 K polarized optical absorption and photoluminescence measurements have been used to determine the energy position of 32, 39 and 36 Pr 3+ Stark levels in NBW, NBMo and LBMo crystals, respectively. These energy levels were labelled with the appropriate irreducible representations corresponding to a C 2 local symmetry of an average optical centre. Single-electron Hamiltonians including free-ion and crystal field interactions have been used in the fitting of experimental energy levels and in the simulation of the full sequence of the 4f 2 Pr 3+ configuration. 300 K absorption spectra of different 2S+1 L J Pr 3+ multiplets were determined and used in the context of the Judd-Ofelt theory and for the calculation of the 1 D 2 -related emission cross sections of this average Pr 3+ centre. Non-radiative electron relaxation from the 3 P 0 level feeds the 1 D 2 multiplet. This latter level efficiently decays radiatively to the ground 3 H 4 multiplet but still there is a significant rate of radiative decay to the 1 D 2 → 3 F 3 praseodymium laser channel. For [Pr] 2 × 10 19 cm −3 , non-radiative electric dipole-dipole Pr pair energy transfer limits the radiative yield.
Because of the incongruent melting of LiYb(MoO 4 ) 2 compound at 912 °C, Li 2 MoO 4 and Li 2 Mo 2 O 7 fluxes have been used to nucleate single crystals of this compound. From the latter flux, crystals of about 1 cm 3 have been pulled by the top-seeded solution-growth method. Details on the preparation and growth procedures are provided. The Li concentration in the crystal has been assessed using 7 Li(p,R) 4 He nuclear reaction induced by protons. The [Yb]/[Li] molar composition ratio obtained in several samples is in the 1.01-1.05 range. This result compares well with the ratio [Yb]/[Li] ) 1 expected from the above chemical formula, therefore nonextensive Li loss is experienced during the crystal growth. Polymorphic transformations to phases with symmetry lower than tetragonal have not been observed upon cooling. At room temperature the crystal structure shows the tetragonal space group I4 h (No. 82), with lattice parameters a ) 5.1191 (9) Å and c ) 11.109 (3) Å, V ) 291.11(10) Å 3 , and Z ) 2. This implies a high ytterbium density, namely [Yb] ) 6.87 × 10 21 cm -3 . The optical absorption and photoluminescence properties are described in detail consistently with the anisotropic character of the tetragonal phase. The relative energies of the Yb 3+ Stark levels have been determined and the perspectives for applications as a laser material are evaluated.
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