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.

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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.

Double tungstate and molybdate compounds with the general formulae MT(WO 4 ) 2 and MT(MoO 4 ) 2 where M is a monovalent alkali cation (Li-Cs) and T is a trivalent cation (Y, La or rare earth Ln) exhibit ordered phases with separate sites for M and T cations and disordered phases where M and T cations are randomly distributed over the same cationic sublattice [1]. Some of the optically passive ordered phases like the monoclinic KGd(WO 4 ) 2 , KY(WO 4 ) 2 , and KLu(WO 4 ) 2 are established laser hosts with very large absorption and emission cross sections of the active dopant. The sodium compounds NaT(WO 4 ) 2 and NaT(MoO 4 ) 2 represent disordered phases with tetragonal structure at room temperature. For T = Y, La, Ce-Er, they exhibit also a congruent melting character. Hence their growth with active Ln-dopants by the Czochralski method is very attractive for the synthesis of novel crystalline laser materials.The revived interest in tetragonal sodium double tungstates and molybdates is due to their potential to ensure larger tunability and bandwidths in mode-locked diodepumped solid-state lasers in comparison to ordered crystals. This is especially true for doping with Yb 3+ which, due to the stronger electron-phonon coupling to the lattice, exhibits intrinsically broader linewidths than the Nd 3+ ion. Hence, the requirements to the pump laser diodes are reduced and Yb-doped disordered hosts hold a greater promise for the generation of mode-locked pulses shorter than 100 fs. It should be added that Yb 3+ possesses longer energy-storage lifetime and smaller quantum defect than Nd 3+ , and that it can be pumped by the optically more robust InGaAs laser diodes operating in the 900 -1000 nm spectral range [2]. Finally, the relatively simple two-manifold structure of Yb 3+ prohibits excited state absorption, up-conversion and cross-relaxation processes.The sodium lanthanum crystals NaLa(WO 4 ) 2 (NaLaW) and NaLa(MoO 4 ) 2 (NaLaMo) were studied in the past as room temperature hosts only for Nd 3+ lasers. Lamp-pumped pulsed laser operation of Nd: NaLaW was reported for the 4F 3/2 -4I 11/2 transition at 1063.5 nm [3,4] and for the 4F 3/2 -4I 13/2 transition at 1335.5 nm [5]. Lamp-pumped operation of Nd:NaLaMo was demonstrated not only in the pulsed regime at 1059.5 -1065.3 nm [6, 7] and 1338 -1344 nm [5] but also in the continuous-wave (cw) regime at 1065.3 nm [8]. In addition NaLaMo, which is an efficient Raman active medium [9], when doped with Nd 3+ , was shown to be an efficient self-converting Raman crystal both in the picosecond [10] and in the nanosecond [11] regime.Here we report on the cw laser performance of Yb :NaLaW and Yb:NaLaMo crystals at room temperature. Note that the isostructural NaGd(WO 4 ) 2 or NaGdW, was the first and so far only disordered laser crystal of this type for which room-temperature cw laser operation could be demonstrated with Yb 3+ doping [12].Single crystals of disordered NaLa(WO 4 ) 2 and NaLa(MoO 4 ) 2 doped with Yb 3+ are grown by the Czochralski method from the melt. Continuous-wave...

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The preparation and characterization of two polymorphic phases of AgNd(WO 4 ) 2 are described. The high-temperature phase of AgNd(WO 4 ) 2 is prepared as a polycrystalline powder and as a single crystal. X-ray diffraction analysis indicates that the crystal has at 300 K the tetragonal symmetry of the space group (SG) I4 h (No. 82), with two independent crystal sites, 2b and 2d, for Nd 3+ cations and structural disorder around them. The 5 K ground state optical absorption of this tetragonal crystal clearly differs from that corresponding to the monoclinic SG C2/m (No. 12) ordered phase found in polycrystalline samples prepared below 800 °C. Four times larger bandwidths and a weaker crystal field (CF), that is, lesser CF splitting for all Nd 3+ 2S+1 L J manifolds, are observed for the tetragonal phase. Well-defined S 4 polarization rules have been determined in the tetragonal phase, and then the observed 99 Nd 3+ energy levels were labeled with the appropriate Γ 7,8 or Γ 5,6 irreducible representations. A detailed Hamiltonian of 26 free ion and CF parameters have been used in the simulation of the phenomenological energy levels and associated wave functions of the 4f 3 configuration of Nd 3+ in the tetragonal AgNd(WO 4 ) 2 single crystal, with final σ ) 12.6 cm -1 . The validity of the above set of CF parameters and wave functions has been established through the good reproduction of the thermal variation of the measured anisotropic paramagnetic susceptibility χ. As a result of this simulation it is shown that the larger bandwidths of the tetragonal phase contain nonresolved contributions from the two Nd 3+ sites. A method to control overheating events is proposed on the basis of the nonreversibility of the tetragonal phase into the monoclinic one.

Finding similarities in sequences and/or structures is one of the fastest ways of characterizing proteins. Similarity in atomic structure of proteins is generally more recognizable than similarity in sequence and may be more closely related to similarity in function. If the atomic structure is not available, useful information can be obtained by X-ray solution scattering. Ab initio methods to analyse the scattering data require longer computation time and yield low resolution models only. We present an approach to rapidly characterize proteins with unknown structure based on comparison of experimental scattering profiles with a database of scattering patterns calculated from known structures.