Crystals of Fe3R(BO3)4 (R = Y, La, Nd) have been grown. Their structure and properties show close relations. The structures of these materials have been established by single-crystal X-ray diffraction. They crystallize in the huntite, CaMg3(CO3)4, structure type, trigonal system, space group R32 (No. 155), Z = 3, with unit-cell parameters (Å) a = 9.592(1), 9.578(1), and 9.563(2) and c = 7.597(7), 7.605(3), and 7.575(2), for R = La, Nd, and Y0.5Bi0.5, respectively. They contain RO6 trigonal prisms and smaller FeO6 octahedra forming layers and giving rise to Fe−R distances of ≃3.78 Å. RO6 polyhedra are interconnected within the layers by corner sharing with triangular BO3 groups of two types and FeO6 octahedra. FeO6 octahedra form 1D helicoidal chains with relatively short Fe−Fe distances, ≃3.18 Å. The temperature dependence of the magnetic susceptibility from 350 to 1.8 K for Fe3R(BO3)4 shows two maxima at about 260 and 35 K and, for R = Nd, another maximum at 6 K. The presence of magnetic order is attributed to magnetic Fe−Fe or Fe−O−Fe interactions. Infrared spectra in the range 4000−240 cm-1 show the ν1, ν3, and ν4 IR-active bands of the B(1)O3 3- triangular groups as well as ν2 of less symmetrical B(2)O3 3-.

A comparison of the suitability of two Tm 3+ -doped monoclinic double tungstate KRE(WO 4 ) 2 (RE = Gd 3+ or Lu 3+ ) laser crystals was carried out based on crystal growth conditions and the strength of crystal field interactions provided by the corresponding host at the Tm 3+ site. For the same 3% Tm 3+ substitution level, macrodefect-free single crystals can be grown, with higher cooling rates and lower temperatures for the KLu(WO 4 ) 2 host. Furthermore, the information provided by the phenomenological crystal field analysis of low temperature polarized spectroscopic data for both hosts indicates that KLu(WO 4 ) 2 exhibits the stronger crystal field and thus an enhanced 3 H 6 splitting compared to KGd(WO 4 ) 2 . Considering these factors as well as its calculated higher emission cross sections, it is concluded that KLu(WO 4 ) 2 is the most suitable of the two hosts for Tm 3+ doping.

Single crystals of Yb 3+ -doped NaGd͑WO 4 ͒ 2 with up to 20 mol % ytterbium content have been grown by the Czochralski technique in air or in N 2 +O 2 atmosphere and cooled to room temperature at different rates ͑4-250°C/h͒. Only the noncentrosymmetric tetragonal space group I4 accounts for all reflections observed in the single crystal x-ray diffraction analysis. The distortion of this symmetry with respect to the centrosymmetric tetragonal space group I4 1 / a is much lower for crystals cooled at a fast rate. Na + , Gd 3+ , and Yb 3+ ions share the two nonequivalent 2b and 2d sites of the I4 structure, but Yb 3+ ͑and Gd 3+ ͒ ions are found preferentially in the 2b site. Optical spectroscopy at low ͑5 K͒ temperature provides additional evidence of the existence of these two sites contributing to the line broadening. The comparison with the 2 F 7/2 ͑n͒ and 2 F 5/2 ͑nЈ͒ Stark energy levels calculated using the crystallographic Yb-O bond distances allows to correlate the experimental optical bands with the 2b and 2d sites. As a novel uniaxial laser host for Yb 3+ , NaGd͑WO 4 ͒ 2 is characterized also with respect to its transparency, band-edge, refractive indices, and main optical phonons. Continuous-wave Yb 3+ -laser operation is studied at room temperature both under Ti:sapphire and diode laser pumping. A maximum slope efficiency of 77% with respect to the absorbed power is achieved for the polarization by Ti:sapphire laser pumping in a three-mirror cavity with Brewster geometry. The emission is tunable in the 1014-1079 nm spectral range with an intracavity Lyot filter. Passive mode locking of this laser produces 120 fs long pulses at 1037.5 nm with an average power of 360 mW at Ϸ97 MHz repetition rate. Using uncoated samples of Yb: NaGd͑WO 4 ͒ 2 at normal incidence in simple two-mirror cavities, output powers as high as 1.45 W and slope efficiencies as high as 51% are achieved with different diode laser pump sources.