RESUMO
We report a joint experimental and theoretical investigation of the high pressure structural and vibrational properties of terbium sesquioxide (Tb2O3). Powder X-ray diffraction and Raman scattering measurements show that cubic Ia3Ì (C-type) Tb2O3 undergoes two phase transitions up to 25 GPa. We observe a first irreversible reconstructive transition to the monoclinic C2/m (B-type) phase at â¼7 GPa and a subsequent reversible displacive transition from the monoclinic to the trigonal P3Ì m1 (A-type) phase at â¼12 GPa. Thus, Tb2O3 is found to follow the well-known C â B â A phase transition sequence found in other cubic rare earth sesquioxides with cations of larger atomic mass than Tb. Our ab initio theoretical calculations predict phase transition pressures and bulk moduli for the three phases in rather good agreement with experimental results. Moreover, Raman-active modes of the three phases have been monitored as a function of pressure, while lattice-dynamics calculations have allowed us to confirm the assignment of the experimental phonon modes in the C- and A-type phases as well as to make a tentative assignment of the symmetry of most vibrational modes in the B-type phase. Finally, we extract the bulk moduli and the Raman-active mode frequencies together with their pressure coefficients for the three phases of Tb2O3. These results are thoroughly compared and discussed in relation to those reported for rare earth and other related sesquioxides as well as with new calculations for selected sesquioxides. It is concluded that the evolution of the volume and bulk modulus of all the three phases of these technologically relevant compounds exhibit a nearly linear trend with respect to the third power of the ionic radii of the cations and that the values of the bulk moduli for the three phases depend on the filling of the f orbitals.
RESUMO
We present, to the best of our knowledge, the first laser demonstration of an Yb-doped Gd(2)O(3) cubic crystal. This crystal was obtained by the flux method using an original borate-based solvent, which was particularly well suited to the growth of rare earth sesquioxide crystals at half the working temperature of classical growth techniques. This flux method is a very interesting alternative for the production of laser sesquioxide crystals, not only because it provides access to new matrices of the cubic polymorph, but also because it permits high Yb(3+)-doping levels for these crystals. The first laser results of two highly Yb(3+)-doped sesquioxides, namely Gd(2)O(3) and Y(2)O(3), grown by this flux method are presented here, including the Ti:sapphire and diode pumping configurations. Laser efficiencies and emission spectra for these two crystals were studied and compared.
RESUMO
Single crystals of lithium iron tungstate, LiFe(WO(4))(2), were obtained using a high-temperature solution growth method. The analysis was conducted using the monoclinic space group C2/c, with ß = 90.597 (2)°, giving R1 = 0.0177. The Li and Fe atoms lie on twofold axes. The structure can also be refined using the orthorhombic space group Cmcm, giving slightly higher residuals. The experimental value of ß and the residuals mitigate in favour of the monoclinic description of the structure. Calculated bond-valence sums for the present results are closer to expected values than those obtained using the results of a previously reported analysis of this structure.
RESUMO
Synthesized powders and grown single crystals of nominal compositions Li(6)Ln(BO(3))(3):Yb(3+) (Ln=Y, Gd) were investigated by means of powder and single-crystal X-ray diffraction (XRD), as well as optical near-IR spectroscopy in conjunction with electron paramagnetic resonance (EPR) spectroscopy. The appearance of two distinct zero-phonon lines suggests the existence of two kinds of Yb(3+) ions in the single crystals. The XRD results exclude the possibility of a phase transition occurring between room and low temperatures. EPR spectra of single crystals show the presence of both isolated ions and pairs of ytterbium ions substituted for Y(3+). A strong temperature dependence of the intensity of Yb-Yb pairs resonance lines coincides with temperature dependence of emission peak at 978 nm, confirming a common origin of the defect giving rise to these spectra. Calculated from EPR spectra, the distance between pairs of Yb(3+) is in good agreement with crystallographic ones: R=3.856 Å, R(cryst) =3.849 Å.
