C. Jacoboni

Crystal structure of the metastable form of aluminum trifluoride β-AlF3 and the gallium and indium homologs

Abstract The crystal structure of the metastable phase β-AlF3, which is related to the hexagonal tungsten bronze structure, has been solved by X-ray powder and single-crystal diffraction methods. The crystal habit is pseudo-hexagonal with systematic twinning (rotation of 120° around the c axis), but the true symmetry is orthorhombic with space group Cmcm, Z = 12, a = 6.931(3), b = 12.002(6), c = 7.134(2), A (R = 0.044 and Rw = 0.051) from 929 independent reflections). The network is built from very regular AlF6 octahedra rotated by approximately 7.2° from the positions of the ideal HTB structure. A similar network, with the same propagation of the tilting, was observed in the compound (H2O)0.33FeF3 and in the metastable polymorphs of CrF3 and of VF3. Our reinvestigation of the structures of β-GaF3 and β-InF3 using powder data shows that they are isotypic with the aluminum compound, with a = 7.210(1), b = 12.398(2), c = 7.333(1) and a = 7.875(2), b = 13.499(4), c = 7.956(2), A, respectively.

Optical transition probabilities of Er3+ in fluoride glasses☆

Abstract Lead-based fluoride glasses of the system PbF 2 GaF 3  M F 2 ( M = Zn or Mn) doped by trivalent erbium were prepared by melting and quenching of the appropriate fluorides under inert atmosphere. Optical spectra of the glasses with and without manganese reveal identical characteristics in the near ir region of the spectrum. In the near uv and visible part, the manganese-containing samples show higher oscillator strength, probably due to interaction between Mn 2+ and Er 3+ . Radiative and nonradiative characteristics of the glasses were computed, using the Judd-Ofelt method. Laser transitions of the glasses were predicted.

New transition metal fluoride glasses isolated in the PbF2MtIIF2MtIIIF3 systems

Abstract New glasses have been prepared in the PbF 2  M t II F 2  M t III F 3 systems ( m t II = Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ ; M t III = Fe 3+ , V 3+ , Cr 3+ , Ga 3+ ). The extent of the vitreous area is shown in a PbF 2 MnF 2 FeF 3 diagram. Thermal properties have been measured for all samples. Some of these glasses are very transparent over a wide range of wavelengths (from 250–12 000 nm). The sixfold coordination of transition metal ions has been established by spectroscopic investigations. The structure of the glasses is discussed on the basis of a random corner-sharing of MF 6 octahedra.

The local field distribution of in transition metal fluoride glasses investigated by electron paramagnetic resonance

The EPR spectra of in two transition metal fluoride glasses (PZG and PBI) have been extensively studied as functions of temperature, concentration and microwave frequency (S, X, K and Q bands). The marked changes observed in the EPR spectra with temperature and concentration imply the existence of pairs of ions. The spectra of isolated ions were obtained at low temperature for samples with low doping concentration. Isolated ions are characterized by a fine-structure -parameter distribution within the range . The simulations of these spectra are computed with a Czjzek fine-structure parameter distribution (this solution prohibits the existence of axial symmetry sites). It indicates a decrease of the number of geometrical constraints compared to those for and (octahedral coordination in a fluoride medium), in agreement with the higher coordination numbers. -ion sites are found to be more distorted in PZG than in PBI, suggesting that the glass network has an influence on the polyhedron distortion amplitude.

The comparison of calculated transition probabilities with luminescence characteristics of erbium(III) in fluoride glasses and in the mixed yttrium-zirconium oxide crystal

Abstract Fluorozirconate glasses containing 2 mole% ErF3 were prepared by melting the binary fluorides with ammonium bifluoride under an atmosphere of carbon tetrachloride and argon at 850°C. Absorption spectra of these glasses were obtained and the Judd-Ofelt parameters were calculated. Emission spectra and lifetimes of erbium in fluorozirconate glass, in lead-gallium-zinc fluoride glass, and in yttrium-zirconium oxide crystal were measured and compared with the theoretical calculations. Laser emission lines in these materials are deduced from these measurements. It is suggested that materials doped with erbium may serve as light sources for fiber optic waveguides made from the undoped materials.

Croissance cristalline des composés fluorés de type RbCoF3 perovskite et RbCoCrF6 pyrochlore

Abstract Studies of crystal growth of fluorinated perovskites AMe II F 3 and pyrochlores AMe II Me III F 6 (A = K, Rb, Cs; Me II = Fe, Co, Ni, Zn; Me III = Al, V, Cr, Fe) are reported. In order to avoid high temperature hydrolysis of these compounds thus giving MeO or Me 2 O 3 , two techniques of growth have been selected: 1) flux growth from a mixture containing melted chlorides and 2) the Bridgman-Stockbarger method in sealed platinum crucible. Phase diagrams are used in order to obtain maximum yield of crystals and avoid side crystallization.

Infrared to visible conversion in rare-earth-doped planar waveguides

The first observation of optical properties of new rare-earth-doped vitreous fluoride waveguides has been recently reported. These vitreous fluoride films have been deposited by a physical vapour deposition technique on fluoride substrates. Several PZG films and sandwiches deposited on various substrates have been prepared with thickness of a few μm and doped in the guiding layer by various amounts of erbium and other rare-earth ions. High δn/n (10−2) has been achieved as a result of the glass compositions. Infrared as well as green and blue upconversion pumped fluorescences have been recorded under side light injection.

Energy transfer between managanese(II) and erbium(III) in various fluoride glasses

The fluorescence spectra and lifetimes of fluoride glasses of molar composition 36PbF2, 24MnF2 (or ZnF2), 35GaF3, 5 (or 7) Al(PO3)3, doped by ErF3 were investigated. The emission of Mn(II) in absence of Er(III) consists of a broad band centered around 630 nm and an integrated lifetime of 1.4 msec. In the presence of Er(III) the intensity and lifetimes are decreased as a result of energy transfer to the 4F92 level of Er(III). The fluorescence of Er(III) arising from 4S32 at 543 nm has an integrated lifetime of 0.06 ms in the absence of Mn(II) and is decreased to 0.01 ms in the presence of Mn(II) as a result of energy transfer to Mn(II). The 666-nm luminescence of Er(III) due to 4F92 emission under excitation at 370 nm (4G112) is about 20 times weaker than the 543-nm emission when Mn(II) is absent. However, in the presence of Mn(II) this emission becomes 5 times stronger than the 543-nm emission. This intensified emission has a non-exponential time dependence. The longer component corresponds to the transfer of stored energy in Mn(II) to Er(III) while the short-lived component is probably due to cascading down Er(III) → Mn(II) → Er(III) through states above the Stokes threshold of Mn(II). This interpretation is backed up by weaker 543-nm emission and stronger 630-nm broad-band emission when the mixed system is excited in one of the upper excited states, of Mn(II) at 395 nm, or of Er(III).

Les pyrochlores AIB2X6: Mise en evidence de l'occupation par le cation AI de nouvelles positions cristallographiques dans le groupe d'espace fd3m

Abstract In A I B 2 X 6 pyrochlores—A I = Pb, Tl; X = O, F— the A I ions occupe 32e positions with a probability of 25% instead of the 8b positions as it was previously described; this results from the observations of very weak X-diffraction peaks provided by planes such as h = 4n, k = 4n, l = 4n + 2, visible on single crystals photographs of RbCoCrF 6 , RbNb 2 O 5 F, TlNb 2 O 5 F; this displacement of A I with respect to the 8b position -about 0,6 A for Tl + and 0,4 A for Rb + - is in agreement with the great size of the 8b cavity -about 1,80 A-. However Cs + , the radius of which is well adapted to this cavity, is not significantly displaced in homologous compounds.

Investigation of fluorine octahedron connectivities in transition metal fluoride glasses by solid-state magic-angle-spinning nuclear magnetic resonance spectroscopy

High-resolution magic-angle-spinning (MAS) NMR spectroscopy is used to investigate the structural properties of some transition metal fluoride glasses ( - - ) related to the fluorine network. Several glass compositions are investigated in order to vary the ratios. MAS NMR experiments are carried out on certain crystalline compounds selected as being the initial constituents or recrystallization compounds of glassy phases, or because they have some specific particular connectivities of the fluorine octahedron network (e.g. ). It is shown that three types of fluorine are involved in the glass network: free fluorines which are not connected to transition metal ions, and shared and unshared fluorines belonging to and octahedra. Quantitative information on these three different fluorine sites, their relative ratios in the glassy networks, and the degree of cross-linking of the fluorine octahedra is obtained. Our results prove the validity of the previously adopted assumption according to which the glass network is built up from corner-sharing fluorine octahedra centred on transition metal ions.