R. De Pape

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.

Hexagonal tungsten bronze-type FeIII fluoride: (H2O)0.33FeF3; crystal structure, magnetic properties, dehydration to a new form of iron trifluoride

(H2O)0.33FeF3, grown by hydrothermal synthesis, crystallizes in the orthorhombic system with cell dimensions a = 7.423(3) A, b = 12.730(4) A, c = 7.526(3)A, and space group Cmcm, Z = 12. The structure, derived from single crystal X-ray diffraction data (605 independent reflections) is refined to R = 0.019 (Rω = 0.021). The framework of the FeIIIF6 octahedra is related to that of hexagonal tungsten bronze (HTB) Rb0.29WO3. At 122°C, zeolithic water is evolved from hexagonal tunnels without any noticeable change of the fluorine skeleton. The related anhydrous compound represents a new form of iron trifluoride which is denoted HTBFeF3; at 525°C, it transforms into the cubic form of ReO3-type. (H2O)0.33FeF3 and HTBFeF3 are antiferromagnetic, with Neel temperatures of TN = 128°7 ± 0.5 K and TN = 97 ± 2 K, respectively.

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.

A new form of FeF3 with the pyrochlore structure : Soft chemistry synthesis, crystal structure, thermal transitions and structural correlations with the other forms of FeF3

The topotactic oxidization of the mixed valence fluoride NH4Fe2F6, which exhibits the ordered modified pyrochlore structure leads to a cubic pyrochlore form of FeF3, hereafter noted Pyr-FeF3 (S.G. Fd3m, Z = 16 a = 10,325(2) A). Fe3+ and F− ions occupy positions 16 c (0 0 0) and 48 f (x 1818 with x = 0,3104 (5)) respectively. The best oxidization reaction occurs in a boiling solution of Br2 in acetonitrile (T = 81°C). The treatment leads to a zeolitic association whose composition varies from (NH3)0.10FeF3 to (NH3)0.25FeF3 according to the experimental conditions. A similar reaction allows the preparation of HTBFeF3 (3) from the fluorinated bronze (NH4)0.25FeF3. PyrFeF3 orders magnetically at 20(2) K ; this low value is related to the highly frustrating character of the Fe3+ cationic sublattice.

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.

Crystal structure of the ordered pyrochlore NH4FeIIFeIIIF6 structural correlations with Fe2F5 · 2H2O and its dehydration product Fe2F5 · H2O

Abstract Crystal structure of NH4FeIIFeIIIF6 is studied in order to explain further its peculiar antiferromagnetic behavior compared to the spin glass one of the pyrochlore family. NH4FeIIFeIIIF6 is orthorhombic, space group Pnma with a = 7.045 (4) A , b = 7.454 (4) A , c = 10.116 (6) A , Z = 4. Diffraction data on single crystals obtained by hydrothermal synthesis, collected on an automatic four circle diffractometer, have been refined by full matrix least-squares calculations to a weighted value of 0.029 (unweighted R = 0.024) for 798 observed reflections. This structure is derived from the pyrochlore structure, with a cationic order between Fe2+ and Fe3+ ions. (FeIIF6)4− octahedra form infinite trans chains along [100] by sharing corners although similar chains of (FeIIIF6)3−) octahedra lie along [010]. This type of FeIIFeIII order is related to a similar one existing in Fe2F5 · 2H2O, the dehydration of which leads to the pyrochlore Fe2F5 · H2O. A mechanism is proposed to explain the formation of this compound.

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.

Structures et proprietes magnetiques de quelques composes de formule M Fe F3 (M Na, K, Rb, Cs, NH4, T1)

Abstract The relations between structures and magnetic properties of some compounds with formula M Fe F 3 are discussed. The exchange interaction integrals are calculated by the Bethe-Peierls-Weiss methods.