Lanthanide distribution in some doped alkaline earth aluminates and gallates

High-resolution neutron and synchrotron X-ray powder diffraction studies are reported for the six oxides AB₂O₄ (A=Ca²⁺, Sr²⁺ and Ba²⁺ and B=Al³⁺ and Ga³⁺). These oxides all adopt a stuffed tridymite type structure, the precise nature of which depends on both the A- and B-type cations. Bond valence calculations reveal a range of values for the various A-type cations, in all cases at least one site is significantly underbonded. Conversely the tetrahedral B-type sites invariably exhibit unexceptional bond valencies. Attempts to dope the oxides with various lanthanides to the 1% level invariably resulted in some segregation into alternate phases located at the grain boundaries. The identity of the impurity phases is presented and the importance of bond valencies in understanding this segregation is highlighted.     

Structural refractometry of β- and β″-Al2O3-type aluminates and gallates

The ionicity of oxide and β- and β″-Al₂O₃ crystals is identified via Lorentz–Lorenz molar refractivity. The investigation was extended to sodium gallates, the refractivity and average refractive index of which are estimated.     

Hydrodefluorination of non-activated C-F bonds by diisobutyl-aluminiumhydride via the aluminium cation [i-Bu2Al]

A novel system for the hydrodefluorination (HDF) of non-activated C-F bonds at room-temperature is described. The reaction of i-Bu₂AlH with [Ph₃C][B(C₆F₅)₄] (1), [Ph₃C][Al(C₆F₅)₄] (2) and [Ph₃C][Al{OC(CF₃)₃}₄] (3) as precatalysts leads under formation of triphenylmethane to the aluminium cation [i-Bu₂Al]⁺ and the non-coordinating anions [M(C₆F₅)₄]⁻ (M = B, Al) and [Al{OC(CF₃)₃}₄]⁻. The formed aluminium cation is very reactive towards C-F bonds and easily forms i-Bu₂AlF releasing a carbocation that abstracts the hydride of excess i-Bu₂AlH and yields the corresponding hydrocarbon. Thereby, the active species [i-Bu₂Al]⁺ is regenerated and can realize a catalytic cycle. For 1-fluorohexane as an example including non-activated C-F bonds different activities were found (TON: 1: 20; 2: 12; 3: 30) in cyclohexane as solvent.     

Strukturchemische und magnetische Untersuchungen an Ho3+‐β″‐Al2O3(Ho0, 5Mg0, 5Al10, 5O17)

Structural Chemistry and Magnetic Properties of Ho³⁺‐β″‐Al₂O₃(Ho_{0, 5}Mg_{0, 5}Al_{10, 5}O₁₇) The crystal structure of Ho³⁺‐β″‐Al₂O₃(Ho_{0, 5}Mg_{0, 5}Al_{10, 5}O₁₇) was determined by single crystal X‐ray diffraction methods at room temperature (trigonal, R3¯m, Z = 3, a = 561.43(12) pm, c = 3353.7(11) pm). The structural chemical results are correlated with magnetic measurements, where ligand field calculations applying the angular overlap model have been taken into account.     

Preparation and luminescence characterization of fine-sized LaSr2AlO5:Ce phosphor prepared by spray pyrolysis

Fine-sized (La_1−x,Ce_x)Sr₂AlO₅ (LSA:Ce) yellow phosphor particles were prepared by spray pyrolysis. The crystal structure and optical properties of prepared LSA:Ce particles were studied by changing the reduction temperature and the Ce content. The LSA:Ce particles had spherical-like shape and fine size of less than ∼2 μm in the reduction temperature range from 1300 to 1450 °C. The luminescence intensity was steadily improved by elevating the reduction temperature due to the increase of crystallinity. A redshift in the emission peak was observed on increasing the Ce content due to the reduction of Ce-O distance. Concentration quenching in the luminescence intensity was observed when the Ce concentration was 1% (x=0.01), which was caused mainly by the electric dipole-dipole interaction. The critical transfer distance was calculated as 29 Å, which is in good agreement with the value estimated from the spectral data.     

Vacuum ultraviolet spectroscopy of Pr in CaAl4O7, LaMgAl11O19 and SrLaAlO4

The vacuum ultraviolet spectroscopy of Pr³⁺ doped CaAl₄O₇, LaMgAl₁₁O₁₉ and SrLaAlO₄ is reported. It appears that whenever the aluminate host lattice is excited directly, mainly exciton and 4f²–4f² [³P₀] Pr³⁺ emission are observed. When the excitation energy is lower, Pr³⁺ ions are excited selectively and 4f5d–4f² emission dominates. These observations can be explained by assuming that energy transfer from the host lattice to the Pr³⁺ ion takes place preferentially via an intermediate exciton state with an energy too low to reach the energetic Pr³⁺ 4f5d excited states.     

5d-level energies of Ce and the crystalline environment. IV. Aluminates and “simple” oxides

Information on the energy of 5d-levels of Ce³⁺ ions in aluminates and “simple” oxides has been collected. The crystal field splitting of the 5d-levels is interpreted in terms of the type and size of anion polyhedron coordinating the Ce³⁺ ion. The centroid (barycenter) shift of the 5d-configuration is analyzed by a ligand polarization model providing values for the spectroscopic polarizability α_sp of the anion ligands. The data provide evidence that the centroid shift behaves independently from the crystal field splitting. By combining centroid shift and crystal field splitting, the “spectroscopic” redshift of the first electric dipole-allowed fd transition of Ce³⁺-doped in the compounds will be interpreted. The large crystal field splitting in garnet compounds and the small splitting in perovskite compounds will be discussed.     

Synthesis and Structures of some Lithium and Sodium (Aminoalcoholato)hydrido Aluminates

LiAlH₄ and NaAlH₄ were allowed to react in a 1 : 1, 1 : 2 and 1 : 3 ratio with 2-dimethylaminoethanol, 1-dimethylamino-2-propanol, and (–)-N-methylephedrine. Reactions in the 1 : 1 ratio leads to mixtures of M(AlH_4–n(OR′NMe₂)_n) in thf solution showing a temperature dependent composition. From the solutions obtained in a 1 : 2 or 1 : 3 ratio the following compounds were isolated as single crystals: (thf)NaAlH₂ · (OCH₂–CH₂NMe₂)₂ ( 2 ), LiAlH₂(OCHMe–CH₂NMe₂)₂ ( 3 ), LiAlH₂(OCHPh–CHMe–NMe₂)₂ ( 4 ), LiAlH(OCH₂CH₂NMe₂)₃ ( 5 ), NaAlH(OCH₂CH₂NMe₂)₃ ( 6 ). All compounds are dimers in the solid state and their Al atoms are pentacoordinated. M–H–Al bridges were found only in the dihydridoaluminate  2 .     

Low-temperature synthesis of SrAl2O4 by a modified sol-gel route: XRD and Raman characterization

Among other alkaline-earth aluminates, the monoclinic (M) polymorph of SrAl₂O₄ can be used as host material for Eu²⁺ luminescence based phosphors. With the aim of reducing the synthesis temperature of this polymorph, we have produced and characterized by XRD and Raman scattering solid solutions of the SrAl_2−xB_xO₄ system (x?0.3) obtained by two different methods, a ceramic route and a modified sol-gel synthesis. Though the addition of boron lowers the temperature of obtention of the M polymorph in both type of samples, lower B contents are needed to stabilize the M form as single phase for samples prepared by the sol-gel method than through the ceramic route. In the sol-gel method, the M polymorph can be obtained at temperatures as low as 1200 °C, with a Boron content of just 1%. Rietveld profile analysis allows us to conclude that coexistence of the monoclinic and hexagonal polymorphs of SrAl₂O₄ occurs for samples synthesized below an onset temperature of about 1000-1100 °C, that depends on the sample composition. Above those temperatures, only the monoclinic phase is formed.     

Multiband orange-red photoluminescence of Eu ions in new “114” LnBaZn3GaO7 and LnBaZn3AlO7 oxides

A new series of gallozincates LnBaZn₃GaO₇ (Ln=La, Nd, Sm, Eu, Gd, Dy, Y) and new aluminozincates LnBaZn₃AlO₇ (Ln=Y, Eu, Dy) have been synthesized. Their structure refinements show that these phases belong to the “114” series, with hexagonal P6₃mc space group previously described for SmBaZn₃AlO₇. The photoluminescence study of these oxides shows that the Eu³⁺ activated LnBaZn₃MO₇ oxides with Ln=Y, La, Gd; and M=Al, Ga exhibit strong magnetic and electric dipole transitions (multiband emission) which is of interest for white light production. These results also confirm that the site occupied by Eu³⁺ is not strictly centrosymmetric. The electric dipole transition intensity is the highest in GdBaZn₃MO₇ [M=Al, Ga]: 0.05Eu³⁺ as compared with other Eu³⁺ activated compositions. This is due to the layer distortion around GdO₆ octahedra when compared with YO₆ and LaO₆ octahedra.