Cesium Oxo-fluoro-aluminates in the CsF-Al2O3 System: Synthesis and Structural Characterization

In an experiment combining various approaches, a precise examination of a portion of the phase diagram of a CsF-Al2O3 system was carried out up to 40 mol% Al2O3. CsF-Al2O3 solidified mixtures have been investigated using high-field solid-state NMR (133Cs, 27Al, and 19F) spectroscopy and X-ray powder diffraction over a broad range of compositions with synchrotron powder diffraction and Rietveld analysis. A new cesium oxo-fluoro-aluminate, Cs2Al2O3F2, was discovered, prepared, and structurally analyzed by synchrotron diffraction analysis. In addition to Cs2Al2O3F2, we have synthesized the following pure compounds in order to aid in the interpretation of NMR spectra of the solidified samples: CsAlF4, Cs3AlF6, and CsAlO2.

KLi2RE(BO3)2 (RE = Dy, Ho, Er, Tm, Yb, and Y): Structural, Spectroscopic, And Thermogravimetric Studies on a Series of Mixed-Alkali Rare-Earth Orthoborates

We report a detailed structural, spectroscopic, and thermogravimetric investigation of a new series of mixed-alkali rare-earth orthoborates KLi₂RE(BO₃)₂ (RE = Dy, Ho, Er, Tm, Yb, and Y). Single crystals were directly prepared by a flux method as well as mechanically separated from the polycrystalline powder obtained from the conventional solid-state reactions. All KLi₂RE(BO₃)₂ members are isotypic and crystallize in the space group P2₁/n. The novel structure type is comprised of [RE₂(BO₃)₄O₄]^14- anionic clusters where the edge-sharing REO₇ pentagonal bipyramids are connected by BO₃ groups and both K⁺ and Li⁺ cations are located at the interstitial voids of the 3D network. The metric parameters and crystal structural features obtained from the single-crystal data are in excellent agreement with those refined from the powder data. The change of the lattice parameters and unit cell volumes can be explained in terms of the lanthanide contraction effect. A comparison between KLi₂RE(BO₃)₂ and other rare-earth borates with similar chemical compositions indicates that the sum of the ionic radii of the alkali-metal cations governs the symmetry of the crystals. Diffuse reflectance UV-vis spectra display the characteristic absorption behaviors of the RE³⁺ cations and the fundamental absorption edge. Both the Tauc's and derivation of absorption spectrum fitting (DASF) methods were used to identify the magnitude and type of bandgap, respectively, which are compared with those obtained from density functional theory (DFT) calculations. The calculated phonon density of states and the vibrational frequency at the gamma point help explain the Fourier transform infrared and Raman spectra of KLi₂RE(BO₃)₂. The incongruent melting behavior and the thermal stability of each member of the KLi₂RE(BO₃)₂ series were also studied by thermogravimetric analyses.

Oxo- and Oxofluoroaluminates in the RbF-Al2O3 System: Synthesis and Structural Characterization

Precise research on the RbF-Al₂O₃ system was carried out by means of combining X-ray powder diffraction, high-field solid-state NMR spectroscopy, and thermal analysis methods. α-Rb₃AlF₆, RbAlO₂, Rb₂Al₂₂O₃₄, and new phase, Rb₂Al₂O₃F₂, were identified in the system. The structure of this new rubidium oxofluoroaluminate was determined. It is built up from single layers of oxygen-connected AlO₃F tetrahedra, those layers beeing separated by fluorine atoms. This type of structure exhibits a decent ionic conductivity at ambient temperature, 1.74 × 10^-6 S cm^-1. The similar structural arrangement of O₃Al-O-AlO₃ and FO₂Al-O-AlO₂F tetrahedra of the conduction planes in Rb₂Al₂₂O₃₄ and Rb₂Al₂O₃F₂ were confirmed by ²⁷Al NMR measurements. A thermal analysis of the RbF-Al₂O₃ system revealed that it can be defined as a pseudobinary subsystem of the more general quaternary RbF-AlF₃-Al₂O₃-Rb₂O phase diagram. From a phase analysis of individual phase fields, the mutual metastable behavior of all founded phases can be considered. It was observed that fluoro- and oxoaluminates exist together. Rb₂Al₂O₃F₂ is more stable under high temperature. Rubidium fluoro- and oxoaluminates are metastable precursors of the thermodynamically more stable structure of rubidium oxofluoroaluminate.

(Oxo)(Fluoro)-Aluminates in KF-Al2O3 System: Thermal Stability and Structural Correlation

Precise investigation of part of the phase diagram of KF-Al₂O₃ system was performed in an experiment combining different techniques. Solidified mixtures of KF-Al₂O₃ were studied by X-ray powder diffraction and high-field solid-state NMR spectroscopy over a wide range of compositions. To help with the interpretation of the NMR spectra of the solidified samples found as complex admixtures, we synthesized the following pure compounds: KAlO₂, K₂Al₂₂O₃₄, α-K₃AlF₆, KAlF₄, and K₂Al₂O₃F₂. These compounds were then characterized using various solid-state NMR techniques, including MQ-MAS and D-HMQC. NMR parameters of the pure compounds were finally determined using first-principles density functional theory calculations. The phase diagram of KF-Al₂O₃ with the alumina content up to 30 mol % was determined by means of thermal analysis. Thermal analysis was also used for the description of the thermal stability of one synthesized compound, K₂Al₂O₃F₂.

Reductive cyclization of 2-nitro-2′-hydroxy-5′-methylazobenzene to benzotriazole over K-doped Pd/γ-Al2O3

A series of Pd/γ-Al₂O₃ catalysts modified by potassium salts were prepared and evaluated in the reductive cyclization of 2-nitro-2′-hydroxy-5′-methylazobenzene without additional base.

The natural tiling approach to cation conductivity in KAlO2 polymorphs

A detailed analysis of correlations between structural features and cation conductivity is performed for KAlO2 polymorphs in a wide temperature range of 300–1023 K. To explore the migration maps of K+ cations we have used neutron diffraction data for low- and high-temperature KAlO2 polymorphs and applied for the first time a novel algorithm based on the natural tiling concept and implemented it into the program package TOPOS. Five independent elementary channels for the K+ cation migration have been revealed whose cross-sections were found to be essentially different in the low-temperature form, indicating a high anisotropy of the cation conductivity. During the transition to the cubic high-temperature phase all five channels become equivalent with sharply increased cross-sections that account for the jump-like increase of the cation conductivity and gives rise to its three-dimensional character.

New phases in the Na3PO4-Mg3(PO4)2 binary quenched from 1000 °C

A reinvestigation of the Na3PO4-Mg3(PO4)2 binary at 1000 °C has identified two new ternary oxide phases and confirmed the existence of an orthorhombic solid solution phase in the vicinity of NaMg4(PO4)3. Cubic γ-Na3PO4 shows a solid solution up to ~20 mol. % Mg3(PO4)2. Between ~24 and ~29 mol. % a new γ-Na3PO4-related rhombohedral superstructure phase is observed. At ~44 mol. % Mg3(PO4)2 a new phase with a cristobalite-related structure is observed, which on cooling rapidly to room-temperature transforms to KGaO2-type (Pbca: a = 10.409(2) Å, b = 14.492(2) Å, c = 5.211(1) Å). The NaMg4(PO4)3 solid-solution phase extends between ~72 and 80 mol. % Mg3(PO4)2. Surprisingly, and contrary to previous investigations, there is no phase at 50 mol. %, i.e. with stoichiometry NaMgPO4. All high-temperature annealing was performed in sealed Pt tubes to prevent loss of components to the atmosphere, which is suspected as the reason for the quite different results reported by some previous workers.