In situ X-ray diffraction (XRD) and NMR methods were used to follow the structural changes that occur during the dismutation reaction of hydrochlorofluorocarbon-22 (CHClF2) over γ-alumina. Use of a flow cell allowed diffraction patterns to be recorded, while the reaction products were simultaneously monitored downstream of the catalyst bed, by gas chromatography. No visible structural changes of γ-Al2O3 were observed at 300 °C, the temperature at which this material becomes active for catalysis. A new phase began to form at 360 °C, which by 500 °C completely dominated the XRD powder pattern. 19F/27Al cross-polarization (CP) experiments of γ-Al2O3 activated at 300 °C showed that AlF3 had already begun to form at this temperature. By 400 °C, resonances from a phase that resembles α-AlF3 dominate both the 19F and 27Al NMR spectra of the used catalyst. In situ XRD experiments of the catalytically inactive α-AlF3 phase were performed to investigate the structural changes of this material, associated with the e...

In Situ X-ray Diffraction and Solid-State NMR Study of the Fluorination of γ-Al2O3 with HCF2Cl

The database on structural phase transitions is presented in the simple and easy accessible form of table. The symmetry changes and temperature for 3446 phase transitions and the references for each of 2242 registered crystalline materials with well defined stoichoimetry are the main data in this table. The rules for constructing and reading the database table are also presented.

Structural phase transitions in crystals. I. Database

Relative to its extremely low abundance in the Earth's crust, tellurium is the most mineralogically diverse chemical element, with over 160 mineral species known that contain essential Te, many of them with unique crystal structures. We review the crystal structures of 703 tellurium oxysalts for which good refinements exist, including 55 that are known to occur as minerals. The dataset is restricted to compounds where oxygen is the only ligand that is strongly bound to Te, but most of the Periodic Table is represented in the compounds that are reviewed. The dataset contains 375 structures that contain only Te cations and 302 with only Te, with 26 of the compounds containing Te in both valence states. Te was almost exclusively in rather regular octahedral coordination by oxygen ligands, with only two instances each of 4- and 5-coordination. Conversely, the lone-pair cation Te displayed irregular coordination, with a broad range of coordination numbers and bond distances. A threshold was applied for Te-O links of ~2.45 A or 0.3 valence units with some flexibility, as a criterion to define strongly bound Te-O polymers and larger structural units. Using this criterion, Te cations display one-sided 3-, 4- or 5-coordination by oxygen (with rare examples of coordination numbers 2 and 6). For both valence states of Te, examples are known of TeO complexes which are monomeric (m = 1; neso), noncyclic finite oligomers (soro), rings (cyclo), infinite chains (ino), layers (phyllo) and frameworks (tecto tellurates). There is a clear analogy to the polymerization classes that are known for silicate anions, but the behaviour of Te is much richer than that of Si for several reasons: (1) the existence of two cationic valence states for Te; (2) the occurrence of multiple coordination numbers; (3) the possibility of edge-sharing by TeO polyhedra; (4) the possibility for oxygen ligands to be 3-coordinated by Te; and (5) the occurrence of TeO polymers that are cationic, as well as neutral or anionic. While most compounds contain only one or two symmetrically distinct types of Te atom, Pauling's Fifth Rule is frequently violated, and stoichiometrically simple compounds such as CaTeO can have polymorphs with up to 18 distinct Te sites. There is a tendency for local symmetry features such as the threefold axis of a TeO octahedron or the acentric symmetry of a TeO polyhedron to be inherited by the host structure; the latter in particular can lead to useful physical properties such as nonlinear optical behaviour. We develop for the first time a hierarchical taxonomy of Te-oxysalt structures, based upon (1) valence state of Te; (2) polymerization state of TeO complexes; (3) polymerization state of larger strongly-bound structural units that include non-Te cations. Structures are readily located and compared within this classification.

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A review of the structural architecture of tellurium oxycompounds

We have investigated the structures of a series of pristine and Zn2+-impregnated aluminas following fluorination with HF, by using both solid-state NMR and X-ray powder diffraction methods. In the ...

A combined solid-state NMR and diffraction study of the structures and acidity of fluorinated aluminas: Implications for catalysis

The very strong solid Lewis acids aluminium chlorofluoride (ACF) and bromofluoride (ABF)—Synthesis, structure, and Lewis acidity

The cubic-rhombohedral phase transition at 450 degrees C of AlF3 is studied by DSC, X-ray powder diffraction and Raman scattering. It is demonstrated that the transition is of first order with a hysteresis of about 6 degrees. It is established by X-ray powder diffraction patterns that the room temperature space group is R3c. A temperature study of the Raman scattering spectra (that confirms the above conclusion) evidences the presence of two soft modes. It is shown from group theory that the transition can be imputed to the condensation of the R5 mode of the cubic Brillouin zone and the attribution of the Raman lines is deduced on the basis of the compatibility diagram between the cubic and rhombohedral symmetries. The frequencies of the Raman lines are used to adjust the parameters of a rigid ion model and to calculate the phonon spectrum in the cubic phase. The calculated phonon density of states appears to be strongly dependent on the soft phonon frequency.

https://iopscience.iop.org/article/10.1088/0953-8984/2/26/003/pdf

A study of the structural phase transitions in AlF3: X-ray powder diffraction, differential scanning calorimetry (DSC) and Raman scattering investigations of the lattice dynamics and phonon spectrum

R Burriel

Papers

A study of the structural phase transitions in AlF3: X-ray powder diffraction, differential scanning calorimetry (DSC) and Raman scattering investigations of the lattice dynamics and phonon spectrum