The Relationship Between ²⁷Al Quadrupolar Parameters and AlF₆^3– Octahedron Connectivity in Crystalline and Glassy Fluoroaluminates

Abstract

Abstract27Al SATRAS and MQ‐MAS spectra were recorded for eight crystalline compounds, from the CaF2–AlF3 and BaF2–AlF3 binary and BaF2–CaF2–AlF3 ternary systems, and four glass compositions with different CaF2/BaF2/AlF3 contents. For the crystalline phases, the reconstruction of the spectra leads to the precise determination of the NMR parameters. For the glassy phases, the 27Al SATRAS spectra have been reconstructed using quadrupolar parameter distributions. The main finding of this study is the dependence of the quadrupolar frequency on the type of AlF63– octahedron connectivity. For crystalline phases the experimental νQ values range between 75 kHz and 510 kHz for structures built up from isolated octahedra, and are between 560 kHz and 1250 kHz for structures built up from isolated chains of cis‐connected octahedra, and are between 1530 kHz and 1580 kHz for structures built up from isolated chains of trans‐connected octahedra. In the glassy phases the maximum of the quadrupolar frequency distribution shifts toward larger values and its width increases with increasing AlF3 content, and subsequently with the number of connected octahedra. The range of the νQ values seems to indicate that when the octahedra are connected, cis connections occur most frequently. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Crystalline aluminium hydroxy fluorides—Suitable reference compounds for 19F chemical shift trend analysis of related amorphous solids

“…Both characteristics are mainly caused by distribution-effects: the superimposition and distribution of more than one certain AlF x O 6Àx -species and a distribution of bond lengths and angles. Different connection modes of the involved AlF x O 6Àx -octahedra (isolated, chains, planes or 3d-network) may also lead to several distinguishable species in the matrix and so to different smaller deviations in the certain spectra, as, for example, recently shown by Body for certain complex aluminium fluorides [48]. As all the maxima of the compounds lie within the range from 0 to À20 ppm, the conclusion is, that the main coordination is octahedral and is a mixed (hydr$/ alk$)oxidic and fluoridic one: AlF x O 6Àx (x = 2-5).…”

On the assignment of19F MAS NMR spectra of fluoroaluminates using through-space spectral edition of19F–27Al and19F–19F connectivities

“…2b Each of the ten peaks corresponds to one fluorine site, but none can be assigned based on relative intensity. The 27 Al MAS spectrum exhibits two resonances 12 which were assigned using ab initio calculations: 13 d iso = À2 ppm, n Q = 550 kHz and Z Q = 0.45 for Al1 and d iso = À0.5 ppm, n Q = 750 kHz and Z Q = 0.10 for Al2.…”

“…Those three compounds were previously studied by 1D 19 F MAS NMR. 2b Body et al have also determined the 27 Al NMR quadrupolar parameters 12 and optimized the structures by coupling the 27 Al NMR results and ab initio electric field gradient (EFG) calculations 13 with the WIEN2k code. 14 Structure optimization is an essential step for compounds whose structure was resolved from X-ray powder diffraction data, which is the case of only b-BaAlF 5 .…”

“…Fig.2(a) 2D19 F-27 Al MAS (25 kHz) CP-HETCOR correlation spectrum of b-BaAlF 5 . Top spectra are the full projection onto the27 Al dimension and simulation of Al1 and Al2 resonances using quadrupolar parameters previously determined 12. Dash lines indicate position where the cross-sections of Al1 and Al2 were extracted from.…”

19F high speed MAS NMR investigation of octahedron connectivity in fluoroaluminate glasses

“…The structural description of these glasses has been recently completed by a 27 Al NMR investigation which shows that octahedron connections are mainly cis ones [21].…”