Influence of F content on the composition of Al-rich synthetic phlogopite: Part II. Probing the structural arrangement of aluminum in tetrahedral and octahedral layers by27Al MQMAS and1H/19F-27Al HETCOR and REDOR experiments

Abstract

The influence of F substitution on the local structure of Al in the tetrahedral and octahedral sheets of synthetic Al-rich phlogopite samples with nominal gel compositions of K(Mg 3-x Al x )[Al 1+x Si 3-x O 10 ] (OH) y (F) 2-y between 0.0 ≤ x ≤ 0.8 and 0.5 ≤ y ≤ 1.8, was studied by 27 Al MAS, MQMAS, { 1 H/ 19 F} → 27 Al 2 D CPMAS (HETCOR) and { 1 H/ 19 F} 27 Al REDOR solid-state NMR and by IR spectroscopy. Changes in intensity of the absorption bands in the OH-stretching region of the IR spectra clearly indicate the incorporation of octahedral Al. Signals from the different phases can be separated in the 27 Al MQMAS NMR spectra by generation of an isotropic dimension in F1. The 27 Al quadrupolar parameters of the four phases were estimated from 27 Al MAS NMR spectra obtained at 104.26 and 208.42 MHz. The quadrupolar coupling constant and isotropic chemical shift increases with increasing Al content for the IV Al site in phlogopite. The VI Al site shows a clear increase of the asymmetry parameter and C Q with increasing F content. The estimated 27 Al signal areas show the lowest amount of impurity phases at high OH contents and a stabilization of VI Al sites by hydroxyl groups. The {1H} → 27 Al 2D CPMAS (HETCOR) NMR experiment at short contact times provides information about site neighborhoods of tetrahedral Al sites and Mg 3 OH as well as Mg 2 AlOH sites, whereas magnetization is only transferred to the octahedral Al sites from hydroxyl groups in Mg 2 AlOH sites. The { 19 F} → 27 Al 2D CPMAS (HETCOR) NMR spectrum is dominated by IV Al sites coupled to the Mg 3 F complex in phlogopite. Resonances from Mg 2 AlF complexes are not observed. Finally, the { 1 H/ 19 F} 27 Al REDOR experiments support the results of the 2D CPMAS (HETCOR) experiments.