Characterization of the α-β phase transition in Friedels salt (Ca2Al(OH)6Cl•2H2O) by variable-temperature 27Al MAS NMR spectroscopy

The first application of variable-temperature Al-27 MAS NMR spectroscopy to the satellite transitions in the characterization of a structural phase transition is presented by an investigation of Friedels salt (Ca2Al(OH)(6)Cl2H2O) over the temperature range from - 121 to 109 degreesC. Accurate values for the Al-27 quadrupole coupling parameters (C-Q and eta(Q)) and the isotropic chemical shifts are obtained from either the manifold of spinning sidebands for the satellite transitions or the line shape observed for the central transition. These data demonstrate that the quadrupole coupling parameters clearly reflect the structural alpha-beta phase transition for Friedels salt at about 34 degreesC, whereas the isotropic chemical shift is invariant over the studied temperature range. The variations in C-Q and eta(Q) with temperature show a parabolic decrease in C-Q and an increase in eta(Q) with increasing temperature for the alpha form of Friedels salt, while only a small linear decrease in C-Q with increasing temperature and a temperature-independent value for eta(Q) are observed for the beta form. The nonlinear variations of C-Q and eta(Q) with temperature for the alpha form and their relationships with changes in the structural parameters associated with thermal expansion of the unit cell are discussed. Finally, the abrupt changes in C-Q and eta(Q) at the temperature for the phase transition are investigated by point-monopole calculations of the Al-27 electric-field gradient tensor, which strongly suggest that hydrogen bonding plays an important role in the structural changes that occur during the phase transition.