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 27 Al MAS NMR spectroscopy to the satellite transitions in the characterization of a structural phase transition is presented by an investigation of Friedels salt (Ca 2 Al(OH) 6 Cl·2H 2 O) over the temperature range from −121 to 109 °C. Accurate values for the 27 Al quadrupole coupling parameters ( C Q and η 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 α−β phase transition for Friedels salt at about 34 °C, whereas the isotropic chemical shift is invariant over the studied temperature range. The variations in C Q and η Q with temperature show a parabolic decrease in C Q and an increase in η Q with increasing temperature for the α form of Friedels salt, while only a small linear decrease in C Q with increasing temperature and a temperature-independent value for η Q are observed for the β form. The nonlinear variations of C Q and η Q with temperature for the α 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 η Q at the temperature for the phase transition are investigated by point-monopole calculations of the 27 Al electric-field gradient tensor, which strongly suggest that hydrogen bonding plays an important role in the structural changes that occur during the phase transition.