Characterization of the α−β Phase Transition in Friedels Salt (Ca2Al(OH)6Cl·2H2O) by Variable-Temperature 27Al MAS NMR Spectroscopy

Abstract

The first application of variable-temperature 27Al 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)6Cl·2H2O) over the temperature range from −121 to 109 °C. Accurate values for the 27Al quadrupole coupling parameters (CQ and ηQ) 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 CQ and ηQ with temperature show a parabolic decrease in CQ and an increase in ηQ with increasing temperature for the α form of Friedels salt, while only a small linear decrease in CQ with increasing temperature and a temperature-independent value for ηQ are observed for the β form. The nonlinear variations of CQ and ηQ 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 CQ and ηQ at the temperature for the phase transition are investigated by point-monopole calculations of the 27Al electric-field gradient tensor, which strongly suggest that hydrogen bonding plays an important role in the structural changes that occur during the phase transition.