Phase transitions and ionic motions in ammonium potassium hexafluoroaluminates(III) with compositions of (NH 4) 3−xK x[AlF 6] ( x = 1,2) as studied by 1H, 19F, and 27Al nuclear magnetic resonance

Abstract

Phase transitions and ionic motions in ammonium potassium hexafluoroaluminates (III) with compositions of (NH 4) 3− x K x [AlF 6] ( x = 1,2) were characterized by means of differential thermal analysis (DTA) and NMR of 1H, 19F, and 27Al nuclei. It was found by powder X-ray diffraction that the x = 2 composition ( a = 8.64 2 A ̊ ) as well as the x = 1 composition crystallize in a face-centered cubic structure at room temperature. From DTA experiments, phase transitions were located at 250 and 186 K for the x = 1 composition and at 170 K for the x = 2 composition. The temperature dependences of the spin-lattice relaxation times T 1 of 1H, 19F, and 27Al NMR were well interpreted in terms of overall reorientations of NH + 4 and [AlF 6] 3− ions. The phase transition mechanisms are briefly discussed in relation to the crystal structures and the molecular motions of the NH 4 + and [AlF 6] 3− ions. Preliminary results of DTA and powder X-ray diffraction are reported for several nonstoichiometric compositions of (NH 4) 3− x K x [AlF 6](0 < x < 2).