Nuclear magnetic resonance study of the phase transitions and local environments of alpha-alum NH4Al(SO4)(2)center dot 12H(2)O single crystals

Abstract

The NMR spectrum, the spin-lattice relaxation time, T-1, and the spin-spin relaxation time, T-2, of NH4Al-(SO4)(2)center dot 12H(2)O crystals were studied as a function of temperature; this crystal was found to undergo increasing loss of H2O with increasing temperature. Two groups of Al resonance lines were observed below T-d (=335 K), whereas above T-d the resonance lines of the two groups were reduced to one Al signal. The presence of only one Al-27 resonance line T-d is due to the structural phase transition. The changes in the temperature dependences of the T-1 and T-2 near T-d are related to the loss of H2O, which probably disrupts the forms of the octahedral of water molecules surrounding Al3+. In addition, the alpha-alum NH4Al-(SO4)(2)center dot 12H(2)O can be distinguished from the alpha-alum RbAl(SO4)(2)center dot 12H(2)O and the beta-alum CsAl(SO4)(2)center dot 12H(2)O on the basis of the temperature dependences of T-1. (C) 2010 Elsevier B.V. All rights reserved.