Nuclear magnetic resonance study of the phase transitions and local environments of α-alum NH₄Al(SO₄)₂.12H₂O single crystals

다국어 초록 (Multilingual Abstract)

The NMR spectrum, the spin-lattice relaxation time, T<SUB>1</SUB>, and the spin-spin relaxation time, T<SUB>2</SUB>, of NH<SUB>4</SUB>Al(SO<SUB>4</SUB>)<SUB>2</SUB>.12H<SUB>2</SUB>O crystals were studied as a function of temperature; this crystal was found to undergo increasing loss of H<SUB>2</SUB>O with increasing temperature. Two groups of Al resonance lines were observed below T<SUB>d</SUB> (=335K), whereas above T<SUB>d</SUB> the resonance lines of the two groups were reduced to one Al signal. The presence of only one <SUP>27</SUP>Al resonance line T<SUB>d</SUB> is due to the structural phase transition. The changes in the temperature dependences of the T<SUB>1</SUB> and T<SUB>2</SUB> near T<SUB>d</SUB> are related to the loss of H<SUB>2</SUB>O, which probably disrupts the forms of the octahedral of water molecules surrounding Al<SUP>3+</SUP>. In addition, the α-alum NH<SUB>4</SUB>Al(SO<SUB>4</SUB>)<SUB>2</SUB>.12H<SUB>2</SUB>O can be distinguished from the α-alum RbAl(SO<SUB>4</SUB>)<SUB>2</SUB>.12H<SUB>2</SUB>O and the β-alum CsAl(SO<SUB>4</SUB>)<SUB>2</SUB>.12H<SUB>2</SUB>O on the basis of the temperature dependences of T<SUB>1</SUB>.