The properties of salt-filled sodalites part 2. Synthesis, decomposition reactions and phase transitions of nitrate sodalite Na 8[AlSiO 4] 6(NO 3) 2

Abstract

Nitrate sodalites, Na 8[AlSiO 4] 6(NO 3) 2, have been synthesized hydrothermally (570 K, 0.1 GPa) using zeolite A, sodium nitrate and 8 M NaOH. IR spectroscopy indicates clearly the enclathration of NO 3 − exclusively within the sodalite cages (absorption band at 1380 cm −1). 29Si-MAS-NMR confirms the alternating ordering of silicon and aluminium in the nitrate sodalite framework, according to a single peak at δ = −86.7 ppm for Si(4Al) units. From the DTA and calorimetric investigations a reversible structural phase transition has been identified for Na 8[AlsiO 4] 6(NO 3) 2, starting at temperatures between 920 K (DTA) and 930 K (DSC). High temperature Guinier-Simon photographs reveal the onset of a maximal framework expansion of the nitrate sodalite at temperatures T tr ⩾ > 920 K. The observed thermal decomposition behaviour and the corresponding evaluation of the unit-cell volume indicate special host-guest interactions due to the dynamics of the enclosed NO 3 −.