Abstract
Abstract Nitrate sodalites, Na8[AlSiO4]6(NO3)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 NO3− 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 Na8[AlsiO4]6(NO3)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 Ttr ⩾ > 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 NO3−.
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