THE PROPERTIES OF SALT-FILLED SODALITES .2. SYNTHESIS, DECOMPOSITION REACTIONS AND PHASE-TRANSITIONS OF NITRATE SODALITE NA8[ALSIO4]6(NO3)2

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). Si-29-MAS-NMR confirms the alternating ordering of silicon and aluminium in the nitrate sodalite framework, according to a single peak at delta = -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 T(tr) greater-than-or-equal-to 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-.