CHANGES IN THE CRYSTAL-STRUCTURE OF TSAREGORODTSEVITE [N(CH3)(4)][SI-2(SI0.5AL0.5)O-6](2) ON HEATING

Tsaregorodtsevite [N(CH3)(4)][Si-2(Si0.5Al0.5)0(6)](2) is a unique fel dspathoid with a tetramethylammonium (TMA(+)) organic cation in an ord ered, sodalite-like framework of orthorhombic symmetry (I222) with a=8 .984(3), b=8.937(2), and c=8.927(2) Angstrom. Here, C-13 cross polariz ation MAS and H-1 MAS NMR spectra give direct evidence that the organi c material in the cavities in tsaregorodtsevite is TMA(+). The Al-27 a nd Si-29 MAS NMR spectra, and XRD data show that the framework is well ordered with Si in the T1 and T2 sites and both Si and Al in the T3 s ite. Upon heating, the colorless tsaregorodtsevite crystals change col or becoming yellow, then brown, and finally black. In this study H-1, C-13, Al-27. and Si-29 MAS NMR, powder XRD, and electron microprobe an alyses are used to investigate the structural changes in tsaregorodtse vite on heating. The eel dimensions and T-O bond lengths decrease, and there are two phase changes. At 690 degrees C, the TMA(+) molecule br eaks down to form aromatic rings and acidic groups, possibly including benzene and the pyridinium ion, with ammonia and other gases produced . The Al-Si framework changes slightly to give a less well-ordered tet ragonal structure (1422), with a=8.925(1), c=8.908(1) Angstrom, and a small amount of Al enters a separate phase. After heating at 970 degre es C, the organic material has further broken down with little C and H visible by NMR spectroscopy. The Al-Si framework forms at least two n ew phases: one with a cubic crystalline structure (I432) with a=8.817( 3) Angstrom, and an amorphous aluminosilicate phase. There isa small r esidue of the initial structure even after heating for one hour at ove r 900 degrees C.