Abstract
Trigonal crystals of monoguanidinium dioxonium trinitrate have been synthesized and investigated as a function of temperature by X-ray diffraction and by DTA. The strongly anisotropic crystals, built of hydrogen-bonded corrugated sheets with clearly pyramidal H 3O + cations, are stable below 280 K, and show an onset of negative thermal expansion along the hydrogen-bonded sheets at 140 K, analogous to that in H 2O ice Ih below 80 K. We have also detected a similar negative expansion region about 80 K in guanidinium nitrate, [C(NH 2) 3] +·NO 3 −, measured by neutron powder diffraction between 4 and 160 K. Factors destabilizing the [C(NH 2) 3] +·2H 3O +·3NO 3 − structure above 280 K have been discussed, and the role of the dynamics of the H 3O + cations postulated.
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