Phase Transition of Hypoxanthinium Nitrate Monohydrate

Structures of hypoxanthinium nitrate monohydrate crystals were determined at 15 different temperatures ranging from 20 to 285 K. Crystals undergo a phase transition from the twinned, low temperature phase of P21/n symmetry into the high temperature Pmnb phase. Sets of X-ray diffraction data acquired at each temperature were subjected to different types of refinement. We found that, as the temperature increases, the layers of hypoxanthinium nitrate monohydrate shift against each other, leading to the negative thermal expansion in one direction and positive one perpendicular to it. Such a phenomenon stops at the point of phase transition when the asymmetry in interactions in the structure disappears