High-pressure-induced phase transitions in the ferroelectric bis-thiourea pyridinium iodide inclusion compound

The effect of temperature and pressure on physical properties of the ferroelectric bis-thiourea pyridinium iodide inclusion compound has been studied by dielectric spectroscopy, neutron spectroscopy, neutron powder diffractometry, single crystal x-ray diffraction and nuclear magnetic resonance (NMR). At ambient pressure two structural phase transitions have been revealed: at T1 = 161 K between phases I and II and at T2 = 141 K between phases II and III. Phase III with increasing pressure splits into two phases, IIIa and IIIb. The temperatures of the phase transitions TI–II, TII–IIIa and TIIIa–IIIb increase with increasing pressure. The p–T phase diagram constructed for this compound shows two triple points of coordinates 150 K, 100 MPa and 115 K, 100 MPa. (Some figures in this article are in colour only in the electronic version)