Phase transitions in the layered structure of diguanidinium tetraiodoplumbate

Sterical effects and the coupling between the cationic dynamics and the anionic-supralattice distortions lead to a series of phase transitions in the crystals of diguanidinium tetraiodoplumbate, [C(NH₂)₃]₂⁺·PbI₄^2-. The crystals have been characterized by dielectric and calorimetric measurements at ambient and high hydrostatic pressures, as well as by single-crystal x-ray and neutron-powder diffraction at varied temperatures. At 287 K the crystal structure is monoclinic, but the strongly corrugated polyanionic sheets approximate the orthorhombic symmetry of the high-temperature phases. Guanidinium cations are enclosed in voids within the polyanions, and in channels between the neighboring sheets. At 0.1 MPa and 307 K the monoclinic phase undergoes a ferroelastic second-order transition to the orthorhombic phase, and at 356 K another continuous transition to the orthorhombic phase with the unit cell reduced four times in volume. The phase diagram reveals an unusual character of the pressure-induced transition with a critical point at 245 K and 270 MPa. Also a triple point at 358 K and 780 MPa has been detected. The mechanisms of the phase transitions have been postulated.