Pressure-induced decoupling of the order-disorder and displacive contributions to the phase transition in diguanidinium tetrachlorostannate

The crystals of diguanidinium tetrachlorostannate [C(NH2)(3)](2)(+).SnCl4-2, were studied by single-crystal x-ray diffraction at various temperatures and by calorimetric and dielectric measurements at ambient and high hydrostatic pressures. At room temperature the crystal structure is orthorhombic, space group Pbca, and consists of infinite staggered (SnCl4-2), chains built of square pyramids sharing corners and of two crystallographically different guanidinium cations G(1) and G(2), located in two kinds of voids between the chains.

The polyanionic chains are tied into double sheets by the G(1) cations. At ambient pressure the crystals undergo two first-order phase transitions at 354.8 and 395.4 K. The former, between two orthorhombic phases (Pbca --> Cmca), is characterized by antiphase displacement of the double sheets along the b direction of the low-temperature unit cell which is coupled to dynamical disordering of G(2) and transformation of its hydrogen bonding scheme.

At elevated pressures the coupling between the displacive and order-disorder contributions is modified and its breaking near a triple point at 180 MPa and 270 K results in a pressure-induced phase observed between Pbca and Cmca phases.