Five crystalline phases of diguanidinium hexachlorostannate(IV), [C(NH2)(3)](2)SnCl6, have been identified and characterized by calorimetric and dielectric measurements, single crystal X-ray diffraction at atmospheric and high pressure, and synchrotron X-ray powder diffraction. The crystal structures of all phases are built of similar layers in which the tin hexachloride anions are connected to the guanidinium cations by N-H center dot center dot center dot Cl hydrogen bonds, forming a interact primarily by Coulombic forces between the ions from ap. double H-bonded sheets. The layers, neutral as a whole, the opposing H-bonded sheets, and through the van der Waals contacts. From water solution the compound crystallizes at room temperature in phase III of space group C2/c. On heating, this phase transforms between 375 and 455 K to the high temperature phase I of space group P (1) over bar, either immediately or through the intermediate phase II of the same space group P (1) over bar. The temperature range of phase II enhances meaningfully at elevated pressure, which made possible the high-pressure crystallization of this phase and determination of its structure. Different transition paths can be realized when the crystal is cooled from phase I: the reverse transition occurs in the monoclinic phase III or in the monoclinic phase IV (space group C2/m), or in the phase V of space group PT. In all phases the layered structure of the crystal is preserved, while the arrangement of the layers is different. The transitions involve also transformations in the networks of N-H center dot center dot center dot Cl hydrogen bonds. The large volume (similar to 3%) and entropy (similar to R ln 3) change at the transition between phases II and III, and the giant pressure coefficient of -755 K GPa(-1), indicate a great potential of this material for applications in solid-state cooling systems.