Urotropin azelate: A rather unwilling co-crystal

Urotropin (U) and azelaic acid (AA) form 1:1 co-crystals (UA) that give rise to a rather complex diffraction pattern, the main features of which are diffuse rods and-bands in addition to the Bragg reflections. UA is characterized by solvent inclusions, parasite phases, and high vacancy and dislocation densities. These defects compounded, with the pronounced tendency of U to escape from the crystal edifice lead to at least seven exotic phase transitions (many of. which barely manifest themselves in a differential scanning calorimetry trace). These involve different incommensurate phases and a peritectoid reaction in the recrystallization regime. (T_h, > 0.6). The system may be understood as an OD (order-disorder) structure based on a layer with layer group P(c)c2 and cell a_o ≃ 4.7, b ≃ 26.1 and c ≃ 14.4 Å. At 338 K the layer stacking is. random, but with decreasing temperature the build-up of an orthorhombic MDO (maximal degree of order) structure with cell a₁ = 2a_o, b₁ = b, c₁ = c and space group Pcc2 is begun (at ∼301 K). The superposition structure of the OD system at T = 286 (1) K with space group Bmmb and cell a = 2a_o, b = b and c = c/2 owes its cohesion to van der Waals interactions between the AA chains and to three types of hydrogen bonds of varied strength between U-U and U-AA. Before reaching completion, this MDO structure is transformed, at 282 K, into a monoclinic one with cell a_m = -a_o + c/4, b _m = b, c_m = -2(a_o + c/2), space group P2 ₁/c, spontaneous deformation ∼2°, and ferroelastic domains. This transformation is achieved in two steps: first a furtive triggering transition, which is not yet fully understood, and second an improper ferroelastic transition. At ∼233 K, the system reaches its ground state (cell a_M = a_m, b_M = b, c_M = c _m. and space group P2₁/c) via an irreversible transition. The phase transitions below 338 K are described by a model based on the interaction of two thermally activated slip systems. The OD structure is described in terms of a three-dimensional Monte Carlo model that involves first- and second-neighbour interactions along the a axis and first-neighbour interactions along the b and c axes. This model includes random shifts of the chains along their axes and satisfactorily accounts for most features that are seen in the observed diffraction pattern.