N-Salicylideneaniline (SA), C13H11NO, belongs to the large family of aromatic Schiff bases. It is of particular importance owing to its reversible photoreactivity. SA forms two photochromic polymorphs, both with two non-coplanar benzene rings. In addition, we have recently discovered a planar polymorph, named the beta-polymorph, which will be discussed in a subsequent paper. We report here the structure of the alpha2-polymorph in the orthorhombic crystal system. This compound exhibits a strong intramolecular O-H...N hydrogen bond and the dihedral angle between the two rings varies with temperature.

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 re¯ections. 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 edi®ce 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 b 0X6). The system may be understood as an OD (order±disorder) structure based on a layer with layer group Pcc2 and cell a o 9 4.7, b 9 26.1 and c 9 14.4 A Ê . 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 a 1 = 2a o , b 1 = b, c 1 = 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 = ca2 owes its cohesion to van der Waals interactions between the AA chains and to three types of hydrogen bonds of varied strength between UÐU and UÐAA. Before reaching completion, this MDO structure is transformed, at 282 K, into a monoclinic one with cell a m = Àa o c/4, b m = b, c m = À2a o ca2, space group P2 1 ac, spontaneous deformation $2, and ferroelastic domains. This transformation is achieved in two steps: ®rst 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 P2 1 ac) 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 ®rst-and second-neighbour interactions along the a axis and ®rst-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.

The title compound, C(6)H(12)N(4).C(9)H(16)O(4), undergoes several thermotropic phase transitions. The crystalline structure is layered, with sheets of azelaic acid linked to sheets of hexamethylenetetramine by hydrogen bonds. In the room-temperature phase, the azelaic acid molecules are disordered. By lowering the temperature, this disorder partially disappears. The ordering is clearly observed in reciprocal space where on the rods of diffuse scattering, present in the room-temperature phase, a series of superstructure reflections emerges. This phase transition leads to twin-lattice quasi-symmetry (TLQS) twinning. The structure of this twinned phase is explored in this paper. There are two orientational domains linked by a mirror plane which relates disordered orientations of the acid molecules above the phase transition. A single domain has space group P2_1/c. The structure has been solved and refined on the complete set of data to R(1) = 0.0469. The chains remain partially disordered, showing two acid groups with unequal population: the major form corresponding to a carboxylic acid and the minor to a carboxylate. The ordering of the structure, when going through the phase transition, is interpreted in terms of stabilization by C-H.O hydrogen bonding. A least-squares estimator of the twinning volume ratio is developed that gives an expression for the twinning ratio in terms of the intensities of nonoverlapping reflections. The twinning ratio obtained in the structure refinement compares very well with that obtained from this estimator.

The thermotropic phase diagram of 1:1 co-crystals of hexamethylenetetramine and pimelic acid (heptanedioic acid) is investigated. Three crystalline phases are identi®ed at ambient pressure. Phase I is disordered, as revealed by diffuse rods in its diffraction pattern. When the temperature is lowered the diffuse streaks disappear in Phase II, but superstructure re¯ections emerge indicating an ordering process of the structure through a non-ferroic, or at least non-ferroelastic, phase transition. Phase II is mainly characterized by an unusual distribution of its re¯ection intensities. Phase III is reached through a ferroelastic phase transition that induces twinned domains. A model based on the stacking of an elementary layer is proposed with the aim of describing the structures in a uni®ed framework. Depending on the value of the unique stacking parameter , each of the different structures observed can be reproduced by this model. Its validity is then tested by a series of simulations reproducing the main features of the diffraction patterns such as the diffuse scattering streaks, the occurrence of superstructure peaks at lower temperature and twinning.

HMT-C11 belongs to the family of adducts formed by the co-crystallization of N4(CH2)6 molecules (hexamethylenetetramine or HMT) and aliphatic dicarboxylic acids HOOC(CH2) n − 2COOH with 5 ≤ n ≤ 13 (Cn). The adducts exhibit a layered structure in which the packing between HMT and Cn is determined by strong hydrogen bonds. The compounds in this family studied so far present thermotropic structural phase transitions and, depending on the chain length, disordered, twinned and modulated phases. The structure refinement of HMT-C11 based on X-ray diffraction experiments indicates three distinct phases from the melting point down to liquid nitrogen temperature: phase I is not crystalline; phase II is disordered (stacking fault) and its average structure is described in space group Bmmb; phase III is partially disordered and its symmetry is P21/c. The systematic study of the structure evolution of phase III upon cooling revealed that the disorder has a dynamic character (anharmonicity). The main structural change observed from 293 K down to 93 K is the increase of the tilting angle of the C11 chains relative to the layer plane and the rotation of the HMT molecules. Both HMT and C11 behave like rigid bodies in the temperature range investigated. The quality of the refinements leads to a conclusive model for the O—H...N hydrogen bonds linking HMT and C11.