The LT phase of Cs3Bi2I9 | EPFL Graph Search

The low temperature (LT) modification of Cs3Bi2I9 has been refined to R(F) = 0.055 on the basis of X-ray diffraction measurements at T = 130 K from a twin crystal with six fractions connected by 3-fold axis and m-planes symmetry operations. Data were collected on a KM4CCD diffractometer; MoKalpha-radiation; 1769 independent reflections with F > 4 sigma (F) were obtained. Below the LT phase transition each reflection were split. The absorption correction was accounted for by the Gaussian integration method. The parameters of the monoclinic unit cell are: alpha = 8.346(8), b = 14.47(1) and c = 21.10(1) Angstrom, beta = 91.00(7)degrees; space group C2/c. The structure of the monoclinic LT phase is slightly distorted in comparison to the hexagonal RT phase (a(m) = a(h); b(m) = 2a(h) + b(h), c(m) = -c(h)). The phase transition at T = 218 K between the RT acid LT modifications is reversible. The additional maxima observed in the Fourier-syntheses are associated with coherent parts in domains belonging to various twin fractions. The nature, the probable structure and the temperature variations of the domain boundaries are considered and discussed. It is suggested that the specific effect observed on I-127 NQR spectra for the LT modification between 205 < T K < 218 may be due to the incommensurate nature of the domain boundaries. [References: 7]

Urotropin azelate: a rather unwilling co-crystal

Gervais Chapuis, Kurt Schenk

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) similar or equal to 4.7, b similar or equal to 26.1 and c similar or equal to 14.4 Angstrom. 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 similar to 301 K). The superposition structure of the OD system at T = 286 (1) K with space group Bmmb and cell (a) over cap = 2a(o), (b) over cap = b and (c) over cap = 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 similar to2degrees, 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 similar to 233 K, the system reaches its ground state (cell a(M) = a(m), bM = b, c(M) = c(m) and space group P2(1)/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.

2003

One-dimensional composite host-guest structure in BaVS3

Alla Arakcheeva, Neven Barisic, Helmuth Berger, Gervais Chapuis, László Forró, Philip Pattison

A detailed synchrotron X-ray diffraction (XRD) study performed with a single crystal of BaVS3 (barium vanadium trisulfide) in the temperature range between 10 and 295 K is reported. Aside from the known tetragonal–orthorhombic (240 K) and orthorhombic–monoclinic (69 K) phase transitions, in the 130 < T ≤ 295 K range the overall structure can be viewed as a host–guest (H–G) composite. The BaS3 matrix is the host, while the V-chains form the guest. The two subsystems lock in at TLOCK = 130 ± 20 K. This temperature is marked by a symmetry change from orthorhombic to monoclinic. This results in the formation of twins, implying a structural phase transition identified here for the first time. From the refined structural data, it is possible to follow, starting already at 295 K downwards, the stepwise transformation of VS6 octahedra into VS5 tetragonal pyramids as the origin of the structure evolution. The new findings will yield a better understanding of the complex electronic phase diagram of BaVS3.

2021

Phase-transition-induced twinning in the 1 : 1 adduct of hexamethylenetetramine and azelaic acid

Gervais Chapuis, Dieter Schwarzenbach

The title compound, C6H12N4. C9H16O4, 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 quasisymmetry (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. [References: 23]

1999

Urotropin azelate: a rather unwilling co-crystal

Gervais Chapuis, Kurt Schenk

2003