M. Hostettler

Stacking disorder: the hexagonal polymorph of tris(bicyclo[2.1.1]hexeno)benzene and related examples

Abstract X-ray diffractograms of tris(bicyclo[2.1.1]hexeno)benzene, crystallized at the interface between a benzene solution and a layer of acetonitrile, show hexagonal symmetry and streaks of diffuse scattering along c*. The heavily faulted layer stacking is analyzed qualitatively and quantitatively in terms of a systematic protocol. This protocol requires partitioning the crystal structure into layers in such a way that pairs of adjacent layers may be stacked in different, but geometrically equivalent ways, which are dictated by the layer group symmetry. This approach is shown to provide a consistent alternative for analysis of a number of related cases provided the layers are defined on the basis of geometrical criteria rather than chemical intuition.

The structure of orange HgI2. I. Polytypic layer structure

The metastable orange crystals of HgI(2) comprise three different crystal structures, all of which are built from corner-linked Hg(4)I(10) supertetrahedra. Two of them are end members with the maximum degree of order (MDO) of a polytypic layer structure; the third shows a three-dimensional linkage. This paper presents the determination from X-ray diffraction data of the tetragonal polytypic structures and their stacking disorder. Diffraction patterns show sharp Bragg reflections and rods of diffuse intensity with pronounced maxima. In a first step, the diffuse intensity was neglected and all maxima were treated as Bragg reflections. The crystal was supposed to be a conglomerate of the two MDO structures diffracting independently, and their parameters and volume ratio were refined against the single data set. The geometries and anisotropic displacement parameters of the layers in the two structures are shown to be nearly identical. Layer contacts in the two stacking modes are identical. The structures are fractal complications of the stable red form of HgI(2). In a second step, the stacking disorder has been quantitatively analyzed with a Markov chain model. Two probabilities describing next-nearest-layer interactions were visually adjusted to observed intensity profiles extracted from image-plate detector data. Results consistently show that the crystal comprises nearly equal volumes of MDO structures with an average domain thickness of about 5 layers or 30 A

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

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.

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) approximately 4.7, b approximately 26.1 and c approximately 14.4 A. 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 approximately 301 K). The superposition structure of the OD system at T = 286 (1) K with space group Bmmb and cell â = 2a(o), b = b and ĉ = 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 approximately 2 degrees, 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 approximately 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)/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.

Co-crystallized cis and trans isomers of dichloro­bis(2-picolylamine)iron(II)

The octahedral cis and trans isomers of dichlorobis(2-picolylamine)iron(II), [FeCl2(C6H8N2)2], co-crystallize in a 1:1 ratio. The cis isomer lies on a twofold axis, whereas the trans isomer lies on an inversion centre. The structure is fully ordered, with both Fe atoms in a pure high-spin state. The Fe, Cl and N(H2) atoms of both isomers lie in the same plane, allowing all Cl and amine H atoms to be engaged in extensive two-dimensional hydrogen bonding. The hydrogen-bonded layers are interconnected through pi-pi interactions between the pyridine rings. Searches in the Cambridge Structural Database uncover very few examples of such isomer co-existence.

The structure of orange HgI2. II. Diamond-type structure and twinning

The metastable orange crystals of HgI(2) comprise three different crystal structures all of which are built from corner-linked Hg(4)I(10) supertetrahedra. Two of the structures are end members with the maximum degree of order (MDO) of a polytypic layer structure. In this paper, the third structure (D) determined from X-ray diffraction, a crystal chemical discussion of the four known tetrahedral HgI(2) structures, and a twinning model are presented. All the various diffraction results published during the past 70 years are now explained. The Hg(4)I(10) supertetrahedra of the tetragonal structure D are corner-linked into two interpenetrating diamond-type networks. The stable red form and the three orange structures show the same cubic densest packing of I atoms and differ only in the distribution of Hg atoms in the tetrahedral voids. Transformations between the structures may involve only movements of Hg atoms, as implied by larger thermal displacement parameters of Hg than of I. A multiply twinned conglomerate of MDO1, MDO2 and D, each structure occurring in three orientations, results in metrically cubic crystals whose Bragg reflections are very close to reciprocal lattice points.

New boron-oxygen layer in the structure of barium hydrodecaborate Ba-5[B20O33(OH)(4)]center dot H2O

The crystal structure of the new synthetic compound Ba5[B20O33(OH)4] ⋅ H2O was established by the methods of X-ray diffraction (a Stoe IPDS diffractometer, λMoKα?, 1860 independent reflections, anisotropic refinement, R = 1.95%, localization of hydrogen atoms): a = 9.495(2) Å, b = 6.713(1) Å, c = 11.709(2) Å, β = 95.09(1)°, sp. gr. P2, Z = 1. The structure is based on double pseudohexagonal layers consisting of BO4-tetrahedra and BO3 triangles linked into three-membered rings in two mutually perpendicular directions. The double layers adjacent along the [100] direction are linked together through the Ba-polyhedra and hydrogen bonds with the participation of the OH-groups occupying the “end” vertices of two B-triangles. The interlayer space is also filled with a sheet of Ba-polyhedra. The structure of the compound is compared to the structures of topologically similar Ba and Ca borates and hydroborates.

The lock-in phase in the urotropine-sebacic acid system

The 1,10-decanedioic acid-1,3,5,7-tetraazatricyclo[3.3.1.1(3,7)]decane (1/1) system, C(10)H(18)O(4).C(6)H(12)N(4), was studied at 215 (2) K. Its analysis provides important information with regard to the long-standing acid-carboxylate controversy in the urotropine-alkanedioic acid system. In the present structure, all the chain end-groups display a clear acid character. The asymmetric unit of this commensurate modulated phase contains two molecules of diacid as well as two molecules of urotropine. Furthermore, the chain packing suggests a possible order parameter for the lock-in transition.

A complex spin crossover scenario as seen by synchrotron diffraction and small angle neutron scattering

Some octahedral iron(II) complexes assume two spin states low spin (LS, S=0 singlet t2g eg ) at low temperature and high spin (HS, S=2, quintet t2g eg )) at high temperature. Cooperative interactions between spin-active centres will lead to temperature-dependent correlations between spin states in the crystal structure. In order to elucidate the corresponding correlation length, synchrotron diffraction and small-angle scattering experiments have been done for Fe(2-pic)3]Cl2 2-propanol solvate [1]. X-ray diffraction does not reveal any noticeable diffuse scattering. The SANS signal shows no effects other than the dependence of crystal density on temperature. Thus both experiments corroborate the presence of mainly long-range correlations between HS and LS states and support a mean-field scenario of the temperature-induced spin crossover [2, 3].

Anti-wurtzite reoriented

Anti-wurtzite and wurtzite are shown to be the same crystal structure despite the claims of a recent paper describing the crystal structure of the mineral rambergite, Mn(1-x)Fe(x)S, x approximately 0.05. The anti-wurtzite/wurtzite confusion is used as an illustration to help clarify the correct general approach to take in the treatment and presentation of achiral non-centrosymmetric crystal structures.