Showing papers in "Acta Crystallographica Section B-structural Science in 2010"

Bond lengths in organic and metal‐organic compounds revisited: X—H bond lengths from neutron diffraction data

Abstract: This paper is part of a short series that celebrates the addition of the 500 000th crystal structure to the Cambridge Structural Database in November 2009.The number of structures in the Cambridge Structural Database (CSD) has increased by an order of magnitude since the preparation of two major compilations of standard bond lengths in mid-1985. It is now of interest to examine whether this huge increase in data availability has implications for the mean bond-length values published in the late 1980s. Those compilations reported mean X—H bond lengths derived from rather sparse information and for rather few chemical environments. During the intervening years, the number of neutron studies has also increased, although only by a factor of around 2.25, permitting a new analysis of X—H bond-length distributions for (a) organic X = C, N, O, B, and (b) a variety of terminal and homometallic bridging transition metal hydrides. New mean values are reported here and are compared with earlier results. These new overall means are also complemented by an analysis of X—H distances at lower temperatures (T ≤ 140 K), which indicates the general level of librational effects in X—H systems. The study also extends the range of chemical environments for which statistically acceptable mean X—H bond lengths can be obtained, although values from individual structures are also collated to further extend the chemical range of this compilation. Updated default `neutron-normalization' distances for use in hydrogen-bond and deformation-density studies are also proposed for C—H, N—H and O—H, and the low-temperature analysis provides specific values for certain chemical environments and hybridization states of X.

Validation of experimental molecular crystal structures with dispersion-corrected density functional theory calculations

Abstract: This paper describes the validation of a dispersion-corrected density functional theory (d-DFT) method for the purpose of assessing the correctness of experimental organic crystal structures and enhancing the information content of purely experimental data. 241 experimental organic crystal structures from the August 2008 issue of Acta Cryst. Section E were energy-minimized in full, including unit-cell parameters. The differences between the experimental and the minimized crystal structures were subjected to statistical analysis. The r.m.s. Cartesian displacement excluding H atoms upon energy minimization with flexible unit-cell parameters is selected as a pertinent indicator of the correctness of a crystal structure. All 241 experimental crystal structures are reproduced very well: the average r.m.s. Cartesian displacement for the 241 crystal structures, including 16 disordered structures, is only 0.095 A (0.084 A for the 225 ordered structures). R.m.s. Cartesian displacements above 0.25 A either indicate incorrect experimental crystal structures or reveal interesting structural features such as exceptionally large temperature effects, incorrectly modelled disorder or symmetry breaking H atoms. After validation, the method is applied to nine examples that are known to be ambiguous or subtly incorrect.

A list of organic kryptoracemates

Abstract: A list of 181 organic kryptoracemates has been compiled. This class of crystallographic oddities is made up of racemic compounds (i.e. pairs of resolvable enantiomers) that happen to crystallize in Sohnke space groups (i.e. groups that include only proper symmetry operations). Most (151) of the 181 structures could have crystallized as ordered structures in non-Sohnke groups. The remaining 30 structures do not fully meet this criterion but would have been classified as kryptoracemates by previous authors. Examples were found and checked with the aid of available software for searching the Cambridge Structural Database, for generating and comparing InChI strings, and for validating crystal structures. The pairs of enantiomers in the true kryptoracemates usually have very similar conformations; often the match is near-perfect. There is a pseudosymmetric relationship of the enantiomers in about 60% of the kryptoracemate structures, but the deviations from inversion or glide symmetry are usually quite easy to spot. Kryptoracemates were found to account for 0.1% of all organic structures containing either a racemic compound, a meso molecule, or some other achiral molecule. The centroid of a pair of enantiomers is more likely (99.9% versus 99% probability) to be located on an inversion center than is the centroid of a potentially centrosymmetric molecule.

Pressure‐induced phase transitions in l‐alanine, revisited

Abstract: The effect of pressure on l-alanine has been studied by X-ray powder diffraction (up to 12.3 GPa), single-crystal X-ray diffraction, Raman spectroscopy and optical microscopy (up to ∼6 GPa). No structural phase transitions have been observed. At ∼2 GPa the cell parameters a and b become accidentally equal to each other, but without a change in space-group symmetry. Neither of two transitions reported by others (to a tetragonal phase at ∼2 GPa and to a monoclinic phase at ∼9 GPa) was observed. The changes in cell parameters were continuous up to the highest measured pressures and the cells remained orthorhombic. Some important changes in the intermolecular interactions occur, which also manifest themselves in the Raman spectra. Two new orthorhombic phases could be crystallized from a MeOH/EtOH/H2O pressure-transmitting mixture in the pressure range 0.8-4.7 GPa, but only if the sample was kept at these pressures for at least 1-2 d. The new phases converted back to l-alanine on decompression. Judging from the Raman spectra and cell parameters, the new phases are most probably not l-alanine but its solvates. © 2010 International Union of Crystallography Printed in Singapore - all rights reserved.

Orientational ordering, tilting and lone-pair activity in the perovskite methylammonium tin bromide, CH3NH3SnBr3.

Abstract: Synchrotron powder diffraction data from methylammonium tin bromide, CH3NH3SnBr3, taken as a function of temperature, reveal the existence of a phase between 230 and 188 K crystallizing in Pmc21, a = 5.8941 (2), b = 8.3862 (2), c = 8.2406 (2) A. Strong ferroelectric distortions of the octahedra, associated with stereochemical activity of the Sn 5s2 lone pair, are evident. A group analysis and decomposition of the distortion modes of the inorganic framework with respect to the cubic parent is given. The primary order parameters driving this upper transition appear to be an in-phase tilt (rotation) of the octahedra coupled to a ferroelectric mode. The precise nature of the lower-temperature phase remains uncertain, although it appears likely to be triclinic. Density-functional theory calculations on such a triclinic cell suggest that directional bonding of the amine group to the halide cage is coupled to the stereochemical activity of the Sn lone pair via the Br atoms, i.e. that the bonding from the organic component may have a strong effect on the inorganic sublattice (principally via switching the direction of the lone pair with little to no energy cost).

Diffuse scattering study of aspirin forms (I) and (II).

Abstract: Full three-dimensional diffuse scattering data have been recorded for both polymorphic forms [(I) and (II)] of aspirin and these data have been analysed using Monte Carlo computer modelling. The observed scattering in form (I) is well reproduced by a simple harmonic model of thermally induced displacements. The data for form (II) show, in addition to thermal diffuse scattering (TDS) similar to that in form (I), diffuse streaks originating from stacking fault-like defects as well as other effects that can be attributed to strain induced by these defects. The present study has provided strong evidence that the aspirin form (II) structure is a true polymorph with a structure quite distinct from that of form (I). The diffuse scattering evidence presented shows that crystals of form (II) are essentially composed of large single domains of the form (II) lattice with a relatively small volume fraction of intrinsic planar defects or faults comprising misoriented bilayers of molecular dimers. There is evidence of some local aggregation of these defect bilayers to form small included regions of the form (I) structure. Evidence is also presented that shows that the strain effects arise from the mismatch of molecular packing between the defect region and the surrounding form (II) lattice. This occurs at the edges of the planar defects in the b direction only.

Ab initio structure determination of phase II of racemic ibuprofen by X-ray powder diffraction.

Abstract: Annealing of the quenched ibuprofen at 258 K yielded a new crystalline form, called phase II. Powder X-ray diffraction patterns of this phase II were recorded with a laboratory diffractometer equipped with an INEL G3000 goniometer and a curved position-sensitive detector CPS120. The starting structural model was found by a Monte-Carlo simulated annealing method. The final structure was obtained through Rietveld refinements with rigid-body constraints for the phenyl group and soft restraints on the other interatomic bond lengths and bond angles. The cell volume is 5% larger than that of the conventional phase I at 258 K. It is also shown that the orientation of the propanoic acid group is drastically changed with respect to phase I, leading to strong modifications of the orientation of the O—H⋯O hydrogen bonds with respect to the chains of dimers. These structural considerations could explain the metastable character of this phase II.

Structures of dipeptides: the head-to-tail story.

Abstract: The hydrogen-bonding patterns in crystal structures of unprotected, zwitterionic dipeptides are dominated by head-to-tail chains involving the N-terminal amino groups and the C-terminal carboxylate groups. Patterns that include two concomitant chains, thus generating a hydrogen-bonded layer, are of special interest. A comprehensive survey shows that dipeptide structures can conveniently be divided into only four distinct patterns, differing by definition in the symmetry of the head-to-tail chains and amide hydrogen-bonding type, but also in other properties such as peptide conformation and the propensity to include solvent water or various organic guest molecules. Upon crystallization, the choice of pattern for a specific dipeptide is not random, but follows from the amino acid sequence.

The lines-of-force landscape of interactions between molecules in crystals; cohesive versus tolerant and 'collateral damage' contact.

Abstract: A quantitative analysis of relative stabilities in organic crystal structures is possible by means of reliable calculations of interaction energies between pairs of molecules. Such calculations have been performed by the PIXEL method for 1108 non-ionic and 98 ionic organic crystals, yielding total energies and separate Coulombic polarization and dispersive contributions. A classification of molecule–molecule interactions emerges based on pair energy and its first derivative, the interaction force, which is estimated here explicitly along an approximate stretching path. When molecular separation is not at the minimum-energy value, as frequently happens, forces may be attractive or repulsive. This information provides a fine structural fingerprint and may be relevant to the mechanical properties of materials. The calculations show that the first coordination shell includes destabilizing contacts in ∼ 9% of crystal structures for compounds with highly polar chemical groups (e.g. CN, NO2, SO2). Calculations also show many pair contacts with weakly stabilizing (neutral) energies; such fine modulation is presumably what makes crystal structure prediction so difficult. Ionic organic salts or zwitterions, including small peptides, show a Madelung-mode pairing of opposite ions where the total lattice energy is stabilized from sums of strongly repulsive and strongly attractive interactions. No obvious relationships between atom–atom distances and interaction energies emerge, so analyses of crystal packing in terms of geometrical parameters alone should be conducted with due care.

More examples of the 15-crown-5...H2O-M-OH2...15-crown-5 motif, M = Al3+, Cr3+ and Pd2+.

Abstract: Five structures of co-crystals grown from aqueous solutions equimolar in 15-crown-5 (or 15C5) and [M(H(2)O)(6)](NO(3))(n), M = Al(3+), Cr(3+) and Pd(2+), are reported. The hydrogen-bonding patterns in all are similar: metal complexes including the fragment trans-H(2)O-M-OH(2) alternate with 15C5 molecules, to which they are hydrogen bonded, to form stacks. A literature survey shows that this hydrogen-bonding pattern is very common. In each of the two polymorphs of the compound [Al(H(2)O)(6)](NO(3))(3).15C5.4H(2)O there are two independent cations; one forms hydrogen bonds directly to the 15C5 molecules adjacent in the stack, while the other cation is hydrogen-bonded to two water molecules that act as spacers in the stack. These stacks are then crosslinked by hydrogen bonds formed by the three nitrate counterions and the three lattice water molecules. The hydrogen-bonded stacks in [Cr(H(2)O)(5)(NO(3))](NO(3))(2).1.5(15C5).H(2)O are discrete rather than infinite; each unit contains two Cr(3+) complex cations and three 15C5 molecules. These units are again crosslinked by the uncoordinated nitrate ions and a lattice water molecule. In [Pd(H(2)O)(2)(NO(3))(2)].15C5 the infinite stacks are electrically neutral and are not crosslinked. In [Pd(H(2)O)(2)(NO(3))(2)].2(15C5).2H(2)O.2HNO(3) a discrete, uncharged unit containing one Pd complex and two 15C5 molecules is ;capped off' at either end by a lattice water molecule and an included nitric acid molecule. In all five structures the infinite stacks or discrete units form an array that is at least approximately hexagonal.

Structure of pyrrhotite 5C (Fe9S10)

Abstract: The distribution of vacancies throughout the underlying NiAs structure of pyrrhotite 5C was analysed through the application of vacancy avoidance and the closeness condition in conjunction with order–disorder layering. Two crystallographically equivalent structure solutions (chiral enantiomers) were produced consisting of layers containing one vacancy in every eight iron sites broken by a fully occupied layer every fifth iron layer, and best described by monoclinic statistical models. The statistical 5C structures were verified using synchrotron powder diffraction data as well as published electron-diffraction patterns. An order–disorder structure description is proposed for the intermediate pyrrhotites of which pyrrhotite 5C is an end-member.

Reversible phase transition of pyridinium-3-carboxylic acid perchlorate.

Abstract: Pyridinium-3-carboxylic acid perchlorate was synthesized and separated as crystals. Differential scanning calorimetry (DSC) measurements show that this compound undergoes a reversible phase transition at ∼ 135 K with a wide hysteresis of 15 K. Dielectric measurements confirm the transition at ∼ 127 K. Measurement of the unit-cell parameters versus temperature shows that the values of the c axis and β angle change abruptly and remarkably at 129 (2) K, indicating that the system undergoes a first-order transition at Tc = 129 K. The crystal structures determined at 103 and 298 K are all monoclinic in P21/c, showing that the phase transition is isosymmetric. The crystal contains one-dimensional hydrogen-bonded chains of the pyridinium-3-carboxylic acid cations, which are further linked to perchlorate anions by hydrogen bonds to form well separated infinite planar layers. The most distinct differences between the structures of the higher-temperature phase and the lower-temperature phase are the change of the distance between the adjacent pyridinium ring planes within the hydrogen-bonded chains and the relative displacement between the hydrogen-bonded layers. Structural analysis shows that the driving force of the transition is the reorientation of the pyridinium-3-carboxylic acid cations. The degree of order of the perchlorate anions may be a secondary order parameter.

Validation of experimental charge densities: refinement of the macrolide antibiotic roxithromycin.

Abstract: Multipole refinements of larger organic molecules have so far been limited to a few exceptional cases. We report an investigation of the detailed experimental electron-density distribution (EDD) of roxithromycin, a macrolide antibiotic consisting of 134 atoms. Although the experimental multipole refinement on high-resolution synchrotron data converged smoothly, validation of the electron density by calculation of an `experiment minus invariom' difference density revealed conformational disorder of the H atoms. Hydrogen disorder is shown to affect the EDD, the electrostatic potential and atomic properties as defined by Bader's quantum theory of atoms in molecules. A procedure to obtain the electron density distribution in the presence of disorder is proposed.

Octahedral tilting in cation‐ordered Jahn–Teller distorted perovskites – a group‐theoretical analysis

Abstract: Computer-based group-theoretical methods are used to enumerate structures arising in A(2)BB'X(6) perovskites, with either rock-salt or checkerboard ordering of the B and B' cations, under the additional assumption that one of these two cations is Jahn-Teller active and thereby induces a distortion of the BX(6) (or B'X(6)) octahedron. The requirement to match the pattern of Jahn-Teller distortions to the cation ordering implies that the corresponding irreducible representations should be associated with the same point in the Brillouin zone. Effects of BX(6) (and B'X(6)) octahedral tilting are included in the usual way. Finally, an analysis is presented of more complex models of ordering and distortion as might lead to the doubling of the long axis of the common Pnma perovskite, observed in systems such as Pr(1-x)Ca(x)MnO(3) (x approximately 0.5). The structural hierarchies derived in this work should prove useful in interpreting experimental results.

Structure solution of the new titanate Li4Ti8Ni3O21 using precession electron diffraction.

Abstract: A sample having stoichiometry Li[Ti1.5Ni0.5]O4 has been synthesized to obtain a spinel structure. The resulting crystalline powder revealed a multiphase nature with spinel as the minor phase. The main phase is a new trigonal phase having a = 5.05910 (1), c = 32.5371 (1) A. The structure has been solved by direct methods working on a three-dimensional set of intensities obtained from a precession electron-diffraction experiment, and refined on synchrotron powder diffraction data in the space group P\bar 3c1. The model consists of hexagonal layers of edge-sharing octahedra occupied either by the heavy cations Ti and Ni, or preferentially by Li. On the basis of cation-site occupancies the stoichiometry becomes Li4Ti8Ni3O21, which is compatible with the microanalysis results.

Universal prediction of intramolecular hydrogen bonds in organic crystals.

Abstract: A complete exploration of intramolecular hydrogen bonds (IHBs) has been undertaken using a combination of statistical analyses of the Cambridge Structural Database and computation of ab initio interaction energies for prototypical hydrogen-bonded fragments. Notable correlations have been revealed between computed energies, hydrogen-bond geometries, donor and acceptor chemistry, and frequencies of occurrence. Significantly, we find that 95% of all observed IHBs correspond to the five-, six- or seven-membered rings. Our method to predict a propensity for hydrogen-bond occurrence in a crystal has been adapted for such IHBs, applying topological and chemical descriptors derived from our findings. In contrast to intermolecular hydrogen bonding, it is found that IHBs can be predicted across the complete chemical landscape from a single optimized probability model, which is presented. Predictivity of 85% has been obtained for generic organic structures, which can exceed 90% for discrete classes of IHB.

Persistence of the stereochemical activity of the Bi3+ lone electron pair in Bi2Ga4O9 up to 50 GPa and crystal structure of the high-pressure phase

Abstract: The crystal structure of the high-pressure phase of bismuth gallium oxide, Bi2Ga4O9, was determined up to 30.5 (5) GPa from in situ single-crystal in-house and synchrotron X-ray diffraction. Structures were refined at ambient conditions and at pressures of 3.3 (2), 6.2 (3), 8.9 (1) and 14.9 (3) GPa for the low-pressure phase, and at 21.4 (5) and 30.5 (5) GPa for the high-pressure phase. The mode-Gruneisen parameters for the Raman modes of the low-pressure structure and the changes of the modes induced by the phase transition were obtained from Raman spectroscopic measurements. Complementary quantum-mechanical calculations based on density-functional theory were performed between 0 and 50 GPa. The phase transition is driven by a large spontaneous displacement of one O atom from a fully constrained position. The density-functional theory (DFT) model confirmed the persistence of the stereochemical activity of the lone electron pair up to at least 50 GPa in accordance with the crystal structure of the high-pressure phase. While the stereochemcial activity of the lone electron pair of Bi^{3+} is reduced at increasing pressure, a symmetrization of the bismuth coordination was not observed in this pressure range. This shows an unexpected stability of the localization of the lone electron pair and of its stereochemical activity at high pressure.

Syntheses, spectroscopic study and X-ray crystallography of some new phosphoramidates and lanthanide(III) complexes of N-(4-nitrobenzoyl)-N',N''-bis(morpholino)phosphoric triamide.

Abstract: New phosphoramidates with the formula RC(O)N(H)P(O)X2, R = 2-NO2—C6H4, 3-NO2—C6H4 and 4-NO2—C6H4, X = N(CH2CH3) (1)–(3), NC4H8 (4)–(6), and NC4H8O (7)–(9) were synthesized and characterized by 1H, 13C, 31P NMR and IR spectroscopy, and elemental analysis. The reaction of (9) with hydrated lanthanide(III) nitrate leads to ten- or nine-coordinated complexes, (10)–(13). The crystal structure has been determined for (3), (5), (9), (10) and (13). In contrast to all of the previously reported similar phosphoramidate compounds, the —C(O)—N(H)—P(O) skeleton in the free ligand (9) shows a cisoid conformation, with the C=O and P=O double bonds adopting a nearly syn conformation. Quantum chemical calculations were applied for clarifying this exceptional conformational behavior. The monodentate neutral ligand (9) is coordinated to the metal ions via the phosphoryl O atom, adopting the usual anti conformation between the C=O and P=O groups.

Mullite‐type Ga4B2O9: structure and order–disorder phenomenon

Abstract: Ga4B2O9, an aluminium-free mullite-type compound, was prepared by a boric-acid flux method and its structure was determined using powder X-ray diffraction techniques, in combination with transmission electron microscopy, solid-state 11B MAS-NMR and IR spectroscopies. GaO6 octahedra share edges in a trans-manner forming one-dimensional chains along the b direction, and the chains are further cross-linked by GaO5, BO3 and BO4 groups into a three-dimensional mullite-type structure. The disorder of the inter-chain groups results in a small unit cell for Ga4B2O9 compared with that for Al4B2O9, an ordered compound with a superstructure. By deconstructing the structure of Ga4B2O9, we were able to identify the fundamental building units and their linking rules which can be used to reconstruct the ordered and disordered structures. For Ga4B2O9, we found that the structure is intrinsically disordered within the ac plane, but ordered along the b axis. The three-dimensional structure can then be constructed by stacking the disordered ac sheets along the b axis (½b) with a ½a shift. The fundamental building units and exclusivity rules identified in this gallium borate mullite may also be useful for understanding other related mullite phases. The structure analysis applying the proposed method is used to recognize the structural features of Al4B2O9 and Al18B4O33.

Towards a generalized vision of oxides: disclosing the role of cations and anions in determining unit-cell dimensions

Abstract: Theoretical calculations of the electron-localization function show that, at the volumes of the two CaO phases (rocksalt and CsCl type), the parent Ca structures (fcc: face-centred cubic and sc: simple cubic, respectively) exhibit charge concentration zones which coincide with the positions occupied by the O atoms in their oxides. Similar features, also observed for the pairs Ca/CaF2 and BaSn/BaSnO3, are supported by recent high-pressure experiments as well as electron-localization function (ELF) calculations, carried out on elemental K. At very high pressures, the elemental K adopts the hP4 structure, topologically identical to that of the K atoms in high-pressure K2S and high-temperature α-K2SO4. Moreover, the ELF for the hP4 structure shows charge concentration (∼ 2 electrons) at the sites occupied by the S atoms in the high-pressure K2S phase. All these features confirm the oxidation/high-pressure equivalence as well as the prediction of how cation arrays should be metastable phases of the parent metals. For the first time to our knowledge, the structure type, dimension and topology of several oxides and fluorides (CaO, CaF2 and BaSnO3) are explained in univocal physical terms.

Electron density studies on hydrogen bonding in two chromone derivatives

Abstract: The experimental electron densities of two chromone derivatives have been determined from X-ray synchrotron diffraction data at low temperature (100 K). Topological analysis of the electron density has been used to analyze the formation of resonance-assisted hydrogen bonds (RAHBs). Geometrical and topological parameters confirm π-electron delocalization within the hydrogen-bonded ring. In addition, weak C—H⋯O interactions were identified in both structures. Hydrogen-bond energies allowed medium and weak hydrogen bonds to be distinguished.

Octahedral tilting in Pb-based relaxor ferroelectrics at high pressure

Abstract: We have employed a combination of powder neutron diffraction and single-crystal synchrotron X-ray diffraction to characterize the pressure-induced phase transitions that occur in the perovskite-type relaxor ferroelectric PbSc05Ta05O3 (PST) and Pb078Ba022Sc05Ta05O3 (PST-Ba) At ambient pressure the symmetry of the average structure for both compounds is Fm\bar{3}m as a result of partial ordering of the Sc and Ta cations on the octahedral sites At pressures above the phase transition both the neutron and X-ray diffraction patterns exhibit an increase in the intensities of h,k,l = all odd reflections and no appearance of additional Bragg reflections Synchrotron single-crystal X-ray diffraction data show that the intensity of hhh peaks, h = 2n + 1, does not change with pressure This indicates that the structural distortion arising from the phase transition has a glide-plane pseudo-symmetry along the 〈111〉 cubic directions Rietveld refinement to the neutron powder data shows that the high-pressure phase has either R\bar{3}c or R\bar{3} symmetry, depending on whether the presence of 1:1 octahedral cation ordering is neglected or taken into account, and comprises octahedral tilts of the type a−a−a− that continuously evolve with pressure The cubic-to-rhombohedral transition is also marked by a large increase in the anisotropy of the displacement ellipsoids of the Pb cations, indicating larger displacements of Pb cations along the rhombohedral threefold axis rather than within the perpendicular plane For PST the anisotropy of the Pb displacement parameters decreases at approximately 3 GPa above the phase-transition pressure For both PST and PST-Ba the average magnitudes of Pb-cation displacements expressed in terms of isotropic displacement ellipsoids gradually decrease over the entire pressure range from ambient to 735 GPa

X-ray atomic orbital analysis of 4f and 5d electron configuration of SmB6 at 100, 165, 230 and 298 K

Abstract: Accurate electron-density measurement of SmB6 at 100, 165, 230 and 298 K, and X-ray atomic orbital (XAO) analysis were carried out The 4f-electron density around Sm and 5d electron density at ∼ 1 A from Sm were analysed by XAO analysis The 5d electron density is due to the electrons of the 5dJ = 5/2Γ8 orbitals which stem from the eg orbitals in the strong field approximation The change in electron populations of the 5d5/2Γ8 orbitals with temperature is similar to that of the resistivity Since the conduction band consists of 5d5/2Γ8 and B-2p orbitals according to band theory, this indicates that the larger populations of the 5d5/2Γ8 orbitals correspond to the larger number of localized electrons and are correlated to the resistivity of SmB6 The occupation of the bulky 5d5/2Γ8 orbitals may be the reason for the elongation of the lattice parameter below 150 K The 4f7/2Γ6 orbitals are obviously occupied except at 100 K, which seems to be caused by the energy gap between 4f5/2 and 4f7/2 states, which begins to exist between 100 and 150 K, and may represent one of the properties of a Kondo insulator

Structures of incommensurate and commensurate composite crystals RbxMnO2 (x = 1.3711, 1.3636).

Abstract: Rb1.3711MnO2 (Rb11Mn8O16) has been synthesized via the azide/nitrate route from a stoichiometric mixture of the precursors Mn2O3, RbNO3 and RbN3. The structure of this extremely air- and moisture-sensitive compound can best be described in terms of an incommensurate composite structure, built up by a honeycomb-like framework of Rb ions, as one subsystem and by a second subsystem of chains, consisting of edge-sharing MnO4/2 tetrahedra. These two composite substructures interpenetrate in such a way that the manganate chain polyanions centre the channels of the Rb-honeycomb framework. Crystals transform by an aging process into Rb1.3636MnO2 (Rb15Mn11O22), which has a similar structure but a different commensurate modulation. Two reasons can be established for the origin of the modulations: the charge ordering of Mn2+/Mn3+ on one hand, and the incompatibility of the Mn—Mn and Rb—Rb separations on the other.

Revision of the structure of Cs2CuSi5O12 leucite as orthorhombic Pbca.

Abstract: The crystal structure of a hydrothermally synthesized leucite analogue Cs2CuSi5O12 has been determined and refined using the Rietveld method from high-resolution synchrotron X-ray and neutron powder diffraction data. This structure is based on the topology and cation-ordering scheme of the Pbca leucite structure of Cs2CdSi5O12, and exhibits five ordered Si sites and one ordered Cu tetrahedrally coordinated (T) site. This structure for Cs2CuSi5O12 is topologically identical to other known leucite structures and is different from that originally proposed by Heinrich & Baerlocher [(1991), Acta Cryst. C47, 237–241] in the tetragonal space group P4_12_12. The crystal structure of a dry-synthesized leucite analogue Cs2CuSi5O12 has also been refined; this has the Ia\bar 3d cubic pollucite structure with disordered T sites.

On the accurate bond-valence parameters for the Sb3+/O2- ion pair.

Abstract: The improved values of the bond-valence parameters for the Sb3+/O2– ion pair, r0 = 1.927 A and b = 0.446 A, have been deduced from the crystal structures of the α and β polymorphs of Sb2O3 and from the set of precisely determined complex structures containing [Sb3+On] coordination shells.

Charge-density analysis of the ground state of a photochromic 1,10-phenanthroline zinc(II) bis(thiolate) complex

Abstract: The charge density of the title compound was determined at 90 K, using a spherical crystal of 150 µm diameter. The proper treatment of the Zn atom in the pseudo-tetrahedral environment is considered in detail. A satisfactory refinement is only obtained when anharmonic Gram–Charlier parameters are included as variables in the refinement. A successful combined anharmonic/multipole refinement indicates a small polarization of the 4s shell in the anisotropic environment. One of the two toluenethiols is approximately π-stacked with the phenanthroline ligand. A bond path is found connecting the two ligands. In addition the Zn—S bond to this ligand is slightly extended compared with the same bond to the second toluenethiol. A separate photocrystallographic and theoretical study indicates the long wavelength emission of the title compound to be due to a ligand-to-ligand charge transfer (LLCT) from a toluenethiol to the phenanthroline ligand. The charge-density results do not provide a basis for deciding which of the thiole ligands is the source of the transferred electron density. This result is in agreement with the theoretical calculations, which show comparable oscillator strengths for charge transfer from either of the ligands.

Precursor effects of the orthorhombic to monoclinic phase transition in benzocaine form (II) revealed by X-ray diffuse scattering.

Abstract: We described the development of a Monte Carlo computer model for the room-temperature form (II) polymorph of benzocaine that incorporates, on a local scale, structural features derived from the low-temperature form (III) polymorph The introduction of this extra information convincingly reproduces those observed diffraction features that an earlier harmonic model was unable to achieve In both form (I) and form (II) the hydrogen-bonded chains of molecules that extend along the respective a axes tend to slide backward and forward along their lengths While in form (I) the motion is well modelled by a harmonic potential in form (II) there is a degree of anharmonicity that means that some intermolecular contact vectors, which are identical in the average structure, are distributed bimodally with either longer or shorter vectors being preferred to the average Moreover there is a tendency for these deviations from average to be correlated to give short-range ordered domains that are the precursors of the two twinned variants of the long-range ordered low-temperature form (III) structure

Crystal chemistry of transition metal diarsenates M2As2O7 (M = Mn, Co, Ni, Zn): variants of the thortveitite structure.

Abstract: The structures of the 3d divalent transition-metal diarsenates M(2)As(2)O(7) (M = Mn, Co, Ni, Zn) can be considered as variants of the monoclinic (C2/m) thortveitite [Sc(2)Si(2)O(7)] structure type with a ≃ 6.7, b ≃ 8.5, c ≃ 4.7 A, α ≃ 90, β ≃ 102, γ ≃ 90° and Z = 2. Co(2)As(2)O(7) and Ni(2)As(2)O(7) are dimorphic. Their high-temperature (β) polymorphs adopt the thortveitite aristotype structure in C2/m, whereas their low-temperature (α) polymorphs are hettotypes and crystallize with larger unit cells in the triclinic crystal system in space groups P\bar 1 and P1, respectively. Mn(2)As(2)O(7) undergoes no phase transition and likewise adopts the thortveitite structure type in C2/m. Zn(2)As(2)O(7) has an incommensurately modulated crystal structure [C2/m(α,0,γ)0s] with q = [0.3190 (1), 0, 0.3717 (1)] at ambient conditions and transforms reversibly to a commensurately modulated structure with Z = 12 (I2/c) below 273 K. The Zn phase resembles the structures and phase transitions of Cr(2)P(2)O(7). Besides descriptions of the low-temperature Co(2)As(2)O(7), Ni(2)As(2)O(7) and Zn(2)As(2)O(7) structures as five-, three- and sixfold superstructures of the thortveitite-type basic structure, the superspace approach can also be applied to descriptions of all the commensurate structures. In addition to the ternary M(2)As(2)O(7) phases, the quaternary phase (Ni,Co)(2)As(2)O(7) was prepared and structurally characterized. In contrast to the previously published crystal structure of the mineral petewilliamsite, which has the same idealized formula and has been described as a 15-fold superstructure of the thortveitite-type basic structure in space group C2, synthetic (Ni,Co)(2)As(2)O(7) can be considered as a solid solution adopting the α-Ni(2)As(2)O(7) structure type. Differences of the two structure models for (Ni,Co)(2)As(2)O(7) are discussed.

An experimental charge density of HEPES.

Abstract: We report the experimental charge density of HEPES [4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid], which is a common buffering agent. The structure was refined using the Hansen–Coppens formalism. The ability of the HEPES molecule to form stable intermolecular interactions and intermolecular hydrogen bonds in the crystal structure is discussed in terms of its buffering properties. The protonation mode observed in the crystal structure is different from that expected in solution, suggesting that additional factors must be taken into consideration in order to explain the solution properties of the compound. As ordered HEPES molecules are found in the active sites of proteins in several protein crystal structures, our results will allow for quantitative analysis of the electrostatic potential of the interacting surfaces of those proteins.