Showing papers in "Acta Crystallographica Section B-structural Science in 2001"

Prediction of the crystal structures of perovskites using the software program SPuDS.

Abstract: The software program SPuDS has been developed to predict the crystal structures of perovskites, including those distorted by tilting of the octahedra. The user inputs the composition and SPuDS calculates the optimal structure in ten different Glazer tilt systems. This is performed by distorting the structure to minimize the global instability index, while maintaining rigid octahedra. The location of the A-site cation is chosen so as to maximize the symmetry of its coordination environment. In its current form SPuDS can handle up to four different A-site cations in the same structure, but only one octahedral ion. Structures predicted by SPuDS are compared with a number of previously determined structures to illustrate the accuracy of this approach. SPuDS is also used to examine the prospects for synthesizing new compounds in tilt systems with multiple A-site coordination geometries (a(+)a(+)a(+), a(0)b(+)b(+), a(0)b(-)c(+)).

Relationship between bond valence and bond softness of alkali halides and chalcogenides

Abstract: Established bond-valence parameter tables rely on the assumption that the bond-valence sum of a central atom is fully determined by interactions to atoms in its first coordination shell. In this work the influence of higher coordination shells is tested in detail for bonds between lithium and oxygen. It is demonstrated that the sum of the weak interactions with atoms of the second coordination shells significantly contributes to the valence sum and should therefore not be neglected. Since the independent refinement of the two parameters R0 and b is hardly possible from the limited range of bond lengths occurring in the first coordination shell, the restriction of bond-valence sums to contributions from nearest neighbours implicated another far-reaching simplification: the postulation of a universally fixed value of the bond-valence parameter b which characterizes the shape of the bond-valence pseudopotential for the respective atom pair. However, recent more sophisticated applications of the bond-valence concept, e.g. to model ion-transport pathways in solid electrolytes, demand sensible estimates of the bond-valence sums for mobile ions not only at their equilibrium sites but also at interstitial sites and bottle-necks of transport pathways. Calculations of bond valences at these non-equilibrium sites require the knowledge of the actual shape of the bond-valence pseudopotential. A systematic route to a more realistic estimate of b for alkali halides and chalcogenides is developed in this work from an empirical correlation between b and the absolute softnesses of the interacting particles.

Rules governing the crystal packing of mono- and dialcohols.

Abstract: A new program, Mercury, has been used to survey 144 monoalcohol (CmHnOH) and 101 dialcohol [CmHn(OH)2] crystal structures. Results show that their hydrogen-bonding patterns are strongly correlated with steric effects. Primary monoalcohols have a strong preference to form infinite ⋯OH⋯OH⋯ chains. Secondary monoalcohols form chains and rings of hydrogen bonds with about equal facility. Tertiary monoalcohols very often form isolated OH⋯O hydrogen bonds or structures containing no OH⋯O hydrogen bonds at all. In the latter case, however, the structures almost invariably contain CH⋯O and/or OH⋯π interactions. Substitution on the β-carbon(s) of monoalcohols has a profound effect on packing patterns, with increased substitution disfavouring chains and rings. Dialcohols show a much stronger preference for chains of hydrogen bonds, compared with monoalcohols. This is particularly so when at least one of the hydroxyl groups is primary, in which case chains are overwhelmingly preferred. Once again, substitution on the β-C atoms is influential, heavy substitution tending to lead to packing arrangements that involve isolated or intramolecular OH⋯O hydrogen bonds. Dialcohols almost never crystallize without at least some OH⋯O hydrogen-bond formation. In both monoalcohols and dialcohols, chains show a stronger preference to be helical (usually threefold helices) as steric hindrance increases. Hydrogen-bonded rings usually contain four OH⋯O hydrogen bonds. It is possible that empirical observations such as these may aid crystal-structure prediction.

Competition of hydrogen-bond acceptors for the strong carboxyl donor

Abstract: A database study on the competition of hydrogen-bond acceptors for the strong carboxyl donor in crystals is reported. The relative success in attracting this donor is determined for 34 types of acceptor (O, N, S, halogen and π acceptors), and a correlation between the success in competition and the average hydrogen-bond distance is established.

Ferroelectricity and structure in the YMnO3 family

Abstract: The 1963 discovery of ferroelectricity in YMnO3 was accompanied by an experimental Curie temperature (Tc) reported as 913 K; this value was revised to 1270 K in the following decade. Subsequently, YInO3 was shown to be isostructural with YMnO3 and later demonstrated to satisfy the structural criteria for ferroelectricity; recent unpublished measurements give Tc (YInO3) = 835 (15) K. The experimental Tc value of 913 K for YMnO3 is in satisfactory agreement with the calculated 1220 (100) K value as derived from a very recent structural refinement, the experimental Tc of 835 (15) K for YInO3 with the calculated 760 (120) K. The full YMnO3 family includes the AMnO3 subfamily with A = Y, Ho, Er, Tm, Yb, Lu, Sc, In; the AInO3 subfamily with A = Y, Gd, Dy, Ho, Tb; and the AGaO3 subfamily with A = Y, Ho, Er. The Tc values of six family members with known structure, in addition to YMnO3 and YInO3, have been structurally derived as 1310 (110) K for ErMnO3, 1290 (165) K for LuMnO3, 1270 (110) K for YbMnO3, 1220 (105) K for ScMnO3, 540 (375) K for InMnO3 and 1020 (100) K for YGaO3. The agreement between predicted and experimental Tc values for ErMnO3, LuMnO3 and YbMnO3, in addition to that for YMnO3 and YInO3, leads to the confident prediction that ScMnO3, InMnO3 and YGaO3 are new ferroelectrics. The remaining six members of the full YMnO3 family are also expected to be new ferroelectrics.

Charge distribution as a tool to investigate structural details. II. Extension to hydrogen bonds, distorted and hetero‐ligand polyhedra

Abstract: It is shown that one of the main reasons for most failures of the methods for calculating distance-dependent bond strengths is related to the distortion of the coordination polyhedra. The charge distribution (CD) method which depends on only one universal empirical parameter (contraction parameter) is modified to include: (i) an iterative calculation of the effective coordination number (ECoN), to deal with structures containing very distorted coordination polyhedra; (ii) a specific contraction parameter to treat structures containing any type of hydrogen bond; (iii) scale factors for coordination subshells, to treat structures with hetero-ligand polyhedra. The contraction parameter for the hydrogen bonds was obtained from 119 well refined structures based on neutron diffraction data. Examples of the application of the iterative charge distribution (CD-IT) are presented to show the efficiency of the new method in dealing with distorted (including hydrogen bonding) and hetero-ligand polyhedra. In particular, analysis of a series of 74 structures with pentacoordinated cations shows that deviations from overall trends are related to structure instability. The possible failure of the method with polyionic structures and `dynamic' structures is discussed.

Reversible phase transitions in Na2S under pressure: a comparison with the cation array in Na2SO4

Abstract: The structural behavior of the antifluorite Na2S, disodium sulfide, has been studied under pressure up to 22 GPa by in situ synchrotron X-ray diffraction experiments in a diamond anvil cell at room temperature. At approximately 7 GPa, Na2S undergoes a first phase transition to the orthorhombic anticotunnite (PbCl2) structure (Pnma, Z = 4). The lattice parameters at 8.2 GPa are a = 6.707 (5), b = 4.120 (3), c = 8.025 (4) A. At approximately 16 GPa, Na2S undergoes a second transition adopting the structure of the Ni2In-type (P63/mmc, Z = 2). The lattice parameters at 16.6 GPa are a = 4.376 (18), c = 5.856 (9) A. Both pressure-induced phases have been confirmed by full Rietveld refinements. An inspection of the cation array of Na2SO4 reveals that its Na2S subarray is also of the Ni2In-type. This feature represents a new example of how the cation arrangements in ternary oxides correspond to the topology of the respective binary compounds. We discuss analogies between the insertion of oxygen and the application of pressure.

Migration of the proton in the strong O-H.O hydrogen bond in urea-phosphoric acid (1/1).

Abstract: The structure of urea–phosphoric acid is reported at a large number of temperatures in the range 150–335 K from neutron diffraction data collected using a novel multiple single-crystal data collection method. The work focuses on the behaviour of the H atom involved in the short strong O—H⋯O hydrogen bond in this material. The position of this atom is shown to vary significantly, by around 0.035 A, as a function of temperature, becoming effectively centred at the highest temperatures studied. This result, only accessible due to the accurate determination of H-atom parameters by neutron diffraction, has implications for the potential governing the hydrogen bond.

General rules for the packing of hydrogen-bonded crystals as derived from the analysis of squaric acid anions: aminoaromatic nitrogen base co-crystals

Abstract: Preparation and single-crystal X-ray structure determination of three co-crystals of hydrogen squarate, HSQ−, with 2-aminopyrimidine, 3-aminopyridine and 4-aminopyridine, and one of squarate, SQ2-, with 8-aminoquinoline are reported. Their crystal packings are analyzed and discussed in terms of the intermolecular O—H⋯O, N—H⋯O/N and C—H⋯O hydrogen bonds formed. Although the fine details of the supramolecular architecture are barely rationalizable, the comparative analysis of the data makes it possible to suggest some simple rules that may be of general application for the packing of hydrogen-bonded crystals, i.e. Rule 1: `All hydrogen-bond acceptors available in a molecule will be engaged in hydrogen bonding as far as there are available donors'; Rule 2: `The hydrogen-bond acceptors will be saturated in order of decreasing strength of the hydrogen bonds formed'.

Solid-state structures of group 1 and group 2 metal 1,5-naphthalenedisulfonates: systematic investigation of lamellar three-dimensional networks constructed by metal arenedisulfonate

Abstract: Seven Group 1 and Group 2 1,5-naphthalenedisulfonates (1,5-­nds) have been synthesized and structurally characterized by single-crystal X-ray diffraction, IR spectroscopy and thermal gravimetric analysis For Group 1 metal complexes, with M = Li+ (1), Na+ (2) and K+ (3), all crystallize in the same space group (P21/c) with the same composition, [M2(1,5-­nds)(H2O)2] They adopt similar three-dimensional packing arrangements with the metal–sulfonate inorganic layers pillared by naphthalene rings However, the coordination behavior of three metal cations toward the SO_3^- group and water molecule are different, resulting in different architectures for the inorganic portion For Group 2 complexes with M = Mg2+ (4), Ca2+ (5), Sr2+ (6) and Ba2+ (7), Mg2+ shows no direct coordination by the SO_3^- group while Ca2+ is coordinated by four SO_3^- groups and a two-dimensional network is formed Complexes (6) and (7) are isostructural, adopting the same three-dimensional, inorganic–organic pillared framework as seen for (1)–(3) The coordination behavior of the metal cations in these structures neatly illustrates the increase in coordination strength with decreasing charge/radius ratio for Group 1 and Group 2 metal cations with large organic anions

Five-dimensional structure refinement of natural melilite, (Ca1.89Sr0.01Na0.08K0.02)(Mg0.92Al0.08)(Si1.98Al0.02)O7

Abstract: The structure of a crystal of natural melilite from San Venanzo, Umbria (Italy) of the general formula X2T1(T2)2O7, where X = Ca0.945Sr0.005Na0.04K0.01, T1 = Mg0.92Al0.08 and T2 = Si0.99Al0.01, has been solved and refined as an incommensurate structure in five-dimensional superspace. The structure is tetragonal, superspace group P\bar 421m:p4mg, cell parameters a = 7.860 (1), c = 5.024 (1) A, modulation vectors q1 = 0.2815 (3)(a* + b*), q2 = 0.2815 (3)(−a* + b*). The data collection was performed on a KumaCCD diffractometer. The structure was refined from 7606 reflections to final R = 0.0481. A special modification of the refinement program Jana2000 was necessary to take into account overlapping of satellite reflections m × n = ±1, which could not be properly separated in the integration procedure. The final model includes modulations of the atomic positions as well as modulations of the thermal parameters. The latter are induced by strong differences in the neighbourhood of the actual modulated positions. The occupational modulation was neither significant for X nor for T1 sites and the sites were supposed to be occupied only by Ca and Mg, respectively. As a consequence of the Ca and O positional modulations six-, seven- and eightfold Ca coordination occur throughout the structure and the thermal ellipsoid changes its shape correspondingly. The positional modulation of the atoms causes variations in the interatomic distances which, however, do not affect bond-valence sums considerably, but induce flattening and rotation in T1 and T2 tetrahedra, respectively.

Use of software to search for higher symmetry: space group C2

Abstract: From a search of the October 2000 release of the Cambridge Structural Database we find coordinate data for approximately 1500 entries under space group No 5: C2 or, occasionally, A2, I2 or B112 Software designed to detect cases of missed higher symmetry identified 144 entries for detailed inspection Of these, 50 should, we believe, be revised to space groups of higher symmetry The most common revision is to space group C2/m, which entails adding a center of inversion and usually results in important changes in bond lengths and angles

Associations of squaric acid and its anions as multiform building blocks of hydrogen-bonded molecular crystals.

Abstract: Squaric acid, H2C4O4 (H2SQ), is a completely flat diprotic acid that can crystallize as such, as well as in three different anionic forms, i.e. H2SQ·HSQ−, HSQ− and SQ2−. Its interest for crystal engineering studies arises from three notable factors: (i) its ability of donating and accepting hydrogen bonds strictly confined to the molecular plane; (ii) the remarkable strength of the O—H⋯O bonds it may form with itself which are either of resonance-assisted (RAHB) or negative-charge-assisted [(−)CAHB] types; (iii) the ease with which it may donate a proton to an aromatic base which, in turn, back-links to the anion by strong low-barrier N—H+⋯O1/2− charge-assisted hydrogen bonds. Analysis of all the structures so far known shows that, while H2SQ can only crystallize in an extended RAHB-linked planar arrangement and SQ2− tends to behave much as a monomeric dianion, the monoanion HSQ− displays a number of different supramolecular patterns that are classifiable as β-chains, α-chains, α-dimers and α-tetramers. Partial protonation of these motifs leads to H2SQ·HSQ− anions whose supramolecular patterns include ribbons of dimerized β-chains and chains of emiprotonated α-dimers. The topological similarities between the three-dimensional crystal chemistry of orthosilicic acid, H4SiO4, and the two-dimensional one of squaric acid, H2C4O4, are finally stressed.

Chemical bonding in energetic materials: β-NTO

Abstract: The electron density and related properties of the quasi-stable β form of 5-nitro-2,4-dihydro-3H-1,2,4-triazol-3-one (NTO; space group P21/c) have been determined from a low-temperature [100 (1) K] X-ray diffraction experiment. ­Intensities were measured with a 2K CCD Bruker diffractometer using Ag Kα radiation. Two detector settings, several φ settings, 0.3° ω scans and 160 s exposure time per frame gave Rint = 0.0215 for 68 989 (4080 unique) reflections and (sin θ/­λ)max = 1.23 A−1. The Hansen–Coppens [Acta Cryst. (1978), A34, 909–921] multipole model as implemented in the XD program gave R = 0.0333 (all reflections), which allowed calculation of the electron density, Laplacian and electrostatic potential distributions. The bonding (3,−1) critical points and the molecular dipole moment of 3.2 (1) D were also obtained. Chemical bonding in the molecule is discussed.

Structure of β-trimyristin and β-tristearin from high-resolution X-ray powder diffraction data

Abstract: The crystal structures of β-1,2,3-tritetradecanoylglycerol (β-trimyristin or β-MMM) and β-1,2,3-trioctadecanoylglycerol (β-tristearin or β-SSS) have been determined from high-resolution synchrotron X-ray powder diffraction data. Grid search and Rietveld refinement have been used to determine and refine the structure, respectively. Both substances crystallize in space group P\bar 1 with Z = 2. The unit-cell parameters for β-MMM are a = 12.0626 (6), b = 41.714 (1), c = 5.4588 (3) A, α = 73.388 (4), β = 100.408 (5) and γ = 118.274 (4)°. For β-SSS the unit-cell parameters are a = 12.0053 (7), b = 51.902 (2), c = 5.4450 (3) A, α = 73.752 (5), β = 100.256 (6) and γ = 117.691 (5)°. Soft-distance restraints have been applied to the molecules during refinement. For β-MMM the final Rp value obtained is 0.053 and for β-SSS the final Rp value is 0.041.

Influence of hydrogen bonding on the second harmonic generation effect: neutron diffraction study of 4-nitro-4'-methylbenzylidene aniline.

Abstract: A neutron diffraction study of the non-linear optical (NLO) material 4-nitro-4′-methylbenzylidene aniline (NMBA) is presented. NMBA exhibits a large macroscopic second-order NLO susceptibility, χ(2), and this study shows that hydrogen bonding is, in part, responsible for this. No hydrogen bonding was reported in the X-ray study [Ponomarev et al. (1977). Sov. Phys. Crystallogr. 22, 223–225], whereas the present work shows that C—H⋯X hydrogen bonds (where X = N, O or π) direct the nature of the three-dimensional lattice. C—H⋯X (X = N or O) hydrogen bonds are common; however, C—H⋯π hydrogen-bond motifs are relatively rare. Such intermolecular interactions help extend the molecular charge transfer into the supramolecular realm, the charge transfer originating as a consequence of the high level of molecular planarity and strong donor-to-acceptor interactions. Molecular planarity, coupled with the favourable nature of the hydrogen bonds, results in parallel stacking of molecules in both the a and c crystallographic directions with extremely close interplanar spacings. Such a combination of influential hydrogen-bonding characteristics accounts, in part, for the large second-order NLO output of the material since the phenomenon is so critically dependent upon the nature of the charge transfer.

Systematic ab initio study of the compressibility of silicate garnets.

Abstract: The structural properties of the silicate garnets andradite, Ca3Fe2Si3O12, uvarovite, Ca3Cr2Si3O12, knorringite, Mg3Cr2Si3O12, goldmanite, Ca3V2Si3O12, blythite, Mn^{2+}_3Mn^{3+}_2Si3O12, skiagite, Fe^{2+}_3Fe^{3+}_2Si3O12, calderite, Mn^{2+}_3Fe^{3+}_2Si3O12, and khoharite, Mg3Fe^{3+}_2Si3O12, have been investigated with a quantum-mechanical model as a function of applied pressure. The study has been performed with the density functional theory code CASTEP, which uses pseudopotentials and a plane-wave basis set. All structural parameters have been optimized. The calculated static geometries (cell parameters, internal coordinates of atoms and bond lengths), bulk moduli and their pressure derivatives are in good agreement with the experimental data available. Predictions are made for those cases where no experimental data have been reported. The data clearly indicate that the elastic properties of all silicate garnets are dominated by the compressibility of the dodecahedral site. The compression mechanism is found to be based on a bending of the angle between the centers of the SiO4 tetrahedra and the adjacent octahedra, as in the aluminosilicate garnets. An analysis of the relationship between ionic radii of the cations and the compressibility of silicate garnets is presented.

Dipyrimidine-copper(II) dinitrate complexes showing magnetic interactions.

Abstract: Crystals of CuII(NO3)2(pm)3 (1), and two crystalline forms of CuII(NO3)2(H2O)2(pm)2, (2) and (3), showed ferromagnetic, antiferromagnetic and paramagnetic interactions at extremely low temperatures, respectively Crystal structure analyses revealed that the complexes were catena-dinitrato[μ-pyrimidine-κN1:κN3]-(pyrimidine-N1)copper(II), [Cu(NO3)2(pm)2]n, catena-diaquadinitrato[μ-pyrimidine-κN1:κN3]copper(II), [Cu(NO3)2(H2O)2(pm)]n, and diaquadinitratodipyrimidinecopper(II), Cu(NO3)2(H2O)2(pm)2 for (1), (2) and (3), respectively In (1) the Cu atom is coordinated by the two nitrates and N atoms of the non-bridging pyrimidine and bridging pyrimidine to form a one-dimensional coordination polymer The complex is a five-coordinated square pyramid and can be regarded as a pseudo-seven-coordinated complex, since other short non-bonding Cu⋯O contacts are observed In the crystals of (2) the pyrimidine bridges the Cu atoms to form a one-dimensional coordination chain On the other hand, complex (3) is not a coordination polymer It is important to form a coordination polymer for the appearance of the magnetic interactions Types of coordination of the bridging organic moieties should also play an important role in magnetic properties Magnetic measurements of (1) and (2) show that they are good examples of uniform S = 1/2 ferro- and antiferromagnetic Heisenberg chains with exchange parameters 2J/kB = +18 and −36 K, respectively

Phosphate tungsten bronze series: crystallographic and structural properties of low-dimensional conductors.

Abstract: Phosphate tungsten bronzes have been shown to be conductors of low dimensionality. A review of the crystallographic and structural properties of this huge series of compounds is given here, corresponding to the present knowledge of the different X-ray studies and electron microscopy investigations. Three main families are described, monophosphate tungsten bronzes, Ax(PO2)4(WO3)2m, either with pentagonal tunnels (MPTBp) or with hexagonal tunnels (MPTBh), and diphos­phate tungsten bronzes, Ax(P2O4)2(WO3)2m, mainly with hexagonal tunnels (DPTBh). The general aspect of these crystal structures may be described as a building of polyhedra sharing oxygen corners made of regular stacking of WO3-type slabs with a thickness function of m, joined by slices of tetrahedral PO4 phosphate or P2O7 diphosphate groups. The relations of the different slabs with respect to the basic perovskite structure are mentioned. The structural description is focused on the tilt phenomenon of the WO6 octahedra inside a slab of WO3-type. In this respect, a comparison with the different phases of the WO3 crystal structures is established. The various modes of tilting and the different possible connections between two adjacent WO3-type slabs involve a great variety of structures with different symmetries, as well as the existence of numerous twins in MPTBp's. Several phase transitions, with the appearance of diffuse scattering and modulation phenomena, were analysed by X-ray scattering measurements and through the temperature dependence of various physical properties for the MPTBp's. The role of the W displacements within the WO3-type slabs, in two modulated structures (m = 4 and m = 10), already solved, is discussed. Finally, the complexity of the structural aspects of DPTBh's is explained on the basis of the average structures which are the only ones solved.

Parametrization of perovskite structures: an ab initio study

Abstract: An overview of the pressure, temperature and chemical composition dependence of the lattice distortion in ortho­rhombic ABO3 perovskite structures is presented. Within the framework of the so-called global parametrization method (GPM) [Thomas (1998). Acta Cryst. B54, 585–599] an improved description for the position of the A cation in terms of the AO_{12} and BO_6 polyhedral volume ratio is proposed. The relationship is derived from an extensive ab initio study based on the density functional theory. The applicability of the improved GPM in combination with ab initio total energy calculations in the prediction of changes in the structural distortion under increasing hydrostatic pressure is investigated. Test calculations are performed for the geophysically important magnesium silicate perovskite and the results are compared with the available theoretical and experimental data.

Amino and cyano N atoms in competitive situations: which is the best hydrogen‐bond acceptor? A crystallographic database investigation

Abstract: The relative hydrogen-bond acceptor abilities of amino and cyano N atoms have been investigated using data retrieved from the Cambridge Structural Database and via ab initio molecular orbital calculations. Surveys of the CSD for hydrogen bonds between HX (X = N, O) donors, N—T—C≡N (push–pull nitriles) and N—(Csp3)n—C≡N molecular fragments show that the hydrogen bonds are more abundant on the nitrile than on the amino nitrogen. In the push–pull family, in which T is a transmitter of resonance effects, the hydrogen-bonding ability of the cyano nitrogen is increased by conjugative interactions between the lone pair of the amino substituent and the C≡N group: a clear example of resonance-assisted hydrogen bonding. The strength of the hydrogen-bonds on the cyano nitrogen in this family follows the experimental order of hydrogen-bond basicity, as observed in solution through the pKHB scale. The number of hydrogen bonds established on the amino nitrogen is greater for aliphatic aminonitriles N—(Csp3)n—C≡N, but remains low. This behaviour reflects the greater sensitivity of the amino nitrogen to steric hindrance and the electron-withdrawing inductive effect compared with the cyano nitrogen. Ab initio molecular orbital calculations (B3LYP/6-31+G** level) of electrostatic potentials on the molecular surface around each nitrogen confirm the experimental observations.

Salts of 3,5‐dinitrobenzoic acid with organic diamines: hydrogen‐bonded supramolecular ­structures in one, two and three dimensions

Abstract: The trigonally trisubstituted carboxylic acid 3,5-dinitrobenzoic acid, (O2N)2C6H3COOH, forms 2:1 salts with a range of organic diamines L, with the general composition [LH2]2+·[{(O2N)2C6H3COO}−]2. When L is a bis-tertiary amine the hard N—H⋯O hydrogen bonds generate finite three-component aggregates, anion⋯cation⋯anion, and these aggregates are further linked by soft C—H⋯O hydrogen bonds to form one-dimensional molecular ladders when L is N,N,N′,N′′-tetramethyl-1,2-diaminoethane and chains of rings when L is 4,4′-dipyridylethane or 4,4′-dipyridylethene; two-dimensional sheets are formed when L is 1,4-diazabicyclo[2.2.2]octane and a three-dimensional framework is formed when L is N,N′-dimethylpiperazine. When L is the bis-secondary amine piperazine, the hard N—­H⋯O and soft C—H⋯O hydrogen bonds each generate continuous motifs in the form of distinct chains of rings, the combination of which generates sheets, while when L is the bis-primary amine 1,2-diaminoethane the hard N—H⋯O hydrogen bonds alone generate a three-dimensional framework.

Polymorphism of maleic hydrazide. I.

Abstract: The third polymorph (denoted MH3) of maleic hydrazide (3,6-dihydroxypyridazine in the monolactim form, 6-hydroxy-3-pyridazinone, C4H4N2O2) has been studied by X-ray diffraction and shown to be monoclinic, space group P21/n. Polymorph MH3 was found as the prevailing form along with the rare triclinic polymorph MH1, space group P\bar 1, but they were obtained separately from monoclinic MH2, space group P21/c. The structure of MH1, previously studied by photographic methods, has been redetermined. Polymorph MH3 exhibits the same scheme of molecular association into hydrogen-bonded ribbons as in MH1 and MH2, but the arrangements of the aggregates and details of their supramolecular conformations are different. The accommodation of the supramolecular conformations to the requirements of close packing of the aggregates in crystal lattices, as well as the symmetries of the polymorphs, are analyzed.

Experimental electron density of urea-phosphoric acid (1/1) at 100 K.

Abstract: The deformation electron density of the urea–phosphoric acid adduct has been studied from 100 K X-ray and neutron diffraction experiments. Data were interpreted according to the Hirshfeld model. The long hydrogen bonds show characteristics of electrostatic interaction. Deformation density maps on the short hydrogen bond shows hydrogen more strongly bonded to urea than to phosphoric acid, and peak maxima at almost midway between the two O—H bonds.

Kinetic behaviour investigations and crystal structure of nitric acid dihydrate.

Abstract: X-ray powder diffraction experiments are performed to prove the possible crystallization of nitric acid dihydrate (HNO3·2H2O, further denoted NAD) and to determine the best thermal conditions for growing a single crystal It is shown that the kinetic behaviour of NAD strongly depends on the preliminary thermal treatment One good single crystal obtained by an in situ adapted Bridgman method procedure enabled determination of the crystal structure The intensities of diffracted lines with h odd are all very weak The H atom of nitric acid is delocalized to one water molecule leading to an association of equimolar nitrate (NO3−) and an H5O2+ ionic group The asymmetric unit contains two such molecules These two molecules are related by a pseudo a/2 translation (with a 03 A mean atomic distance difference), except for one H atom of the water molecules (086 A) because of their different orientations in the two molecules The two molecules, linked by very strong hydrogen bonds, are arranged in layers Two layers which are linked by weaker hydrogen bonds are approximately oriented along the c axis The structure may be described by translations of this set of two layers along the c axis without hydrogen bonds leading to a two-dimensional hydrogen-bond network The structures of the monohydrate (NAM) and trihydrate (NAT) are re-determined for comparisons These structures may be described by one- and three-dimensional hydrogen-bond networks, respectively

Structural complexity of a polar perhydrotriphenylene inclusion compound brought to light by synchrotron radiation.

Abstract: The complex diffraction pattern of the heavily disordered co-crystals of perhydrotriphenylene and 1-(4-nitrophenyl)piperazine (5C18H30·C10H13N3O2) has been investigated with synchrotron radiation and an area detector. Five (almost) complete, three-dimensional data sets have been obtained from the tips and the centre of a needle-like crystal at room temperature and 120 K. They revealed a rich variety of features including one,- two- and three-dimensional diffuse scattering, as well as incommensurate satellites. At the centre and one tip of the crystal the symmetry appears to be orthorhombic, whereas at the other tip the symmetry of the satellites and of some of the diffuse scattering is clearly monoclinic, indicating that the crystal is not homogeneous. Most of the scattering could be assigned to R/S occupational disorder of the chiral host molecules, to positional disorder of the guest molecules or to local distortions of the average structure. Assignments are based on the disorder deduced from the average structure and the molecular form factors of host and guest molecules which show characteristic patterns in reciprocal space. Two smaller, orthorhombic twin fragments and an additional phase with hexagonal symmetry have also been found.

High-pressure structures of α- and δ-ZrMo2O8

Abstract: In situ high-pressure synchrotron X-ray powder diffraction studies of trigonal α-ZrMo2O8, zirconium molybdate, have been performed from ambient conditions to 1.9 GPa, over the α–δ phase transition at 1.06–1.11 GPa. The monoclinic structure of δ-ZrMo2O8, stable between 1.1 and 2.5 GPa at 298 K, has been solved by direct methods and refined using the Rietveld method. Significant distortions of the ZrO6 and MoO4 polyhedral elements are observed for δ-ZrMo2O8, as compared to the ambient conditions of the α-phase, while the packing of anions becomes more symmetric at high pressure.

Quantifying distortion from ideal closest-packing in a crystal structure with analysis and application

Abstract: A parameter, Ucp, that quantifies the distortion of the anion skeleton in a crystal from ideal closest-packing has been developed. It is a measure of the average isotropic displacement of the observed anions from their ideal equivalents. An ideal closest-packed structure can be fit to an observed structure by varying the radius of the ideal spheres, orientation and translation, such that Ucp is minimized. Ideal structures were fit to the M1M2TO4 polymorphs, pyroxenes and kyanite. The distortions of these crystals were analyzed in terms of the two parameters, Ucp and the ideal radius. Changes in structures due to temperature, pressure and compositional effects were characterized in terms of these parameters.

Irreversible single-crystal to polycrystal and ­reversible single-crystal to single-crystal phase transformations in cyanurates

Abstract: 4,6-Dimethoxy-3-methyldihydrotriazine-2-one (1) undergoes a single-crystal to single-crystal reversible phase transformation at 319 K. The low-temperature phase crystallizes in monoclinic space group P21/n with two crystallographically independent molecules in the asymmetric unit. The high-temperature phase is obtained by heating a single crystal of the low-temperature phase. This phase is orthorhombic, space group Pnma, with the molecules occupying a crystallographic mirror plane. The enthalpy of the transformation is 1.34 kJ mol−1. The small energy difference between the two phases and the minimal atomic movement facilitate the single-crystal to single-crystal reversible phase transformation with no destruction of the crystal lattice. On further heating, the high-temperature phase undergoes methyl rearrangement in the solid state. 2,4,6-Trimethoxy-1,3,5-triazine (3), on the other hand, undergoes an irreversible phase transformation from single-crystal to polycrystalline material at 340 K with an enthalpy of 3.9 kJ mol−1; upon further heating it melts and methyl rearrangement takes place.

Commensurate structure of Ca2CoSi2O7, a new twinned orthorhombic structure

Abstract: The crystal structure of the commensurate phase of Ca2CoSi2O7, dicalcium cobalt disilicate, has been derived from the modulated structure described in (3 + 2)-dimensional space. The structure is orthorhombic P21212; a = 23.510 (4), b = 23.510 (4), c = 5.025 (1) A (at 170 K), Z = 18. Since the crystal is twinned and the apparent diffraction symmetry is 4/mmm, the parameters were refined by a newly developed least-squares program for the refinement of twinned crystals. The structure is essentially similar to the known structure of the melilite group, but with regular arrangement of the bundles along [001] formed with four arrays of the sixfold coordinated Ca polyhedra and an array of CoO4 tetrahedra. The distribution of the bundles found in the present structure is different from that reported by Riester et al. [(2000), Z. Kristallogr. 215, 102–109].