Showing papers in "Zeitschrift Fur Kristallographie in 2008"

The “state of the art” of the diffraction analysis of crystallite size and lattice strain.

Abstract: This paper addresses both old, but "renovated" methods and new methods for diffraction line-profile analysis. Classical and even extremely simple single-line methods for separating "size" and "strain" broadening effects have merit for characterization of the material im- perfectness, but it is generally very difficult to interpret the data obtained in terms of microstructure parameters as used in materials science. Developments of recent years, focusing on distinct anisotropic line-broadening effects, as due to the type, orientation and distribution of dislocations and minute compositional variation, will be touched upon. The most promising development may be the synthesis of line profiles on the basis of a microstructure model and application of the (kinematical) diffraction theory without any further assumption, which contrasts with the other methods. This approach can in principle be applied in sin- gle-line and multiple-line variants and also in analyses of the whole diffraction pattern. The advantage is the direct evaluation of microstructure parameters as used in materi- als science. The challenge is to develop microstructure models which are flexible enough to be applicable in more than one case ...

Light metal borohydrides: Crystal structures and beyond

Abstract: Experimental structures of M(BH4)n, where M is a 2nd-4th period element, are reviewed with a particluar emphasize on crystal chemistry. It is shown that except certain cases, the BH4 group has a nearly ideal tetrahedral geometry. Correction of the experimentally determined Hpositions allows to compare directly the results obtained by different diffraction techniques and by theoretical calculations. Analysis of coordination geometries for M and BH4, and of mechanisms of phase transitions in LiBH4, suggests that the directional BH 4... M interaction is at the origin of structural complexity of borohydrides. The ways to influence their stability by chemical modification are discussed. Study of structural evolution with temperature and pressure is shown to be the way to access fundamental information on structural stability of these systems. © by Oldenbourg Wissenschaftsverlag, Munchen.

What is a co-crystal?

Abstract: Abstract What might very well be classified as a revolution in chemical crystallography is the emergence of crystal engineering as a discipline and the pivotal role X-ray crystallography plays in this [1]. Although dating back to the 1950's, the field has exploded in the last decade or so, and now is at the forefront of contemporary chemical and cognisant research. One of the areas in crystal engineering of greatest potential impact relates to the pharmaceutical industry – drug formulation and intellectual property – where issues relating to crystallisation, polymorphism, etc. are paramount [2]. A focus of these crystal engineering studies relates to the preparation of multi-component crystals, often referred to as co-crystals, where, for example utilizing hydrogen bonding synthons, active pharmaceutical ingredients are co-crystallised with other molecules [2]. The motivation for such studies is to improve stability (e.g. shelf-life), bioavailability (e.g. solubility), etc. [3].

Understanding the weakly bonded constituents in oxysalt minerals

Abstract: The crystal structure of a mineral may be divided into two parts: (1) the structural unit, an array of high-bond-valence polyhedra that is usually anionic in character, and (2) the interstitial complex, an array of large low-valence cations, simple anions and (H 2 O) groups that is usually cationic in character. Interstitial complexes link the structural units with weak cation-anion and hydrogen bonds into a continuous structure, and the breakdown of a structure is usually controlled by the strengths of the weak bonds that link the structural units together. The interstitial complex is (usually) a complex cation, and can be characterized by its Lewis acidity, a measure of the electrophilic character of the complex. The structural unit is (usually) a complex oxyanion, and can be characterized by its Lewis basicity. The interaction between the structural unit and the interstitial complex can be examined using the principle of correspondence of Lewis acidity-basicity. If one examines a series of structures with the same structural unit, it is evident that the average coordination of the O atoms of the structural unit varies slightly from one structure to another, producing a range of Lewis basicity for this specific structural unit. In this way, a specific structural unit can be stable over a range of Lewis basicity (i.e., over a specific pH range). The formula of an interstitial complex may be written in the following way: { [m] M + a [n] M 2+ b [l] M 3+ c (H 2 O) d (H 2 O)(OH) f (H 2 O) g } (a+2b+3c-f)+ , where [m], [n] and [l] are coordination numbers, a, b and c are the numbers of monovalent, divalent and trivalent cations, d is the number of transformer (H 2 O) groups, e is the number of (H 2 O) groups bonded to two interstitial cations or one interstitial cation and one hydrogen bond, f is the number of interstitial (OH) groups, and g is the number of (H 2 O) groups not bonded to any cation. The number of transformer (H 2 O) groups strongly affects the Lewis acidity of the interstitial complex, and the variation in Lewis acidity of a generalized interstitial complex can be graphically represented as a function of the number of transformer (H 2 O) groups. Where the Lewis acidity of a generalized interstitial complex overlaps the range of Lewis basicity of a specific structural unit, the principle of correspondence of Lewis acidity-basicity is satisfied and a stable structural arrangement is possible. Detailed predictions of the compositions of interstitial complexes are made for the borate, sulfate and uranyl-oxide-hydroxy-hydrate minerals. There is fairly close agreement between the predicted ranges of interstitial complex and those observed in Nature.

Does porous mean soft? On the elastic behaviour and structural evolution of zeolites under pressure

Abstract: This is a comparative study on lattice compressibility, pressure (P)-induced structural deformation mechanisms and influence of the framework and extra-framework content on the elastic behaviour of zeolites, based on previously published data obtained by in situ HP-single crystal and powder diffraction experiments. The elastic data of zeolites reported so far allow us to infer that: 1) the peculiar characteristics of the zeolitic structure, with large channels and a flexible framework built of rigid units (i.e. the tetrahedra), implies that the main deformation mechanisms at high-pressure (HP) are controlled by rigid (Si,Al)O 4 -tetrahedral tilting; 2) the structural rearrangement at HP is mainly driven by framework geometry and its topological symmetry; 3) the compressibility of zeolites appears not to be directly related to the microporosity, represented by the "framework density"; 4) the elastic parameters available for natural zeolites demonstrate that microporosity does not necessarily imply high compressibility. Several zeolites appear to be less compressible than many rock-forming minerals. A high compressibility is generally expected for open-framework structures due to the tetrahedral tilting, which produces inter-tetrahedral angle variations and accommodates the effect of pressure. However, the bonding between the host zeolitic framework and the stuffed guest species (cations and H 2 O molecules) affect the overall compression behaviour, making this class of porous material unexpectedly less compressible than other silicates.

Minerals with antiperovskite structure: a review

Abstract: Antiperovskites or inverse perovskites are inorganic compounds with a perovskite structure but with cations replaced by anions and vice versa. It is shown that there exists a number of minerals whose structures can be regarded as based upon antiperovskite units composed of face-and comer-sharing octahedra. The structures of sulphohalite, galeite, schairerite, kogarkoite, nacaphite, arctite, polyphite, hatrurite and related compounds are considered. The 3C, 2H, 5H, 7H, 9R and 15R antiperovskite polytypes are described. In some cases, antiperovskite modules are intercalated with intermediate ions and complexes.

Exploiting EXAFS and XANES for time-resolved molecular structures in liquids

Abstract: A review. Time-resolved X-ray absorption fine structure (XAFS) spectroscopy with picosecond temporal resoln. is a new method to observe electronic and geometric structures of short-lived reaction intermediates. It combines an intense femtosecond laser source synchronized to the X-ray pulses delivered by a synchrotron light source (SLS). We present key expts. on charge transfer reactions as well as spin-crossover processes in coordination chem. compds. next to solvation dynamics studies of photogenerated at. radicals. These examples emphasize the observables at hand using ultrafast XAFS techniques, which include the d. of states, full and even partial changes in oxidn. state, and internuclear distances with milli-Angstrom accuracy. An outlook towards femtosecond studies and biol. relevant systems stresses the high potential of XAFS methods using new femtosecond X-ray sources like free electron lasers (XFELs).

Structures of aluminium-based light weight hydrides

Abstract: Light weight hydrides based on aluminium are among the most promising materials for hydrogen storage applications since the hydrogen content is up to 11 wt%. The crystal structures of many promising aluminium-based compounds have been determined and refined during the last years. This review gives an overview of structures of tetra-alanates (with AlH 4 tetrahedra), hexa-alanates (with AlH 6 octahedra) and different modifications of AlH 3 . The structure determinations are based on powder diffraction with X-rays, synchrotron X-rays and neutrons. In most structural work neutron diffraction has been used in order to determine accurate hydrogen/deuterium positions. In addition, density functional theory has been used both to predict new structures among these compounds and in combination with experimental work.

A crystallographic review of arsenic sulfides : effects of chemical variations and changes induced by exposure to light

Abstract: Crystal data for natural and synthetic arsenic sulfides are reported and discussed. Most of them [a- and β-dimorphite, realgar, β-As 4 S 4 phase, pararealgar, Kutoglu's As 4 S 4 (II) phase, alacranite, uzonite, orthorhombic As 4 S 5 phase] have a crystal structure consisting of a packing of cage-like, covalently bonded As 4 S n (n = 3, 4 and 5) molecules held together by weak interactions of van der Waals character. Their structures are compared in terms of molecular packing and molecular parameters. The layered structural arrangement of orpiment, As 2 S 3 , is described and the effects of the incorporation of Se replacing for S is discussed. The structures of wakabayashilite and getchellite, which contain mixed (As, Sb) coordination polyhedra, are also described to outline the geometric effects of the Sb → As substitution. The results of recent studies dealing with the effects of the exposure of realgar or other arsenic sulfides to visible light are reported and discussed. Their interest in the study of arsenical pigments and their preservation in artwork is outlined with some examples of application.

Minerals and synthetic Pb(II) compounds with oxocentered tetrahedra: review and classification

Abstract: The crystal structures of minerals and inorgan- ic compounds with OPb4 oxocentered tetrahedra are re- viewed. It is shown that the OPb4 tetrahedral units may link by sharing common Pb atoms to form structural units of various shape and dimensionality. These units deter- mine basic topology of the structures and influence their stability and properties. The high strength of the OPb4 te- trahedral units involves interplay between high basicity of additional O 2� anions and stereochemical activity of the 6s 2 lone electron pairs on Pb 2þ cations. The structural chemistry of polycations based upon OPb4 tetrahedra, in general, follows major trends previously observed for ca- tion-centered tetrahedral units (silicates, phosphates, metal sulphides with MS4 tetrahedra, etc.). One may conclude that the basic structural correlations depend upon size and charge parameters of the ions only, irrespective of their positive or negative sign.

Bonded interactions and the crystal chemistry of minerals: a review

Abstract: Connections established during last century between bond length, radii, bond strength, bond valence and crystal and molecular chemistry are briefly reviewed followed by a survey of the physical properties of the electron density distributions for a variety of minerals and representative molecules, recently generated with first-prin- ciples local energy density quantum mechanical methods. The structures for several minerals, geometry-optimized at zero pressure and at a variety of pressures were found to agree with the experimental structures within a few per- cent. The experimental Si- -O bond lengths and the Si--O- -Si angle, the Si- -O bond energy and the bond criti- cal point properties for crystal quartz are comparable with those calculated for the H6Si2O7 disilicic acid molecule, an indication that the bonded interactions in silica are lar- gely short ranged and local in nature. The topology of model experimental electron density distributions for first and second row metal M atoms bonded to O, determined with high resolution and high energy synchrotron single crystal X-ray diffraction data are compared with the topol- ogy of theoretical distributions calculated with first princi- ples methods. As the electron density is progressively accumulated between pairs of bonded atoms, the distribu- tions show that the nuclei are progressively shielded as the bond lengths and the bonded radii of the atoms decrease. Concomitant with the decrease in the M- -O bond lengths, the local kinetic energy, G(rc), the local potential energy, V(rc), and the electronic energy density, H(rc) ¼ G(rc) þ V(rc), evaluated at the bond critical points, rc, each in- creases in magnitude with the local potential energy domi- nating the kinetic energy density in the internuclear region for intermediate and shared interactions. The shorter the bonds, the more negative the local electronic energy den- sity, the greater the stabilization and the greater the shared character of the intermediate and shared bonded interac- tions. In contrast, the local kinetic energy density in- creases with decreasing bond length for closed shell inter- actions with G(rc) dominating V(rc) in the internuclear region, typical of an ionic bond. Notwithstanding its origin in Pauling's electrostatic bond strength rule, the Brown-Shannon bond valence for Si- -O bonded interactions agrees with the value of the electron density, r(rc), on a one-to-one basis, indicating that the Pauling bond strength is a direct measure of r(rc), the greater the bond strength, the more shared the interac- tion. Mappings of the Laplacian, the deformation electron density distribution and the electron localization function for several silicates are reviewed. The maps display hemi- spherical domains ascribed to bond pair electrons along the bond vectors and larger kidney-shaped domains as- cribed to lone pair electrons on the reflex sides of the Si--O- -Si angles. In the case of the nonbridging Si--O bonded interactions, the O atoms are capped by mush- room shaped domains. With few exceptions, the domains agree in number and location with those embodied in the VSEPR model for closed-shell molecules, defining reac- tive sites of potential electrophilic attack and centers of protonation. The electrophilicity of the O atoms compris- ing the Si--O- -Si bonded interactions in coesite is indi- cated to increase with decreasing angle, providing a basis for understanding the protonization of the structure. The shapes and arrangements of the bond and lone pair features displayed by the bridging O atoms in quartz and and the nonbridging O atoms in forsterite are transferable on an one-to-one basis to sheet and chain magnesiosili- cates that possess both bridging and nonbridging O atoms. The G(rc)/r(rc) ratio increases for each of the M--O bonds along separate trends with decreasing bond length and the coordination number of the M atom, suggesting that the ratio is a measure of bond character. An examina- tion of the interactions in terms of the jV(rc)j=G(rc) ratio indicates that the Li--O, Na- -O and Mg- -O bonds are closed shell ionic interactions, that the C- -O bond and one of the S- -O bonds is shared covalent and that the Be--O, Al--O, Si--O, B--O, P- -O and S- -O bonds are intermedi-

Reaction of rhenium and carbon at high pressures and temperatures

Abstract: A combined X-ray diffraction, EELS and DFT study of the reaction of rhenium with carbon at high-(P, T) conditions up to P(max) = 67 GPa and T(max) = 3800 K is presented. A hexagonal rhenium carbide, ReC(x), was identified as the stable phase at high-(P, T) conditions. A composition of ReC(0.5) is proposed. No evidence for a cubic ReC polymorph with rocksalt structure, as suggested in the literature, or for any other phase was found at the P-T conditions explored. A preliminary P-T rhenium-carbon phase diagram has been derived and properties such as bulk moduli and elastic stiffness coefficients were obtained.

Crystal chemistry of the mendipite-type system Pb3O2Cl2-Pb3O2Br2

Abstract: Lead oxyhalides / Mendipite / Oxocentered tetrahedra / Conformation / Single crystal structure analysis / X-ray diffraction Abstract. The crystal structures of the mendipite series Pb3O2Cl2- -Pb3O2Br2 have been refined. The structures are based upon (O2Pb3) 2+ double chains of edge-sharing OPb4 tetrahedra. There are three symmetrically independent Pb 2+ cations. The number of nonequivalent halogen sites is two (X1, X2). Short Pb- -O bonds are located on one side of the Pb 2+ cations and weak Pb-X bonds are located on the other side of the Pb 2+ coordination sphere. The evident strong distortion of the Pb 2+ coordination polyhedra is due to the stereoactivity of the 6s 2 lone electron pairs of the Pb 2+ cations. Pb1-X2 and Pb2-X2 bonds are the most sen- sitive to the X site occupancy, which is in agreement with the non-linear behavior of the a and c parameters. Deter- mination of unit-cell parameters by single crystal studies showed strong deviation from Vegard's rule. Nonlinearity of the lattice parameters is caused by selective ordering of the halide anions over X1 and X2 sites. Br atoms prefer the X2 position, whereas Cl prefers the X1 site. The angle between two adjacent OPb4 tetrahedra was determined to analyze the influence of halogen atoms on the structure of the (O2Pb3) 2+ chain. Different occupancy of the X1 site by Cl and Br atoms leads to most pronounced angular changes. These observations may be interpreted as adapta- tion of the (O2Pb3) 2+ double chains to the large halide ions in the crystal structures of the mendipite series com- pounds.

Compounds of glycine with halogen or metal halogenides: review and comparison

Abstract: The large family of glycine halogenides and metal halogenides encompasses a high variety of different crystal structures. The role of the glycine molecule as a structural unit as a mono-, bi- or tridentate ligand, of the halogenide ions as well as the composition and condensation of the cationic polyhedra is discussed. With respect to the connectivity of the building units, five different structure families of glycine halogenides and glycine-metal halogenides can be distinguished. Parameters such as symmetry, ionic radii, electronegativity of cation and anion and their difference, charge, coordination number, hydration states, stoichiometric ratios and connectivity were weighed against each other in order to see if any correlations exist. The data suggest that cations with high electro-negativities seem to promote formation of structures with a low grade of connectivity, i.e., isolated units, and vice versa.

Out-of-equilibrium charge density distribution of spin crossover complexes from steady-state photocrystallographic measurements: experimental methodology and results

Abstract: The electron density distribution of a light-in- duced molecular excited state, i.e. the high spin metastable state of (Fe(phen)2(NCS)2), was determined from steady- state photocrystallographic measurements. We defined the experimental conditions under which the accuracy of the measured diffraction data is compatible with an electron density analysis. These include: (i) a large structural and electronic contrast between high spin (HS) and low spin (LS) states, (ii) an efficient photoconversion under light irradiation and (iii) slow relaxation of the HS metastable state. Multipolar modeling of the electron density yielded a deformation density and 3d-orbital populations for Fe(II) characteristic of a high spin (t2g 4 eg 2 ) electron configuration and support the assumption of significant s-donation and p-backbonding of the Fe- -N interactions. The electron density distribution in the intermolecular regions confirms anisotropic intermolecular interactions with possibly a layer topology parallel to the orthorhombic (ab) plane, re- lated to the system cooperativity.

Hydrogen induced site depopulation in the LaMgNi4-hydrogen system

Abstract: The LaMgNi 4 -hydrogen system was investigated by in-situ neutron powder diffraction and pressure-composition isotherm measurements at 100 °C and hydrogen (deuterium) pressures of up 50 bar. The system displays three hydride phases that have distinctly different hydrogen plateau pressures and H atom distributions. The cubic α-LaMgNi 4 H 0.75 phase forms below 0.01 bar hydrogen pressure and H atoms fill one type of tetrahedral Ni 4 interstices. The orthorhombic distorted β-LaMgNiH 3.7 phase forms at ∼3 bar hydrogen pressure and H atoms fill both tetrahedral LaNi 3 and triangular bi-pyramidal La 2 MgNi 2 interstices. Interestingly, tetrahedral Ni 4 interstices are no longer occupied. Finally, the most hydrogen rich γ-LaMgNi 4 H 4.85 phase forms above 20 bar. It has again cubic symmetry and H atoms continue to occupy triangular bi-pyramidal La 2 MgNi 2 interstices while filling a new type of tetrahedral Ni 4 interstices that are neither occupied in the α- nor in the β-phase. The tetrahedral LaNi 3 interstices occupied in the /3-phase are empty. Hydrogen induced depopulations of interstitial sites in metal hydrides are relatively rare and consistent with, but not entirely due to, the onset of repulsive H-H interactions at increasing hydrogen concentrations.

Coexistence of hydrogen and polyanions in multinary main group element hydrides

Abstract: Multinary hydrides based on mixtures of sand p-block metals are often not fully hydrogenated, but possess a polymeric anion which is formed by the p-block component upon reduction by the s-block metals. Hydrogen may occur hydridic and exclusively coordinated by s-block metal ions (Zintl phase hydrides), or be incorporated in the polymeric anion where it acts as a covalently bonded terminating ligand (polyanionic hydrides). The current state of Zintl phase hydrides and polyanionic hydrides is reviewed with an emphasis on crystal structures and bonding principles.

Crystal chemistry and metal-hydrogen bonding in anisotropic and interstitial hydrides of intermetallics of rare earth (R) and transition metals (T), RT3 and R2T7

Abstract: Hydrides of the RNi 3 - and R 2 Ni 7 -based (R = light rare earth element) intermetallics exhibit novel structural features Structures of these hydrides, including CeNi 3 D 28 , La 2 Ni 7 D 65 , LaNi 3 D 28 , and Ce 2 Ni 7 D 47 , are formed via a huge volume expansion occurring along a single crystallographic direction Unique structural features during the formation of the hydrides include: (a) The lattice expansion proceeds exclusively within the RNi 2 slabs leaving the RNi 5 slabs unmodified Such expansion, ∼60% along [001] for the Laves layers, is associated with occupation of these slabs by D atoms; (b) New types of interstitial sites occupied by D are formed; (c) An ordered hydrogen sublattice is observed In the present work we give (a) a review of the crystal chemistry of the conventional, interstitial type hydrides formed by RT 3 and R 2 T 7 intermetallic compounds (R = rare earths; T = Fe, Co, Ni) as compared to the unusual features of the crystal chemistry of anisotropic hydrides formed by the RNi 3 and R 2 Ni 7 intermetallics and (b) studies of the interrelation between structure and bonding in anisotropic hydrides by performing density functional calculations for CeNi 3 and Ce 2 Ni 7 intermetallic alloys and their corresponding hydrides These studies provide an understanding of the bonding mechanism in the hydrogenated compounds which causes a complete anisotropic rebuilding of their structures From DOS analysis, both initial intermetallics and their related hydrides were found to be metallic Bader topological analysis for the non-hydrogenated intermetallics showed that Ce atoms donate in average of almost 12 electrons to the Ni sites Hydrogenation increases electron transfer from Ce; its atoms donate 12-16 electrons to Ni and H The Charge Density Distribution and Electron Localization Function for the Ce 2 Ni1 7 D 47 7 phase clearly confirm that the interaction between the Ce and Ni does not have any significant covalent bonding Ni is bonded with H via forming spatial frameworks -H-Ni-H-Ni- where H atoms accumulate an excess electron density of ∼05e - Thus, the tetrahedral or open saddle-type NiH 4 coordination observed in the structures of these hydrides is not associated with the formation of [Ni 0 H 4 1- 1] 4- complexes containing a hydrido-ion H -1 In the structural frameworks there are terminal bonds Ni-H, bridges Ni-H-Ni, and the bonds where one H is bound to three different Ni These spatial ordered frameworks stand as the principal reason for the anisotropic changes in the structural parameters on hydrogenation Another unique feature of anisotropic hydrides is the donation of electrons from nonhydrogenated RNi 5 parts to hydrogen in RNi 2 slabs stabilising these fragments

New compounds of glycine with metal halogenides

Abstract: The crystal structures of five new glycine (H 2 N-CH 2 -COOH) metal halogenide compounds were determined by single crystal X-ray diffraction. Three of them are monoclinic phases, GlycineCaCl 2 · 3 H 2 O (space group P2 1 /c, a = 9.964(2), b = 6.868(1), c = 13.959(3) A, β= 104.11(3)°, R1= 0.024), GlycineZnCl 2 · H 2 O (space group P2 1 /a, a = 7.868(1), b = 9.124(1), c = 10.645(1) A, β= 103.51(1)°, R1= 0.026) and Glycine 2 ZnCl 2 ·2 H 2 O (space group C2/c, a = 14.444(1), b= 6.916(1), c= 12.968(1) A, β = 117.90(3)° R1 = 0.033), two of them are orthorhombic phases Glycine 3 CeCl 3 ·3H 2 O (space group P2 1 2 1 2 1 , a = 4.788(1), b = 12.074(2), c = 30.949(6) A R1 = 0.034) and Glycine 2 CaI 2 ·3H 2 O (space group Pca2 1 , a = 13.059(3), b = 9.862(2), c = 22.724(5) A R1 = 0.023). The atomic arrangements as well as aspects of the crystal chemistry of these compounds are presented.

Static and time-resolved photocrystallographic studies in supramolecular solids

Abstract: The supramolecular solid state is a medium par excellence for photochemical studies of molecules in a well-defined environment. If the photoactive molecule is embedded within a strong photochemically-inert frame- work, reactions can proceed without breakdown of the crystal lattice. A number of examples of trans-cis and cis- trans reactions are presented. A variable temperature study of Zn-coordinated tiglic acid shows that the reaction is en- ergy-controlled with a small activation energy of � 2 kJ/ mol, dependent on the size of the cavity in which the mo- lecule is located. Time-resolved studies of the instanta- neous dimer-formation of an organic and an inorganic spe- cies are presented. The Cu(I)(NH3)2 cation forms a dimeric species with a contracted Cu(I)- -Cu(I) distance through promotion of an electron from an antibonding to a weakly bonding orbital, whereas xanthone forms an excimer with decrease interplanar spacing. The concept of the photodif- ference map is defined and used thoughout the article.

Reducing the positional modulation of NbO6-octahedra in SrxBa1–xNb2O6 by increasing the barium content: A single crystal neutron diffraction study at ambient temperature for x = 0.61 and x = 0.34

Abstract: We report on the influence of the barium content on the modulation amplitude in Sr x Ba 1-x Nb 2 O 6 compounds by comparing Sr 0.61 Ba 0.39 Nb 2 O 6 (SBN61) and Sr 0.34 Ba 0.66 Nb 2 O 6 (SBN34). Our single crystal neutron diffraction results demonstrate that the amplitude of the positional modulation of the NbO 6 octahedra is reduced with increasing barium content, indicating that the origin of the modulation is the partial occupation of the pentagonal channels by Sr and Ba atoms. By increasing the Sr content the bigger Ba atoms are replaced by the smaller Sr atoms, which leads to a larger deformation of the surrounding lattice and hence to a larger modulation amplitude. The more homogeneous the filling of these channels with one atomic type (Ba) the lower the modulation amplitude. Our results also show that the structure can be described with a two-dimensional incommensurate harmonic modulation. No second order modulation has been observed, both by single crystal diffraction measurements and q-scans. The positional modulation of the Nb atoms is much smaller than that of the oxygen atoms, such that the modulation can be seen as a rotational modulation of almost rigid NbO 6 -octahedra.

The nature of deuterium arrangements in YD3 and other rare-earth trideuterides

Abstract: The efficacy of different structural models for describing the observed neutron-powder-diffraction (NPD) measurements of bulk polycrystalline YD 3 as well as other hexagonal rare-earth (i.e., Nd, Tb, Dy, Ho, Er, and Tm) trideuteride powders has been investigated via Rietveld refinement. Between the two possible structural configurations, centrosymmetric P3cl and noncentrosymmetric P6 3 cm, the latter can be excluded due to very high correlations found between the positions of the D sites. Hence, the true "diffraction-average" structure for YD 3 and all other rare-earth deuterides studied is centrosymmetric (P3cl). This seems to contrast with the prior evidence from first-principles calculations and various spectroscopic probes suggesting that the true local symmetry is not P3cl, but rather, noncentrosymmetric. A possible way to reconcile the apparently conflicting conclusions from NPD and spectroscopic measurements is by assuming that the real structure is a twinned arrangement of nanosized, noncentrosymmetric configurations. For example, we demonstrate that the diffraction-average centrosymmetric P3cl structure can result from a superposition of individual, noncentrosymmetric P3cl twins. A comparison of neutron vibrational spectra for YH 3 and YD 3 confirms that both compounds share similar structural arrangements.

Applications of photocrystallography: a future perspective

Abstract: The applied potential of photocrystallography is discussed herein. In particular, its ability to help realise new generation 'smart' materials for the opto-electronics industry, alternative photovoltaic solar cells, and health science, is discussed. Where relevant photocrystallographic studies have already been reported, examples are given and put into the context of potential device applicability. In certain areas of photonics research, there are no current photocrystallographic results specifically pertaining to the perceived application described. Here, the likely type of results is projected since this perspective is designed deliberately to look ahead as to what advances photocrystallography could bring to the photonics research area that continues to develop so rapidly. Brief reference to global economic, environmental and health considerations are also made in relevant places in order to put the subject onto a world setting.

Similarity versus coincidence rotations of lattices

Abstract: The groups of similarity and coincidence rotations of an arbitrary lattice r in d-dimensional Euclidean space are considered. It is shown that the group of similarity rotations contains the coincidence rotations as a normal subgroup. Furthermore, the structure of the corresponding factor group is examined. If the dimension d is a prime number, this factor group is an elementary Abelian d-group. Moreover, if Γ is a rational lattice, the factor group is trivial (d odd) or an elementary Abelian 2-group (d even).

Excited state isomerization in photochromic ruthenium complexes

Abstract: Photochromic ruthenium polypyridyl complexes whose action is based on an excited state linkage isomerization are discussed. Picosecond transient absoption spectroscopic data indicate that isomerization occurs along the charge-transfer manifold. These studies indicate that rotation about the Ru-S bond is an important vibration along this coordinate. These complexes are of interest in photonic molecular devices, specifically molecular information storage applications.

Synthesis of single phase i-AlCuFe bulk quasicrystal by spray forming

Abstract: In the present study, an icosahedral single phase bulk quasicrystalline material based on Al 62 Cu 25.5 Fe 12.5 has been synthesised by a spray forming route. Microstructural characterization showed an average grain size of 10 μm. The oversprayed fine powder showed the presence of β- and λ-phases, whereas, the deposit consited of the fully single phase bulk quasicrystalline material with compositional homogeneity. The hardness and fracture toughness measurements were carried out at different indentation loads of 50-500 g. The hardness values varied in the range 10.4-8.1 GPa and fracture toughness was seen to decrease with increasing load. The varionan of hardness with load, which is known as indentation size effect (ISE), has been established clearly. Fracture toughness value was constant in the load range from 200 to 500 g at 1.2 MPa m 1/2 . The cracking pattern after indentation at higher load has been observed to be intergranular as well as transgranular. The evolution of the single phase bulk quasicrystalline material has been discussed in light of the unique combination of atomization and deposition process elements in spray forming technique.

Hydrogen-bonded frameworks in molecular metal-organic crystals: the network approach

Abstract: Supramolecular architectures in molecular metal-organic crystals with three-dimensional networks of hydrogen bonds have been studied from the topological point of view. 636 hydrogen-bonded frameworks have been classified into 78 types of three-dimensional nets. It is shown that the resulting type of a framework is predetermined by prevailing octahedral coordination of metals in complexes that leads to the preference of 6-and 8-coordinated net topologies and those framework types compatible with the inversion symmetry of a complex. The study of molecular packings in metal-organic crystals with three-dimensional hydrogen-bonded frameworks and in those without hydrogen bonding has confirmed the trend to form close-packed motifs, irrespective of the presence of hydrogen bonds. Finally, a comparative analysis of organic and metal-organic hydrogen-bonded molecular frameworks as well as valence-bonded metal-organic frameworks is presented.

Nano quasicrystal formation and local atomic structure in Zr–Pd and Zr–Pt binary metallic glasses

Abstract: Formation of the nanoscale icosahedral quasicrystalline phase (I-phase) in the melt-spun Zr 70 Pd 30 and Zr 80 Pt 20 binary metallic glasses were reported. Local atomic structure in the glassy and quasicrystal (QC)-formed states were also analyzed by XRD and EXAFS measurements in order to investigate the formation mechanism of QC phase. The distorted icosahedral-like local structure can be identified around Zr atom in the Zr 70 Pd 30 metallic glass. In the QC formation process, a change of local environment around Zr is detected, in which the approximately one Zr atom substitutes for one Pd atom. In contrast, since the local environment around Pt atom is remaining during the QC precipitation, it is suggested that the stable icosahedral local structure is mainly formed around a center Pt atom in the glassy state in Zr 80 Pt 20 . We also found that the local environment around Zr atom significantly changes during the quasicrystallization in the alloy. These results differ from those in the Zr 70 Pd 30 metallic glass.

Gallium substitution in the alumosilicate framework: synthesis and structural studies of hydro sodalites

Abstract: A series of hydro sodalites of composition Na6[Ali-yGaySiO 4 ] 6 (H 2 O) 8 (0 < y < 1) have been prepared from Na 6+x [Al 1-y Ga y SiO 4 ] 6 (OH H 2 O) x (H 2 O) 8-4x sodalites by NaOH/H 2 O exchange methods carried out in acidic aqueous media. One of the end members (y = 1) Na 6 [GaSiO 4 ] 6 (H 2 O) 8 is obtained from Na 8 [GaSiO 4 ] 6 I 2 by a hydrothermal transformation process. The polycrystalline samples are characterized by Rietveld refinements using X-ray powder diffraction data. The cell parameter fluctuates slightly from the [AlSiO 4 ] 6 to the [GaSiO 4 ] 6 framework with an average value of 884.8 pm. The occurrence of an almost constant cell parameter has been explained as a combined effect of hydrogen bonding between the oxygen atoms of the entrapped water (or the occluded [Na 3 □(H 2 O) 4 ] 3+ complex in the /3-cage as a whole), and the framework together with six-ring window deformation. During XRD refinements both aluminium and gallium atoms have been converged to a single crystallographic site. The Al/Ga-O bond distance therefore indicates the mean of Al-O and Ga-O distances, which increases with increasing gallium content in the framework. However, the Si-O distance remains almost constant at 162.9 pm at any Ga/Al ratio. The coupled twisting of the (Al,Ga)O 4 and SiO 4 tetrahedra confirms the increase of tilt angles and decrease of six-ring pore dimension upon aluminium replacement by gallium in the framework. Results of 1 H and 29 Si MAS NMR as well as FTIR spectroscopy are also discussed in terms of successive gallium incorporation in the alumosilicate hydro sodalite frameworks.

Homometric point sets and inverse problems

Abstract: The inverse problem of diffraction theory in essence amounts to the reconstruction of the atomic positions of a solid from its diffraction image. From a mathematical perspective, this is a notoriously difficult problem, even in the idealised situation of perfect diffraction from an infinite structure. Here, the problem is analysed via the autocorrelation measure of the underlying point set, where two point sets are called homometric when they share the same autocorrelation. For the class of mathematical quasicrystals within a given cut and project scheme, the homometry problem becomes equivalent to Matheron´s covariogram problem, in the sense of determining the window from its covariogram. Although certain uniqueness results are known for convex windows, interesting examples of distinct homometric model sets already emerge in the plane. The uncertainty level increases in the presence of diffuse scattering. Already in one dimension, a mixed spectrum can be compatible with structures of different entropy. We expand on this example by constructing a family of mixed systems with fixed diffraction image but varying entropy. We also outline how this generalises to higher dimension.