bluuebunny/crossref_metadata_2025_split huggingface dataset:
Crystallographic point and space group symmetries in the one-and two-body density of crystals and generalized Patterson functions: Fourier path integral Monte Carlo case studies of novel high-temperature/high-pressure4He quantum crystals
Authors: Gernoth Klaus A. 
Published: December 2003

AbstractWe present a complete formal analysis of crystallographic point and space group symmetries in the local one- and two-body densityρ(x) andρ2(x1,x2) of crystals and from there develop the mathematical theory of generalized Patterson functions. The two-body densityρ2(x1,x2) andρ(x1)ρ(x2) may be written as symmetrized lattice Fourier series over all reciprocal lattice vectorsKwith Fourier transforms that are functions of the relative vectorr=x1–x2. In the new formalism derived here the usual Patterson function turns out to be the lowest-lyingK= 0 lattice Fourier transform in the Fourier series representation of the uncorrelated productρ(x1)ρ(x2) of one-body densities. All otherK≠ 0 Fourier transforms in the Fourier series ofρ(x1)ρ(x2) may be regarded as generalized Patterson functions. In complete analogy to these generalized Patterson functions the Fourier transform functions in the Fourier series of the fully correlated two-body densityρ2(x1,x2) may be regarded asK= 0 andK≠ 0 fully correlated generalized Patterson functions. It is shown that the former generalized Patterson functions represent the uncorrelated long-range limits for large interparticle distances of the latter, fully correlated, generalized Patterson functions. Both types of generalized Patterson functions may be cast in the form of symmetry-adapted trigonometric series with expansion functions that depend on the relative distancer= |r| and on the polar angle ofr. The symmetrized trigonometric series representations of the generalized Patterson functions are given here explicitly in specific applications of the new formalism to crystallographic space groupP63/mmc. In exact path integral Monte Carlo case studies of novel high-temperature/high-pressure4He quantum crystals in the hexagonal close-packed structure we present and discuss exact numerical results for the one-body densityρ(x) and for expansion functions, for both types of generalized Patterson functions, in the symmetrized trigonometric series representations of the generalized Patterson functions. The numerical results for the two types of generalized Patterson functions are compared to each other.
In situandex situinvestigation of the displacive phase transformations Ag3Sn(h) → Ag3Sn(l) and Ag3Sb(h) → Ag3Sb(l)
Authors: Ellner M. , Mittemeijer E. J. 
Published: December 2003

AbstractThe occurence of the polymorpic phase transformations Ag3Sn(h) → Ag3Sn(l) and Ag3Sb(h) → Ag3Sb(l) was demostratedin situprimarily by means of high-temperature X-ray powder diffracion and thermal analysis as well as byex situX-ray powder diffraction. Both solid- and liquid-quenched aloys were investigated. The structure of the high-temperature phases Ag3Sn(h) and Ag3Sb(h) is isotypic with Mg (Pearson symbolhP2, space groupP63/mmc); unit cell parameters are:a= 2.9863(8) Å,c= 4.784(2) Å for Ag3Sn(h) anda= 2.9944(9) Å,c= 4.812(3) Å for Ag3Sb(h). The low temperature phases Ag3Sn(l) and Ag3Sb(l) are orthorhombic displacive homeotypes of the hexagonal close-packed structure, isotypic with withβ-TiCu (oC4,Cmcm). The unit cell parameters are for Ag3Sn(l):a= 2.9973(5) Å,b= 5.1618(8) Å,c= 4.786(1) Å,b/a= 1.722 < √3; and for Ag3Sb(l):a= 2.9947(3) Å,b= 5.2497(5) Å,c= 4.8495(7) Å,b/a= 1.753 > √3. The crystal structures and the phase transformations werde discussed in terms of the valence electron concentration.
Geometrical isomerism in uranyl chromates II. Crystal structures of Mg2[(UO2)3(CrO4)5](H2O)17 and Ca2[(UO2)3(CrO4)5](H2O)19
Authors: Krivovichev Sergey V. , Burns Peter C. 
Published: December 2003

Abstract
Geometrical isomerism of uranyl chromate sheets occurs in the structures of Mg2[(UO2)3(CrO4)5](H2O)17 and Ca2[(UO2)3(CrO4)5](H2O)19. Both compounds were prepared by evaporation from aqueous solutions. Compound 1 is orthorhombic, Pbca, a = 19.9206(10), b = 21.0526(10), c = 18.4966(9) Å, V = 7757.1(7) Å3; R
1 = 0.050 for 6332 unique reflections with |F
o| ≥ 4σF
. Compound 2 is monoclinic, P21/m, a = 11.0359(5), b = 17.5364(7), c = 11.5056(5) Å, β = 118.178(1)°, V = 1962.8(2) Å3; R
1 = 0.037 for 6209 unique reflections with |F
o| = 4σ
F. Both structures contain sheets of composition [(UO2)3(CrO4)5] involving corner-sharing UO7 pentagonal bipyramids and CrO4 tetrahedra. The sheets are parallel to (100) in both structures. Mg2+ and Ca2+ cations are coordinated by H2O molecules and are located in the interlayer. The topology of the [(UO2)3(CrO4)5] sheets in both structures is the same, whereas their geometrical structures are different, and they are geometrical isomers. The sheets differ in the orientations of non-shared corners of chromate tetrahedra relative to the plane of the sheet. In order to describe the geometrical structure of the isomers, the concept of orientation matrix of tetrahedra is introduced. It has been shown that there are at least three geometrical isomers of the [(UO2)3(CrO4)5] sheet in uranyl chromates that differ in their orientation matrices of the tetrahedra.
Dimorphism in mercury(II) tellurite(IV) tellurate(VI): preparation and crystal structures of α- and β-Hg2Te2O7
Authors: Weil Matthias 
Published: December 2003

Abstract
The mixed-valent mercury(II) tellurite(IV) tellurate(VI), Hg2Te2O7, is dimorphous and crystallizes in two modifications with slightly different density, named α-(high density) and β-(low density). Colourless single crystals of both modifications were prepared simultaneously by chemical transport reactions, starting from stoichiometric mixtures of HgO, TeO2 and TeO3 in sealed and evacuated silica glass ampoules. A temperature gradient of 600 → 550°C was applied and small amounts of HgCl2 served as transport agent. The crystal structures (α-Hg2Te2O7: C2/c, Z = 8, a = 12.910(4), b = 7.407(2), c = 13.256(4) Å, β = 112.044(5)°, 1711 structure factors, 73 parameters, R[F
2 > 2σ(F
2)] = 0.0325, wR(F
2 all) = 0.0796; β-Hg2Te2O7: Aea2, Z = 16, a = 7.4405(12), b = 23.713(4), c = 13.522(2) Å, 3528 structure factors, 132 parameters, R[F
2 > 2σ(F
2)] = 0.0430, wR(F
2 all) = 0.0738) are not directly correlated by a supergroup-subgroup relation but show similar building units which are also found in the structure of the mixed-valent Te(IV,VI) compound BaCuTe2O7. Characteristic structural feature of both polymorphs is a slightly distorted hexagonal array made up of isolated 1
∞[TeVIO4/1O2/2] chains running parallel to [010] for the α- and to [100] for the β-modification. The chains are interconnected via [TeIVO4] polyhedra to form infinite Te2O7
4- layers with a different topology for both structures. In α-Hg2Te2O7 the layers are linearly ordered and spread parallel to (101̅) whereas in β-Hg2Te2O7 the zig-zag arranged layers extend parallel to (001). The lone-pairs of the TeIV atoms are stereochemically active and protrude into the cavities of the structure. For both structure types the mercury atoms are situated in-between the anionic sheets and interconnect the layers into the three-dimensional network. The coordination numbers of the mercury atoms range from 6–8 in both modifications with the typical Hg–O distances between 2.03–3.0 Å.
(Na-15-Krone-5)2ReCl6·4 CH2Cl2, eine Struktur mit CH2Cl2-Molekülen in pseudohexagonalen Kanälen
Authors: Müller Ulrich , Noll Andrea 
Published: December 2003

Abstract
The monoclinic crystal structure of (Na-15-crown-5)2ReCl6·4 CH2Cl2 was determined by X-ray diffraction (P21/c, Z = 4, R
1 = 0.061). It contains octahedral ReCl6
2- ions that are joined on two opposite octahedron faces to two Na+ ions; each Na+ is coordinated with one crown ether molecule. One of the crown ether molecules is disordered in two orientations. The resulting ‘Kro-Re-Kro’ units have a nearly cylindrical shape and are stacked to rods that run parallel to c. The rods form a pseudohexagonal rod packing in which one third of the rods is missing. The channels in the places of the missing rods are filled with solvent molecules showing high thermal motion. Another description is that of a hexagonal-closest packing of Kro-Re-Kro units, the channels resulting from the strings of face-sharing octahedral voids of the packing. Group–subgroup relations show that the deviation of the Kro-Re-Kro packing from hexagonal symmetry is astonishingly small. The crystal structure of ‘(P(C6H5)4)2ReCl6·2 CH3CN’ is confirmed; however, the chemical formula must be corrected, it is (P(C6H5)4)2ReCl6·4 CH3CN).
X-ray study of the pseudopolymorphism of the azithromycin monohydrate
Authors: Montejo-Bernardo J. M. , García-Grande Santiago , Bayod-Jasanada M. S. , Lavona-Díaz L. , Llorente I. 
Published: December 2003

Abstract
A combination of X-ray powder diffraction and single crystal studies on azithomycin pseudopolymorphs give the precise solid state composition of all monohydrate pseudopolymorphs reported. According to the X-ray results the four monohydrates of azithromycin studied have the same crystallographic parameters. Furthermore, the analysis of the relative intensities from the powder patterns points to very similar chemical compositions and crystal structures. This result has been confirmed by the single crystal studies. The single crystal studies show that the solid state conformation of the azithromycin molecules is affected by the presence of solvents. The solvent methanol molecules were found disordered probably due to the existence of many positions where the hydrogen bonding is favoured and the large size of the available space to host the solvents.
Hydrogen in novel solid-state metal hydrides
Authors: Yvon K. 
Published: December 2003

Abstract
Solid-state metal hydrides display hydrogen densities close to that of liquid hydrogen and thus provide a safe and efficient way of storing hydrogen. As a result of recent neutron and synchrotron diffraction work some novel metal hydrides have been characterized that shed new light on the nature of metal-hydrogen interactions. While hydrogen appears as an anion surrounded by a large inventory of cation configurations in ionic hydrides such as Ca4Mg3H14, Ca19Mg8H54, Eu2MgD6, Eu6Mg7D26 and Eu2Mg3D10, it acts as a terminal ligand in covalently bonded hydride complexes based on p-elements such as [BH4]– and d-elements such as [IrH5]4– and [IrH4]5– in the complex hydrides LiBH4 and Mg6Ir2H11, respectively. Surprisingly, hydride complexes and hydride anions can also be discerned in typically metallic (interstitial) hydrides such as NdMgNi4H4 (= Nd3+Mg+2·[Ni4H4]5–) and LaMg2NiD7 (= La3+Mg+2
2·[NiH4]4–·3H–). Some hydrides also reveal other interesting features such as a hydrogenation induced Ce4+ → Ce3+ valence change in CeMn1.8Al0.2H4.4 at room temperature that is accompanied by a Mn/Al metal atom exchange over distances of ~2.6 Å, and a hydrogen induced metal-to-nonmetal transition near ambient conditions that leads from the metallic compound Mg3Ir to the red colored hydride Mg6Ir2H11. In this article recent work and some methodological aspects are highlighted.
Recrystallisation of HDA ice under pressure by in-situ neutron diffraction to 3.9 GPa
Authors: Klotz S. , Hamel G. , Loveday J. S. , Nelmes R. J. , Guthrie M. 
Published: December 2003

Abstract
We have studied by in-situ neutron diffraction the recrystallisation behaviour of HDA ice in the pressure range 0.3–3.9 GPa, i.e. the entire stability range of HDA. We report quantitative and detailed structural information on the various high pressure ice phases formed metastably at low temperatures. We find that between ~0.4–0.7 GPa, HDA transforms at 175 K to mainly phases IV and V, and XII, and at 1–1.2 GPa to a mixture of ice VI and XII. On isothermal compression at 100 K, HDA recrystallises to an ice VII-like structure which is either partially ordered or mixed with ice VIII. Full structural data obtained by Rietveld refinements are reported for all these phases at low temperatures. Phases IV, V and XII are fully hydrogen disordered when obtained from HDA. The transformation behaviour in the 0.5–1.2 GPa range is in good agreement with the picture reported from quenched-recovered samples, although some differences persist in the recrystallisation to IV/XII mixtures. The recrystallisation behaviour of HDA over the entire pressure range appears to follow closely that of liquid water under pressure.
The determination of anharmonic probability densities from static and dynamic disorder by neutron powder diffraction
Authors: Boysen H. 
Published: December 2003

Abstract
Atomic probability densities as derived from a careful analysis of the Debye-Waller factors contain important information about the average static and dynamic disorder in crystals. Although the inclusion of anharmonic terms often requires single crystal data, in favourable cases, in particular at high temperatures, it is possible or even necessary to rely on neutron powder experiments too. This is illustrated by several examples including the anharmonic motion of the atoms in ZnS, the anionic diffusion and the static disorder in zirconia based super-ionic conductors, and the critical behaviour at the structural phase transition in PbTiO3. Anharmonic and split models are compared on the example of Zr—Mg—O—N.
Structural analysis of complex materials using the atomic pair distribution function — a practical guide
Authors: Proffen Th. , Billinge S. J. L. , Egami T. , Louca D. 
Published: December 2003

Abstract
Modern materials and their properties are often characterized by varying degrees of disorder. Routine crystallographic structure solution only reveals the average structure. The study of Bragg and diffuse scattering yields the local atomic arrangements holding the key to understanding increasingly complex materials. In this paper we review the pair distribution function technique used to unravel the local structure. We aim to give a practical overview and make this method easily accessible to the wider scientific community.
Stroboscopic neutron diffraction from spatially modulated systems
Authors: Eckold G. , Gibhardt H. , Caspary D. , Elter P. , Elisbihani K. 
Published: December 2003

AbstractThe combination of stroboscopic techniques and neutron diffraction can be used to study the kinetics of structural changes in condensed matter on a microscopic level. Transient states may be identified and characterized on time-scales down to the microsecond regime. Hence, valuable information about the underlying mechanisms can be obtained from time-resolved experiments. Particularly interesting subjects for this type of investigation are spatially modulated systems which undergo phase transitions or phase separation. The potential of stroboscopic neutron diffraction is demonstrated using three different examples in which samples are periodically perturbed by the variation of temperature, mechanical stress or electric field and their structural response is characterized by time-resolved diffraction. Spinodal decomposition in ionic crystals of the silver-alkali halide type is shown to be dominated by two different processes on different time-scales. The stress-induced phase transition into the incommensurate phase of quartz involves relaxation processes which are reflected by different kinetic behaviours of Bragg peaks and satellite reflections, respectively. Finally, metastable transient states are observed during the field induced lock-in transition in ferroelectric Rb2ZnCl4which are most probably due to strain fields and pinning effects.
Single-crystal neutron diffractometry — Recent developments and trends
Authors: Graf H. A. 
Published: December 2003

Abstract
An overview is given over recent developments and trends in single-crystal neutron diffractometry. Special emphasis is put on instrumental developments which have considerably enhanced the efficiency of neutron scattering experiments over the last years by improving monochromator and/or detector systems. Not discussed are developments in the field of polarised neutrons. Examples from the Berlin Neutron Scattering Center (BENSC) are given for the measurement of short-range magnetic order using the flat-cone diffractometer E2 of BENSC and of the study of long-range magnetic order making use of a high-field cryomagnet at BENSC. The perspectives of neutron diffractometry within the next 5 to 10 years are discussed with view on the new powerful spallation sources SNS (under construction in the United States), JSNS (under construction in Japan) and ESS, the European Spallation Source, planned to be constructed in Europe.
A review of sample environments in neutron scattering
Authors: Bailey I. F. 
Published: December 2003

Abstract
Sample environment equipment, cryostats, furnaces, pressure cells etc. are an essential adjunct to most neutron scattering experiments to induce the sample being studied into a phase or state of particular interest. He we give a brief overview of the diverse range of Sample Environment equipment that is currently in use, a background to its development and also some recent trends.
High resolution neutron protein crystallography. Hydrogen and hydration in proteins
Authors: Niimura N. , Chatake T. , Ostermann A. , Kurihara K. , Tanaka I. 
Published: December 2003

Abstract
Neutron diffraction provides an experimental method of directly locating hydrogen atoms in proteins, and the development of the neutron imaging plate (NIP) became a breakthrough event in neutron protein crystallography. The general features of the NIP are reviewed. A high resolution neutron diffractometer dedicated to biological macromolecules (BIX-3) with the NIP has been constructed at Japan Atomic Energy Research Institute and this has enabled 1.5 Å resolution structural analyses of several proteins to be carried out. The specifications of BIX-3 and LADI (a quasi-Laue type diffractometer installed in the Institut Laue-Langevin) are compared. The crystal structures of myoglobin, wild type rubredoxin and a mutant of rubredoxin have been carried out using BIX-3. From these studies, several topics, such as the location of hydrogen bonds and certain acidic hydrogen atoms, the identification of methyl hydrogen atoms, details of H/D exchange and dynamical behavior of hydration structures have been investigated, and important information has been extracted from the structural results. Finally, a systematic procedure to grow large single crystals of proteins or nucleic acids is described.
Chemical, thermal, and theoretical analysis of the α—β phase transition of Ba3Tt4 compounds
Authors: Zürcher F. , Leoni S. , Nesper R. 
Published: December 2003

Abstract
The α to β phase transition of Ba3Tt4 (Tt = Si, Ge; Ba3Ge4 and Ba3Si4 structure type, respectively) has been investigated experimentally by differential thermal analysis (transition temperature 628 K) and variation of the composition Ba3Si
x
Ge4–

x
which shows that the transition is clearly dependent on the compositional parameter (0.4 ≤ x ≤ 0.5). The five new Zintl compounds Ba3Si1.89Ge2.11, Ba3Si1.05Ge2.95, Ba3Si0.57Ge3.43 (Ba3Si4 structure type) and Ba3Si0.31Ge3.69, Ba3Si0.16Ge3.84 (Ba3Ge4 structure type) were synthesized and structurally characterized. We have also developed a topological model for the description of the phase transition which is based on the Periodic Nodal Surface (PNS) approach.
Geminography — The science of twinning applied to the early-stage derivation of non-merohedric twin laws
Authors: Nespolo Massimo , Ferraris G. 
Published: December 2003

Abstract
The recognition of twinning and the determination of the twin laws are nowadays often entrusted to "black-box" software packages. This approach may be both risky and uneconomical. It is risky because never a procedure should be blindly applied. It is uneconomical if procrastinates the analysis of twinning at the very late stage of structure solution and refinement, whereas in many cases this analysis can and should be applied much earlier. In this paper we present a brief survey of the working strategies to be applied to the structural study of twinned crystals, emphasizing the role of the crystallographer, rather than that of the machine.
Pentagonal and trigonal quasilattices and their approximants
Authors: Subramaniam Anandh , Lord E. A. 
Published: December 2003

Abstract
The Strip Projection Method is used to generate pentagonal and trigonal 2-dimensional quasilattices as rational approximants to the icosahedral quasilattice. Further, 3-dimensionally periodic structures are generated as rational approximants to these 2-dimensional quasilattices. A simplified approach based on uniform distortions of perpendicular space is put forth for the analyses.
Thermal expansion of lattice parameters of La3Nb0.5Ga5.5O14, La3Ta0.5Ga5.5O14 and La3SbZn3Ge2O14
Authors: Pavlovska A. , Schneider J. , Werner S. , Maximov B. , Mill B. , Baetz C. 
Published: December 2003

Abstract
The thermal expansion of trigonal La3Nb0.5Ga5.5O14 (LNG), La3Ta0.5Ga5.5O14 (LTG) and monoclinic La3SbZn3Ge2O14 (LSZG) was studied with X-ray powder diffraction technique. The trigonal crystal structures of LNG and LTG are stable under elevated temperature and no phase transition was detected up to 1123 K, whereas the monoclinic LSZG undergoes a phase transition at temperature 523(50) K, presumably to trigonal space group P321. The thermal expansion of LNG, LTG and LSZG is anisotropic. This can be explained by the layer structure of title compounds. The mean coefficients in the monoclinic a, b and c directions for LSZG from room temperature to 523(50) K are 8.20, 8.47 and 6.22 [10-6/K], respectively. The mean coefficients in the trigonal a and c directions are 8.141 and 6.162 [10-6/K] for LNG; 7.653 and 5.344 [10-6/K] for LTG from room temperature to 1123 K.
A neutron diffraction study of quartz-type FePO4: high-temperature behavior and α–β phase transition
Authors: Haines J. , Cambon O. , Hull S. 
Published: December 2003

Abstract
The structural evolution of quartz-type iron phosphate, FePO4, was studied by time-of-flight neutron powder diffraction from 294 K up to 1073 K. The refined structural parameters of the low-temperature α phase tend towards the values obtained for high-temperature β-quartz-type FePO4. In particular, the Fe—O—P bridging angles increase and the tetrahedral tilt angles decrease strongly as the α–β phase transition at 980 K is approached. Important discontinuities in these structural parameters are observed at this first-order transition. Thermal expansion in the α phase is strongly non-linear and dominated by these angular variations. There is essentially no thermal expansion in the β phase due to the absence of these mechanisms. The behavior in the α phase is distinct from those of other α-quartz homeotypes in that the angular variations are greater than for the other materials, including SiO2 and AlPO4, and do not scale with the initial structural distortion. This may be an indication of a dynamic instability in α-quartz-type FePO4.
The pyroxene-spinel polysomatic system
Authors: Zvyagin Boris B. , Merlino St. 
Published: December 2003

Abstract
Modules 〈P〉 and 〈S〉 which taken separetely constitute the structures of pyroxene and spinel respectively, may give rise, in combination, to a wide polysomatic system ranging between these two minerals as end members. The system is firstly discussed on the basis of complex layers 〈PmSnPm〉, built up by somatic parts of both spinel and pyroxene, from the 〈(P/2)S(P/2)〉 modules, which are the OD layers giving rise to the sapphirine family, to the general 〈Pm/2SnPm/2〉, which includes 〈(P/2)Sn(P/2)〉 (characterizing the structure of calcium aluminium ferrite and SFCA-I compound) and 〈PSP〉 (characterizing the surinamite-type structures). Afterwards the system is discussed on the basis of 〈Pm〉 and 〈Sn〉 layers, with derivation of a new polytypic family closely related to that of sapphirine. Finally the possible stacking sequences of single modules 〈S〉, giving rise to the spinelloid series and the stacking and condensation of single 〈P〉 modules, giving rise to the whole biopyriboles series, are presented and discussed.