bluuebunny/crossref_metadata_2025_split huggingface dataset:
Phonon dispersion curves by inelastic neutron scattering to 12 Gpa
Authors: Klotz S. 
Published: December 2003

Abstract
Recent progress in high pressure techniques allows the measurements of phonon dispersion curves to ~12 GPa by inelastic neutron scattering on triple axis spec-trometers. Provided the structure is not too complex, a vari-ety of low-compressibility solids may be studied over the entire stability range of their ambient pressure forms. This article reviews results obtained during the last five years on the lattice dynamics of a number of “simple” systems (Ge, GaSb, PbTe, FeO, Zn, Fe) where the pressure-induced frequency shifts of the acoustic branches have been studied in considerable detail. In several of these solids pronounced “mode softening” is found under pressure. Grüneisen parameters and elastic constants have been determined and the results were compared to predictions of first-principle calculations.
Crystal structure of kristiansenite: a case of class IIB twinning by metric merohedry
Authors: Ferraris G. , Gula A. , Ivaldi G. , Nespolo Massimo , Raade G. 
Published: December 2003

Abstract
The structure of the new disilicate kristiansenite, Ca2ScSn(Si2O7)(Si2O6OH), has been solved and refined from a crystal polysynthetically twinned by metric merohedry. The Bravais lattice is mC, with parameters a = 10.028(1), b = 8.408(1), c = 13.339(2) Å, α = 90.01(1), β = 109.10(1), γ = 90.00(1)°, but the space-group type is C1 (Z = 4). The twin law is m´, and the two components of the twin have nearly identical volumes: as a consequence, the Laue group of the twin is practically 2/m. By taking into account the twinning, an anisotropic refinement of the structure in C1 converged to R1 = 0.0242 for 259 refined parameters and 4862 observed reflections. The effects of the twinning by metric merohedry and of the volume ratio of the components on the symmetry of the diffraction pattern are discussed. The triclinic structure approximates within about 0.1 Å the monoclinic symmetry, the lower symmetry resulting mainly from cation ordering. Kristiansenite represents a new type of silicate structure and the first known case with the presence of protonated and normal disilicate groups at the same time. The disilicate groups and the other polyhedra centred on cations lie on different alternating (101) planes.
The crystal structures of phenacite-type Li2(MoO4), and scheelite-type LiY(MoO4)2 and LiNd(MoO4)2
Authors: Kolitsch U. 
Published: December 2003

Abstract
The crystal structures of flux-grown Li2(MoO4), LiY(MoO4)2 and LiNd(MoO4)2 were solved and refined from single-crystal intensity data collected with a four-circle diffractometer (MoK
αX-radiation, CCD area detector, room temperature). Li2(MoO4) is isostructural with phenacite, Be2(SiO4) and has space group R3̅, a = 14.330(2), c = 9.584(2) Å, Z = 18 (R1 = 1.6%). The structure consists of a three-dimensional network of corner-linked, slightly distorted LiO4 and fairly regular MoO4 tetrahedra. Average Li–O and Mo–O distances are 1.965 Å (Li1) and 1.967 Å (Li2), and 1.764 Å (Mo). The atomic arrangement is characterised by a narrow open channel along the three-fold axis. Comparisons are drawn with other phenacite-type compounds. Low-temperature single-crystal X-ray studies of Li2(MoO4) in the range from 103 to 293 K gave no evidence of a phase transition, thus contradicting recent literature reports. Both LiY(MoO4)2 and LiNd(MoO4)2 crystallise in the scheelite-type structure, with space group I41/a and Z = 2 (R1 = 1.7 and 2.7 %, re-spectively). The first has a = 5.148(1), c = 11.173(2) Å, V = 296.11(10) Å3, the latter a = 5.243(1), c = 11.440(2) Å, V = 314.47(10) Å3. Both Li and Y/Nd are completely disordered on a jointly occupied site. Mean (Y,Li)– and (Nd,Li)–O distances are 2.40 and 2.47 Å, respectively. In both compounds the unique MoO4 tetrahedron has four identical Mo–O bonds with lengths of 1.779 Å. The stoichiometry of LiRE(MoO4)2 (RE = rare-earth element) compounds is discussed and the relation to structure types of other MRE(XO4)2 (M = alkali metal, X = Mo, W) compounds is briefly addressed.
A synchrotron X-ray diffraction analysis of near-stoichiometric LiNbO3
Authors: Etschmann B. , Ishizawa N. , Streltsov V. , Oishi S. 
Published: December 2003

Abstract
Single-crystal diffraction data was collected at 120 and 294 K for an approximately spherical LiNbO3 crystal, 16 μm in diameter, using focused λ=0.7516 (1) Å wavelength synchrotron X-radiation and a fast avalanche photodiode (APD) detector. Extinction was significantly less than that recorded for previous single-crystal X-ray diffraction experiments. Anharmonic atomic displacements of Nb and Li were refined using a Gram-Charlier expansion to the fourth order. There was no significant change in the topography of the difference electron density (Δρ) map after anharmonic refinement. An accumulation of electrons present at the vacant site in the 294 K Δρ map was not evident in the 120 K Δρ map, indicating a migration of Li atoms to the vacant site as the temperature increased. The orientation of the electron density around Nb flipped by 90° going from 120 to 294 K.
Polymorphism and multiple twinning of (Et4N)2ZnCl4
Authors: Sondergeld P. , Fuess H. , Ishihara H. , Schmahl W. W. 
Published: December 2003

Abstract
The phase sequence of The phase sequence of (Et4N)2ZnCl4 has been determined based on thermal analysis and 35Cl NQR spectroscopy. On cooling two phase transitions are observed at 223.0(5) and 217.7(9)K, respectively. On heating only one phase transition is found at 228.4(3)K. Therefore three phases are distinguished below the decomposition point (556(5)K) with a metastable phase in the temperature region 217-229K, a twinned low-temperature phase below and a disordered high-temperature phase above this region, respectively. The structure of the low-temperature phase has been determined from X-ray 4-circle diffraction data. The space group of this phase is C2ca(Aba2) with a = 11.366(6)Å, b = 13.347(4)Å and c = 15.246(6)Å at T = 190K. From the group-subgroup relations between the high- and the low-temperature phase the structure of the intermediate phase has been derived and confirmed by the analysis of the transition entropies.
Equations of state for solids under strong compression
Authors: Holzapfel Wolfgang 
Published: December 2003

Abstract
Various approaches for the representation of equation of state data for solids under strong compression are discussed. The theoretical background for reasonable extrapolations to higher pressures and higher as well as lower temperatures is described. The distinction between ideal, regular, and anomalous behaviour allows to gain deeper insight into the electronic changes occurring in various solids under strong compression. The discussion of experimental data for various regular solids leads finally to an estimate of the accuracy obtained in the present realisation of a practical pressure scale based on equation of state measurements.
Symmetry and structural relationships among ABW-type materials
Authors: Kahlenberg V. , Fischer R. X. , Baur W. H. 
Published: December 2003

Abstract
The symmetry relationships among zeolite-type ABW compounds are described in terms of the reductions of symmetry from the highest possible topological symmetry in space group Imam to eight possible subgroups. This family of compounds, also known as Icmm-type compounds, covers a wide range of chemical compositions. The distortion modes inducing the lowering of symmetry are described based on rotation and ordering processes. Atomic site relationships with their evolution from the aristotype are given for the recently described monoclinic P1121/n structures with an ABW-type framework.
Compression mechanism of cubic silica sodalite [Si12O24]: a first principles study of theIm3̅mtoI4̅3mphase transition
Authors: Knorr K. , Winkler B. , Milman V. 
Published: December 2003

AbstractThe high pressure behaviour of cubic silica sodalite has been investigated byab initiocalculations based on density functional theory. The experimentally observed phase transition from the fully-expanded ideal sodalite framework (space groupIm3̅m) to a collapsed framework with space groupI4̅3mcould be successfully reproduced to appear at 0.12 GPa. This transition is discontinuous. The mechanism of the transition can be understood in terms of a tilt of the framework and of deformations of the SiO4-tetrahedra. Discontinuities of the enthalpy, cell volume, Si-O bond lengths and angles at 1.15 and 3.01 Gpa point at further possible pressure-induced iso-symmetric phase transitions in the space groupI4̅3m.
Polymorph stability and phase transitions of trigonal Al1-

x
Ga
x
PO4 mixed crystals
Authors: Barz R.-U. , David F. , Schneider J. , Gille P. 
Published: December 2003

Abstract
Single crystals of Al1-

x
Ga
x
PO4 of various mole fractions x were grown from phosphoric acid solutions under hydrothermal conditions using the slow-heating method. In order to relate the parameters of this low-quartz structure analogue and the observed phase transitions to the respective mole fraction, the crystals were investigated by means of electron probe microanalysis, X-ray powder diffraction and differential thermal analysis. The results are discussed in terms of the segregation effects occuring during growth. The limiting geometrical parameters for the existence of the high-quartz phase are compared to the values given in the literature.
X-ray studies of phase transitions of (Ca,Sr)-åkermanite solid solutions
Authors: Kusz J. , Böhm H. 
Published: December 2003

Abstract
The phase transitions of the solid solutions (Ca1-

x
Sr
x
)2MgSi2O7 (0 ≤ x ≤ 0.32) of modulated melilites are studied by X-ray diffraction. The variation of the intensity and the FWHM of satellite reflections reveal different transition temperatures. Two ranges of different ordering are found in the range of incommensurate modulation. The mechanism of the phase transition into the incommensurately modulated structure is discussed by including the diffuse scattering above the transition.
Crystal structures of sulfamates MeLi(NH2SO3)2 (Me: K, Rb and Cs) and physical properties of KLi(NH2SO3)2 (refractive indices, thermal expansion, elastic properties)
Authors: Meinhart A. , Haussühl E. , Bohatý L. , Tillmanns E. 
Published: December 2003

Abstract
New sulfamates KLi(NH2SO3)2, RbLi ⋅ (NH2SO3)2 and CsLi(NH2SO3)2 were prepared from non-stoichiometric (ratio Li: Me = 3:1) aqueous solutions of LiNH2SO3 and MeNH2SO3 (Me = K, Rb, Cs) at room temperature. Large single crystals of KLi(NH2SO3)2 were grown from these solutions by controlled lowering of the temperature between 293 K and 288 K. The crystal structures were investigated using single-crystal X-ray diffraction methods. KLi(NH2SO3)2: space group P21, Z = 2, a = 5.196(1) Å, b = 8.475(1) Å, c = 8.868(1) Å, β = 105.05(1)°, T
meas. = 293 K, R1 = 0.026; RbLi(NH2SO3)2: space group P1̅, Z = 4, a = 7.844(1) Å, b = 9.804(1) Å, c = 10.825(1) Å, α = 108.77(1)°, β = 93.29(1)°, γ = 98.64(1)°, T
meas. = 293 K, R1 = 0.033; CsLi(NH2SO3)2 : space group P1̅, Z = 2, a = 5.535(1) Å, b = 7.877(1) Å, c = 9.857(1) Å, α = 74.07(1)°, β = 76.33(1)°, γ = 87.07(1)°, T
meas. = 200 K; R1 = 0.023. The common feature of the three crystal structures are alternating layers of Me+ and lithium cations together with anionic [NH2SO3]– tetrahedra. Li+ coordination polyhedra and [NH2SO3]– anions, linked via common corners, form one type of these sheets. Additionally, these layers are stabilized by hydrogen bonds. Me+ coordination polyhedra are linked to each other and to the above mentioned layers by common corners and edges. RbLi(NH2SO3)2 and CsLi(NH2SO3)2 possess similar structures. Generally, the observed interatomic distances and angles agree with crystal chemical expectations. Differential scanning calorimetry measurements showed that KLi(NH2SO3)2 undergoes a first-order structural phase transition at 401 K and melts at 423 K. Between 153 K and 423 K no indications of further phase transitions were observed. In addition, the refractive indices and the tensor of thermal expansion were measured. Further, the elastic and thermoelastic constants of KLi(NH2SO3)2 were determined from ultrasonic resonant frequencies of thick plane-parallel plates and their shift upon variation of temperature, respectively. The elastic behaviour of KLi(NH2SO3)2 is similar to the average elastic properties of LiNH2SO3 and KNH2SO3.
Crystal structure of 12H+-2,4-benzo-1,5-dioxa-8,12,16-triaza-cyclooctadec-2-ene-7,17-dione picrate
Authors: Hundal G. , Kumar S. , Hundal M. S. , Singh H. 
Published: December 2003

Abstract
The title compound [C16H24N3O4]+[C6H2N3O7]- resembles 18 membered macrocycle, which has two ether O atoms, two pairs of amide N and O atoms and an amine N as potential donor atoms. The macrocycle has undergone protonation at the amine N atom. The picrate ion interacts through its phenolic O atom with NH2
+ group via N-H⋅⋅⋅O hydrogen bond. NH2
+ also shows a strong intramolecular H bonding with one of the amide oxygens of the macrocyclic ring. This results in significant conformational changes in one half of the macrocyclic ring. Apart from it, an increased flexibility of the system and a -CH2⋅⋅⋅π interaction renders the cavity non-suitable for accepting small molecules like water or alcohol in it.
ACORN — theory and practice
Authors: Jia-xing Y. , Woolfson M. M. , Wilson K. S. , Dodson E. J. 
Published: December 2003

AbstractACORN is a flexible and general procedure for theab initiosolution of protein structures when atomic resolution data are available. It has two components — ACORN-MR that locates a single atom or a fragment of the structure and ACORN-PHASE that refines the structure starting with the phase estimates from the fragment. ACORN-MR depends heavily on a correlation coefficient (CC) between the observed normalized structure factors and those from the fragment to find the fragment‘s correct orientation and/or position. ACORN-PHASE has three components, a Patterson-superposition sum function, real-space Sayre-equation refinement and, most importantly, Dynamic Density Modification (DDM). The concepts that underline DDM are explained by a theoretical approach. ACORN has been shown in trials to be able to solve structures with up to 5000 independent non-hydrogen atoms in the asymmetric unit. Several previously unknown structures have also been solved. At lower resolution, if anomalous scattering or isomorphous replacement data are available, then substructures can be determined using the ACORN approach. The quality of the final maps from ACORN, without further processing, enables automatic interpretation in terms of structure to be readily made.
Qantitative determination of phase for macromolecular crystals using multiple diffraction methods and dynamical theory
Authors: Chang S.-L. , Stetsko Y. P. , Lee Y.- R. 
Published: December 2003

Abstract
Multiple diffraction techniques for quantitative phase determination of macromolecular crystals are briefly described and overviewed. The modified second-order iterative approximation [Chang, S. L.; Tang, M. T.: Acta Cryst. A44 (1988) 1065] is employed to derive analytic expressions for the phase sensitivity of multiple diffraction and the visibility of multiple diffraction line profiles. Calculated visibility and sensitivity of three-beam cases of the protein, aldose reductase, are illustrated. The functions used for intensity profile fitting in the quantitative phase determination procedure are also modified accordingly. This calculation provides an effective way of choosing appropriate reflections for experimentally generating useful three-beam diffractions for phase analysis. Other practical issues are also addressed.
Structural variations in the aegirine solid-solution series (Na,Li)FeSi2O6 at 298 K and 80 K
Authors: Redhammer G. , Roth G. 
Published: December 2003

Abstract
The structural variations within the aegirine (NaFeSi2O6) – Li–aegirine (LiFeSi2O6) solid-solution series have been determined from structure refinements of single–crystal X-ray diffraction data of synthetic compounds at 298 K and 85 K. At 298 K, all compounds have space group C2/c. Replacing Na+ by Li+ in aegirine causes a distinct decrease of the b lattice parameter by 1.54% and an increase of the monoclinic angle β by 2.60%. The oxygen coordination of the M2 cation decreases from 6 + 2–fold in aegirine to 6–fold in pure LiFeSi2O6, the bond length decreases by 7.1% with increasing replacement of Na+ by Li+. The decrease of the M2–oxygen coordination number correlates with a straightening of the tetrahedral chains which are slightly kinked in aegirine and fully extended in Li–aegirine (O3-O3-O3 bridging angle = 174.2(1)° and 180.8(1)° respectively). Small variations of individual and mean Si-O bond lengths and distinct changes in O-Si-O bond angles were also found. Within the Fe3+O6 – M1 octahedra, structural rearrangements mainly affect the O-O interatomic distances (= edges of the octahedra). At 85 K, the compounds Na1-

x
Li
x
FeSi2O6 with Li+ concentrations x > 0.75 crystallize in the space group P21/c. For the compounds with x < 0.75, no phase transition from C2/c → P21/c was observed down to low temperatures (30 K). A x – T phase diagram for the stability of the low temperature P21/c structure is provided. The characteristica of the P21/c low temperature form of the (Na,Li)FeSi2O6 compounds agree with those of pure LiFeSi2O6, recently described by Redhammer et al. (2001).
Crystal structures of R-thiocamphor
Authors: Brunelli M. , Fitch A. N. , Mora A. J. 
Published: December 2003

Abstract
The crystal structure of the globular organic molecule R-thiocamphor (C10H16S) has been studied at ambient and low temperatures by high resolution powder X-ray diffraction using synchrotron radiation. At around 260 K thiocamphor transforms on cooling to an ordered orthorhombic structure from the orientationally-disordered body-centered cubic phase with a = 8.1477(1)Å at room temperature. The structure of the low-temperature phase was solved by auto-indexing, then by intensity extraction and direct methods using the EXPO package and refined by the Rietveld Method. The low temperature structure has a = 9.0490(1)Å, b = 14.5887(3)Å, c = 7.2931(1)Å, space group P212121, Z = 4, ρ
calc = 1.161 g cm-3.
Synthesis and X-ray structure of 1,3-dimethylurea-picric acid (1/1) complex
Authors: Artali R. , Bombieri G. , Carvalho C. C. 
Published: December 2003

Abstract
The synthesis and the X-ray crystal and molecular structure of the N,N'-Dimethylurea picrate [C3H9N2
+ C6H2N3O7
-] are reported. It crystallizes in the triclinic system, space group P1̅ with a = 6.882(7), b = 9.050(2), c = 11.149(12) , α = 97.54(13), β = 104.02(9), γ = 96.78(14), V = 659.7(2)Å3, Z = 2, D
calc = 1.60 g/cm3.The structure consists of infinite layers of anions alternating with cations linked together through an extensive hydrogen bonding involving most of the potential donors and acceptors. Semiempirical calculations for the picrate complex reproduce well the observed geometry.
The crystal structure of Na2Ca2Si2O7 - a mixed anion silicate with defect perovskite characteristics
Authors: Kahlenberg V. , Hösch A. 
Published: December 2003

Abstract
Single crystals of a new sodium calcium silicate with composition Na2Ca2Si2O7 have been synthesized. The title compound is monoclinic, space group I12/a1, a = 10.4906(5) Å, b = 10.3762(6) Å, c = 24.5349(12) Å, β = 91.217(6)°, V = 2670.1(2) Å3, Z = 16. The crystal used for the data collection showed twinning by pseudo-merohedry, a feature we took account of by separating the reflections from the different twin individuals. Structure determination was based on direct methods followed by difference Fourier calculations and least squares refinements (R(|F|) = 0.056 for 2414 independent observed reflections (I>2σ(I)) and 245 parameters). Na2Ca2Si2O7 belongs to the group of mixed anion silicates comprising insular [SiO4]-groups and [Si3O10]-trimers. Both anion groups are located in layers perpendicular to [001]. Ten symmetrically independent M-sites occupied by Na and/or Ca crosslink between the tetrahedral anion groups. Another very interesting understanding of the structure can be obtained from the comparison with the ABO3 perovskite structure type. The compound can be regarded as a superstructure of perovskite (a ≈ 2√2a
p, b ≈ 2√2a
p, c ≈ 6a
p) with 22.2% oxygen vacancies. The vacancies are ordered in such a way that the silicon atoms residing on the B-sites of the perovskite structure are coordinated to four oxygens. The relationship with perovskite can be expressed in the following crystal chemical formula: [(Na,Ca)][(Na,Ca)1/4Si3/4][O7/9∀2/9]3.
Calculation of the BO3 triangle to BO4 tetrahedron ratio in borates
Authors: Parthé E. 
Published: December 2003

Abstract
A bonding model, valid for alimited group of borates, is formulated which allows the ratio of triangularly to tetrahedrally coordinated B atoms to be calculated from the chemical formula. The anion complex, which is formed by covalently bound atoms, has non-directed ionic bonds with the surrounding cations. To complete its electron octet, each O atoms of the complex forms exactly two covalent bonds either with 2 B and/or with 1 B and 1 H atom, depending on the number of OH groups. The B atoms have no preference for either triangular or tetrahedral O coordination. However, a change of a B-O base triangle to an isoelectronic base tetrahedron decreases the number of O atoms which can be used for B-O-H bonds and increases the number of shared O atoms engaged in B-O-B bonds - and vice versa. The ratio of B-O triangles to tetrahedra is adjusted such that O atoms can form the proper number of B-O-B and/or B-O-H bonds. Using the model one can also prove that a borate with OH groups and all its dehydration products (independent whether they are crystallized or a glass) should have the same ratio of three- to four-coordinated B atoms. Under the presupposition that the bonding model is applicable, one can specify a simple mixture of base triangles and/or base tetrahedra with which it should be possible to construct an anion complex. The observed complex is either built up with this simple mixture or with a mixture of base polyhedra derived from the simple one by cross-substitution which does not change the triangle to tetrahedron ratio. The realization of the bonding model is in principle possible only within a restricted composition range in which lie one quarter of the known borates. The equation to calculate the triangle to tetrahedron ratio has been tested on all borates within this range. An agreement was found for 85 percent of the 222 investigated crystal structures.
The crystal structure of InYGe2O7 germanate
Authors: Juárez-Arellano E. A. , Bucio L. , Ruvalcaba J. L. , Moreno-Tovar R. , García-Robledo J. , Orozco E. 
Published: December 2003

Abstract
A new indium yttrium germanate presenting the thortveitite structure with symmetry described by the space group C2/m (No. 12) has been prepared by high temperature solid state reaction as polycrystalline powder material. This crystallizes in the monoclinic system, with cell parameters a = 6.8286(1) Å, b = 8.8836(2) Å, c = 4.9045(1) Å, β= 101.8340(7)º, V = 291.195(9) Å3 and Z = 2. The structure was characterized by X-ray powder diffraction and Rietveld refinement of the diffraction pattern. The In3+ and Y+3 cations occupy the same octahedral site forming a hexagonal arrangement on the ab planes. In their turn, the hexagonal arrangements of InO6/YO6 octahedral layers are held together by sheets of isolated diorthogroups constituted by a double tetrahedra sharing a common vertex. In this compound, the Ge2O7 diorthogroup shows the C2h symmetry implying a Ge-O-Ge angle of 180º, being an important feature of the thortveitite structure, which has been controversial in some reported papers. A remarkable photo-luminescence effect (in comparison with glass) was observed when the sample was irradiated with α-particles beam during the RBS experiments employed to analyze the chemical stoichiometry.