Showing papers in "Zeitschrift Fur Kristallographie in 1997"
276 citations
180 citations
TL;DR: In this article, a computer method for generating periodic 4-connected frameworks was described, where the number of unique tetrahedral atoms and the crystallographic space group type were taken into account.
Abstract: We describe a computer method for generating periodic 4-connected frameworks. Given the number of unique tetrahedral atoms and the crystallographic space group type, the algorithm systematically explores all combinations of connected atoms and crystallographic sites, seeking the 4-connected graphs. The resulting symmetry-encoded graphs are relaxed by simulated annealing to identify the regular tetrahedral frameworks. Results are presented for one unique tetrahedral atom in each of the 230 crystallographic space group types. Over 6,400 unique 3-dimensional 4-connected uninodal graphs are found when we restrict our search to those topologies that connect to nearest-neighbour asymmetric units. In any given space group, the number of graphs can depend on the choice of asymmetric unit. About 3% of the 4-connected graphs refine to reasonable tetrahedral conformations, and many are described. There is a combinatorial explosion of graphs as the number of unique vertices is increased, a result which currently restricts this method to consideration of small numbers of unique atoms.
173 citations
TL;DR: A genetic algorithm based method for solving crystal structures directly from powder diffraction data has been developed in this article, which is based on fitting the diffraction samples generated from trial structures against the measured diffraction measurements.
Abstract: A genetic algorithm (GA) based method for solving crystal structures directly from powder diffraction data has been developed. The method is based around fitting the diffraction data generated from trial structures against the measured diffraction data and has the ability to handle flexible molecules and multiple fragments. It is computationally highly efficient and takes full advantage of the implicit parallelism of the GA. The method is illustrated with the solutions of three crystal structures of varying complexity.
149 citations
86 citations
TL;DR: In this paper, the antiferrodistortive phase transition in titanite, CaTiSiO 5, is shown to be a stepwise process and a group-theoretical analysis for the two-step subduction process is given.
Abstract: In this study it is shown that the antiferrodistortive phase transition in titanite, CaTiSiO 5 , is a stepwise process. We report the occurrence of an intermediate phase with Ca-disorder in split positions. Ordering of Ca triggers the displacement of Ti in the antiferroelectric phase at lower temperatures. A group-theoretical analysis for the two-step subduction process is given. The irreducible representation Y 2 + accounts for the intermediate phase A2/a (C2/c) with Ca cations in 8f Wyckoff positions below 825 K as well as for the subduction of the isotropy subgroup P2 1 /a below 496K. A high-symmetry phase with Ca in 4e positions exists only above ca. 825 K. In order to study the stepwise structural changes synchrotron radiation was used to collect X-ray diffraction data at 100 K, 295 K and 530 K on the 4-circle diffractometer at HASYLAB using a wavelength of 0.560(1) A. A total of 2292 reflections were recorded at 100 K, 1540 at 295 K and 1442 at 530 K and reduced to 705 (100 K), 707 (295 K) and 358 (530 K) unique observations respectively (sin θ/λ ≤ 0.611 A -1 for all data sets). Refinements with harmonic displacement parameters converged to R w = 0.038 (100 K), R w = 0.039 (295 K), and R w = 0.045 (530 K). The value for R w at 530 K was further reduced to 0.042 by assuming anharmonic temperature factors for Ca. A final significant improvement of the fit could be achieved for the 530 K data using splitted calcium atoms with occupation probability 0.5 in 8f positions (R w = 0.039). k + l = 2n + 1 Bragg-peaks disapper at 496 K. Between 496 K and 825 K anisotropic diffuse scattering of reflections k + l = odd is observed. APB's and deviations from a statistical distribution of disordered Ca can lead to such diffuse reflections with their intensity extended along b * and c * .
56 citations
TL;DR: In this article, a dipole-dipole model was used to estimate non-linear optical properties from aqueous solutions, including thermal expansion, refractive indices, dielectric constants, optical activity, Faraday effect and electro-optic effects.
Abstract: Large single crystals of optical quality were grown from aqueous solutions. The structure [Berman, Jeffrey and Rosenstein, Acta Crystallogr. B24 (1968) 442] was reinvestigated and refined in space group P2 1 2 1 2 1 to an R(F)-value of 0.0276. Thermal expansion, refractive indices, dielectric constants, optical activity, Faraday effect and electro-optic effects were experimentally determined. Simple model calculations based on structural information and empirical data are compared with the observed anisotropy of the experimental results. Non-linear optical properties were estimated from a dipole-dipole model which uses polarizability volumes of the atoms [Devarajan and Glazer, Acta Crystallogr. A42 (1986) 560] and in which electron-cloud shifting by a simulated electrical field is introduced.
48 citations
TL;DR: In this paper, the structural changes of stoichiometric LiNbO3 at temperatures up to 1470 K have been investigated by neutron powder diffraction, and it was shown that the probability density function of Li is extended along c, that of the oxygens is more or less perpendicular to the Nb-O bonds, and that of Nb is slightly extended in the a - b plane at high temperatures.
Abstract: The structural changes of stoichiometric LiNbO3 at temperatures up to 1470 K have been investigated by neutron powder diffraction. At Tc ≈ 1460 K, LiNbO3 transforms from a paraelectric high-temperature (R3c) to a ferroelectric low temperature structure (R3c). The phase transition is of coupled order-disorder and displacive type. In the high-temperature phase, Li is highly disordered over two off-centre positions, whereas Nb takes a centrosymmetric position within O6 octahedra. An analysis of anisotropic displacement parameters shows that the probability density function of Li is extended along c, that of the oxygens is more or less perpendicular to the Nb-O bonds, and that of Nb is slightly extended in the a - b plane at high temperatures. An increase of the tilt angle of the NbO6 octahedra away from an h.c.p. arrangement towards a perovskite structure was observed. The octahedra become almost regular at high temperatures. Comparison with an earlier investigation of congruent lithium niobate shows a quite similar behaviour, however, all parameters of the latter are closer to those of the perovskite structure at same temperatures. Anomalies in the behaviour of the order parameters, a more complicated disorder and a high mobility of Li at high temperatures are explained by the competition of the order-disorder and displacive character of the phase transition.
45 citations
TL;DR: In this article, several crystalline substances have been found to be transformed into the amorphous state under compressed condition at kinetically low temperature and the irreversible modes are attributed to the plastic deformation by the nucleation of high-pressure form in the parent lattice but thermal energy is not kinetically high enough to provide the large crystallite size coherent to the X-ray radiation.
Abstract: Several crystalline substances have been found to be transformed into the amorphous state under compressed condition at kinetically low temperature. Dynamical lattice-instability due to elastic deformation by shear and stress induces the reversible amorphization, some of which produces memory glass. On the other hand the irreversible modes are attributed to the plastic deformation by the nucleation of high-pressure form in the parent lattice but thermal energy is not kinetically high enough to provide the large crystallite size coherent to the X-ray radiation. They can be defined as X-ray amorphous. These reversible and irreversible transformations arise from the hindrance to sufficient atomic mobility. These pressure-induced amorphizations are the precursor phenomena of the phase transformation to high-pressure polymorphs. Successive structure changes of the pressure-induced amorphization are investigated under various pressure and temperature by X-ray diffractometry, EXAFS and Raman spectroscopy. The amorphization has been also simulated by the molecular dynamics.
45 citations
TL;DR: The structure of clinotobermorite is closely related to the one of so called 11 A tobermorite Ca 5 S1 6 O 16 (OH) 2 2 H 2 O (pseudo-orthorhombic, a = 11.17 A, b = 7.19 A, c = 22.46 A, β = 96.6 A).
Abstract: The substructure of natural clinotobermorite Ca 5 [S1 3 O 8 (OH)] 2 . 4 H 2 O - Ca 5 [Si 6 O 17 ].5H 2 O from the Wessels mine, Kalahari manganese field, South Africa, was determined from single-crystal X-ray data. The diffraction pattern (monoclinic: a = 11.19 A, b = 7.29 A, c = 22.46 A, β = 96.97°) is characterised by rather diffuse reflections of the type hkl with h, k = 2n + 1. The substructure (space group I2/m, Z = 1, a/2 = 5.593(6) A, b/2 = 3.645(4) A, c = 22.46(3) A, β = 96.97(2)°) was solved from the sharp Bragg reflections (hkl, with h, k = 2n) and refined on the basis of 1877 unique reflections to R(F) = 6.7%. The structure of clinotobermorite is closely related to the one of so called 11 A tobermorite Ca 5 S1 6 O 16 (OH) 2 2 H 2 O (pseudo-orthorhombic, a = 11.17 A, b = 7.38 A, c = 22.6 A). In both minerals layers parallel (001), formed by seven-fold co-ordinated Ca, are flanked on either side by wollastonite-like [Si 3 O 9 ] single chains (ca. 7.3 A periodicity along b) creating a sandwich-like assemblage. The short b/2 axis (3.645 A) and the short a/2 axis (5.593 A of the subcell are caused by an overlay of two half-occupied single chains of tetrahedra with one shifted by b/2 against the other. In clinotobermorite the sandwich-like layers are stacked along c yielding an interlayer distance of 4.5 A with a free aperture of 3.2 A where additional Ca ions and H 2 O molecules connect the sandwich units. Clinotobermorite with 4-5 H 2 O represents a much denser structure than 11 A tobermorite with only 2 H 2 O or 14 A tobermorite with ca. 6 H 2 O (pseudo-orthorhombic, a = 11.17 A, b = 7.38 A, c = 27.94 A). The substructure of clinotobermorite can be interpreted as either composed of 'Dreiereinfachketten' leading to the formula Ca 5 [Si 3 O 8 (OH)] 2 .4H 2 O, or of 'Dreierdoppelketten' leading to the formula Ca 5 [Si 6 O 17 ]. 5 H 2 O, or of a mixture of both variants. The surface structure of clinotobermorite was studied by atomic force microscopy (AFM) where low magnifications reveal a complex layering or twinning parallel (001). High resolutions scans parallel (100) establish enhanced contrast fringes with a periodicity of ca. 10 A interpreted as the repeat distance of the Ca polyhedral layers parallel to c.
42 citations
TL;DR: The crystal structures of IrAl 275 (Ir 2 Al 5 ) and RhAl 263 (Rh 2Al 5 ) phases have been studied with single crystal methods as mentioned in this paper, and the overall number of atoms in the unit cell is 30(1) for IrAl275 and 290(3) for RhAl263.
Abstract: The crystal structures of IrAl 275 (Ir 2 Al 5 ) and RhAl 263 (Rh2Al 5 ) phases have been studied with single crystal methods: space group P23, IrAl 275 , a = 7674(1) A, Z = 8, R F = 0053 (457 hkl, 29 parameters) and RhAl 263 , a = 76692(1) A, RF = 0042 (684 hkl, 29 parameters) The unit cell contains three transition metal sites and five aluminum sites Most aluminum sites reveal strong defects in the occupations, some of them are described as being split The overall number of atoms in the unit cell is 30(1) for IrAl 275 and 290(3) for RhAl 263 Ordering and full occupation lead either to the AuZn 3 (space group Pm3n) or to the hypothetical Ir 4 Al 13 structure (space group Pm3)
TL;DR: In this article, the properties of polyions of linked cation-centered polyhedra with face-, edge-, and corner-sharing are discussed, and a new rule of selection of fundamental chains for layers and frameworks is proposed which also considers the property of linking.
Abstract: Systematics and description of polyions of both linked cation- and anion-centered polyhedra with face-, edge-, and corner-sharing are discussed. Some additions to the system introduced by Liebau (1985) for cation-centered polyions are proposed which take into account the property of linking. The concept of linkedness of the polyion, ML, and the scheme of its determination are offered. A new rule of selection of fundamental chains for layers and frameworks is proposed which also considers the property of linking. For the description of topological properties of polyhedra in the polyion a diagram of connectedness (on the base of Schlegel diagrams) and a formula of connectedness are used. For the distinction of polyhedra in the polyion the concept of topological equivalence by Wells (1970) is used and the concept of configurational equivalence is proposed. Some examples of polyions composed of anion-centered tetrahedra and polyions composed of cation-centered tetrahedra are given.
TL;DR: The 3-regular nets with 4 and 6 vertices per unit cell are enumerated and this subset of crystallographically interesting nets is characterized by special topological properties.
Abstract: The 3-regular nets with 4 and 6 vertices per unit cell are enumerated. The large number of nets obtained (5 nets and 269 nets resp.) is in stark contrast to the relatively small number of nets which are of crystallographic interest. This subset of crystallographically interesting nets is characterized by special topological properties
TL;DR: In this paper, the similarity between the lattice parameters of the gallophosphate molecular sieve DAB-2 (orthorhombic with a = 14.170 A, b= 15.959 A, c= 17.156 A) and those of several aluminophosphates with the ZON topology indicated that this material might be GaPO 4 -ZON, so a structural investigation was undertaken.
Abstract: The similarity between the lattice parameters of the gallophosphate molecular sieve DAB-2 (orthorhombic with a = 14.170 A, b= 15.959 A, c= 17.156 A) and those of several aluminophosphates with the ZON topology indicated that this material might be GaPO 4 -ZON, so a structural investigation was undertaken. Rietveld refinement of the structures of the as synthesized and partially calcined forms of DAB-2 assuming this topology converged with R wp = 0.129 (R F = 0.047) and R wp = 0.097 (R F = 0.036), respectively. In the as synthesized form, all cages are occupied by H 2 DABCO 2+ ions, whose charges are balanced by fluoride ions coordinated to framework Ga atoms. One of these F- ions bridges across a 4-ring between two Ga atoms and the other is coordinated to just one of these Ga atoms and hydrogen bonds to the H 2 DABCO 2+ ion in the cage. Thus, of the four Ga sites, two are 4-, one is 5- and one is 6-coordinate. Mild heat treatment at 573 K for six hours results in a partial decomposition of the H 2 DABCO 2+ ion and the loss of the fluoride ion hydrogen bonding to the organic cation. The changes in the unit cell parameters of different materials with the ZON topology can be traced to the influence of the fluoride ions of the framework geometry. The removal of the non-bridging F - ion causes a to increase and b to decrease, while the elimination of the second F - ion causes c to increase and b to decrease.
TL;DR: In this article, the authors deal with X-ray single crystal experiments of modulated cobalt akermanite at temperatures between 30 K and 600 K. The primary objective of these investigations was to study the predicted lock-in phase transition into a commensurate low temperature phase.
Abstract: This paper deals with X-ray single crystal experiments of modulated cobalt akermanite at temperatures between 30 K and 600 K. The primary objective of these investigations was to study the predicted lock-in phase transition into a commensurate low temperature phase. Upon cooling the phase transition occurs at 160 K, provided that the time for thermodynamic equilibration at each temperature step is guaranteed. The low temperature phase turns out to be incommensurate with a modulation wavelength of 3.086(6). a 0 . This is very close to the predicted commensurate value of 3 . a 0 . The standard deviations of the q-values are small enough to determine this low temperature phase as significantly incommensurate. A fourier synthesis of the complementary structure (as a 3-fold superstructure) reveals a tetragonal displacement pattern of the cobalt-atoms at low temperatures. The second order phase transition to the parent structure occurs at 498(1)K. It is characterized by a critical exponent β = 0.31(1)°.
TL;DR: In this article, the crystal structure of the orthorhombic e-Al 4 Cr phase in the Al-Cr system was determined by single crystal X-ray structure analysis.
Abstract: The crystal structure of the orthorhombic e-Al 4 Cr phase in the Al-Cr system was determined by single crystal X-ray structure analysis: space group Cmcm (No. 63), a = 12.521(1) A, b = 34.705(2) A, c = 20.223(1) A, V = 8787.7(9) A 3 , Pearson symbol oC584-20.48, Z = 8; M r = 2256, composition Al 56.23 Cr 14.21 , D x = 3.41 g/cm 3 ; F(000) = 8576, λ (MOKa) = 0.71069 A, μ = 4.510 mm -1 , room temperature, R = 0.068 for 2839 reflections, goodness of fit S = 1.084. There are 55 (13 Cr+42 Al) independent atoms in the unit cell, in which 12 Cr atoms and 28 Al atoms have icosahedral coordination. Interatomic distances cover the ranges of 2.570 A ∼ 2.646 A, 2.421 A ∼ 2.978 A, and 2.480 A ∼ 2.983 A, for Cr-Cr, Cr-Al and Al-Al atomic pairs, respectively. The crystalline phase appears obviously as a layered structure perpendicular to the a-axis. A typical aggregation of the icosahedral clusters in the structure of the e-Al 4 Cr phase has been pointed out.
TL;DR: For a large class of graphs, including in particular the classical root lattices, this article presented the coordination sequences and their generating functions, summarizing and extending recent results of Conway and Sloane.
Abstract: The coordination sequence sΛ(k) of a graph Λ counts the number of its vertices which have distance k from a given vertex, where the distance between two vertices is defined as the minimal number of bonds in any path connecting them. For a large class of graphs, including in particular the classical root lattices, we present the coordination sequences and their generating functions, summarizing and extending recent results of Conway and Sloane [1].
TL;DR: In this paper, topological properties and the Laplacian function of the electron density of 1,2,4-triazole have been determined from X-ray diffraction data collected at 15 K.
Abstract: Topological properties and the Laplacian function of the electron density of 1,2,4-triazole have been determined from X-ray diffraction data collected at 15 K. 1,2,4-Triazole, C 2 H 3 N 3 , orthorhombic, space group Pbca, Z= 8, lattice parameters a = 9.717(4) A, b = 9.304(4) A and c = 6.912(4) A. The results are compared with those derived from ab initio-wavefunctions at the Hartree-Fock and MP2 level of theory. The different C-N bonds are analyzed in terms of bond topological parameters. The result reveals considerable aromaticity which seems to be enhanced by the strong hydrogen bonds.
TL;DR: In this article, a review on diffuse scattering from periodic and aperiodic crystalline solids is given to demonstrate the wide field of applications in basic and applied research, and a short summary of recent developments of experimental methods and evaluation techniques is presented.
Abstract: A (selective) review on diffuse scattering from periodic and aperiodic crystalline solids is given to demonstrate the wide field of applications in basic and applied research. After a general introduction in this field each topic is exemplified by one or two examples. Main emphasis is laid on recent work. More established work, e.g., on diffuse scattering from metals and alloys, poly-types, stacking disorder from layered structures, etc. is omitted due to the availability of excellent textbooks and reviews. Finally a short summary of recent developments of experimental methods and evaluation techniques is presented.
TL;DR: The structure of gordaite is characterized by bl11cite-like sheets of composition (Zn6(OH)1202)4- which are formed by edge-shared Zn( OH)6 octahedra as discussed by the authors.
Abstract: The new mineral gordaite (14) from the SielTa Gorda, Chile, has chemical composition NaZn4(S04)(OH)6CI. 6 H20 and crystallizes trigonal with space group P3. The lattice parameters are a = 8.3556(3) A and c = 13.025(1) A. The structure was determined using direct methods and difference Fourier maps. Structure refinement yielded a final R(F)-value of 8.19%. The structure is characterized by bl11cite-like sheets of composition (Zn6(OH)1202)4- which are formed by edge- shared Zn(OH)6 octahedra. One seventh of the octahedral sites in the sheet is vacant. Above and below these sites (Zn(OH)3CIF- tetrahedra are located sharing their basal hydroxides with six octahedra. Sulfate groups are connected to the sheets on both sides by corner shar- ing. The composition for this modified layer is (Zn6ctOOC\OH)6Zn~t(OH)6CI2(S04)2P-. To compensate for the negative charge Na+ ions which are octahedrally coordinated by water molecules are incorporated between the layers. A complex hydrogen bonding system further links subsequent layers. The structure of gordaite is closely related to that of the synthetic material Zn4(OH)6S04' In H20 with In = 3,5 descIibed by Bear et al. in 1986 and presumably the same material as the one investigated by MacEwan, Cruz Cum- plido and Cano Ruiz (1966) who state an average chemi- cal composition of ZnnCOH)15(S04)3CI3.5 H20. A com- parison to these materials as well as to Zns(OH)sCb . H20 described by Nowacki and Silverman in (12J is drawn.
TL;DR: In this paper, population dynamics and structural properties of the metastable electronic excited state MS II in sodiumnitroprusside Na 2 [Fe(CN) 5 NO] 2 D 2 O.
Abstract: We present population dynamics and structural properties of the metastable electronic excited state MS II in sodiumnitroprusside Na 2 [Fe(CN) 5 NO]. 2 D 2 O. The population of MS II was performed in a two step process: First by irradiation with light of λ = 476.5 nm in order to excite MS I by about 50% and secondly by irradiation with light of λ = 1064 nm to transfer 30% of the anions from MS I to MS II . The population is determined by Mossbauer spectroscopy in dependence on light exposure. Structure analysis of a single crystal with 32% of the anions in MS II, 8% in MS I and 60% in the ground state has been done using neutron single crystal diffraction.
TL;DR: In this paper, the crystal structure of a polyphosphide containing the new polyanionic phosphorus polymer ∞ 1 [P 20 2- ] was reported, which crystallizes in the triclinic system, space group P1 (No. 2).
Abstract: We report about the crystal structure of Cu 12 P 20 Br 10 , a polyphosphide containing the new polyanionic phosphorus polymer ∞ 1 [P 20 2- ]. It crystallizes in the triclinic system, space group P1 (No. 2) with a= 9.886(2) A, b = 13.190(2) A, c = 14.763 (3) A, a = 70.83 (1)°, β = 82. 68 (1)°, γ = 80.13 (1)°, V= 1786.1(6) A 3 and Z= 2. The phosphorus atoms build up one dimensional negatively charged tubes consisting of condensed 5- and 6-membered rings. They are connected via copper atoms to form double tubes oriented parallel [010] and are embedded between wavy layers of copper and bromine atoms. All copper atoms except those that are linked to negatively charged P-atoms are strongly disordered.
TL;DR: In this article, the tensors of bis(guanidinium)zirconium bis(nitrilotriacetate)hydrate have been determined by ultrasonic methods.
Abstract: Large single crystals of optical quality of bis(guanidinium)zirconium bis(nitrilotriacetate)hydrate have been grown from aqueous solutions. Complete elastic, thermoelastic and piezoelastic tensors have been determined by ultrasonic methods. In addition, the tensor of thermal expansion, refractive indices, dielectric and piezoelectric constants have been studied in the range between 210 K and 330 K. Most physical properties investigated show a strong anisotropy. The piezoelectric constant d 123 exceeds d 111 of α-quartz by a factor of 25. Two higher-order phase transitions appear at about 287 K and 290 K. The intermediate phase exhibits strong anomalies of the temperature and pressure dependence of the propagation velocities of quasi-longitudinal elastic waves having a large component of the displacement vector parallel to [010].
TL;DR: The crystal and molecular structure of bis(dimethyldithiocarbamato)mercury(II), [Hg(S 2 CNMe 2 ) 2 ], C 6 H 12 HgN 2 S 4 has been determined at room temperature as mentioned in this paper.
Abstract: The crystal and molecular structure of bis(dimethyldithiocarbamato)mercury(II), [Hg(S 2 CNMe 2 ) 2 ], C 6 H 12 HgN 2 S 4 has been determined at room temperature. The pale-green crystals are monoclinic, space group C2/c with unit cell dimensions a = 17.308(1) A, b = 7.558(3) A, c = 9.978(2) A, β = 113.24(1)°, Z = 4 and D x = 2.442 Mg m -3 . The structure was solved by Patterson methods and refined by a full-matrix least-squares procedure to final R = 0.041 using 729 reflections. The mercury atom in the centrosymmetric molecule exists in a diamond shaped geometry defined by two short Hg-S separations of 2.374(3) A and two weaker interactions of 2.988(3) A. The structure is compared to other [Hg(S 2 CNR 2 ) 2 ] species and it is shown that a wide variety of structural motifs may be adopted by these compounds.
TL;DR: In this paper, a model structure for the crystallisation of intermetallic phases is presented, which shows a remarkable self-similarity and can be regarded as a model of the crystal structure of the lattice structure of a supericosahedron.
Abstract: Single phase samples of the new intermetallic compound Mo 7 Sn 12 Zn 40 are synthesized from the elements at 1023 K. Mo 7 Sn 12 Zn 40 crystallizes cubic (Fm3c) with a = 25.447 A. The complex crystal structure is characterized by two different polyhedra of high symmetry, which are built up by two shells. Besides chiral polyhedra MoZn 14 Sn 24 there are nearly undistorted Mackay polyhedra Mo 13 Zn 42 of symmetry I n , which are connected to a 3D net via Zn 6 octahedra. Thereby twelve icosahedra MoZn 10 Sn 2 , which form the homogeneous cubic bauverband cP2[12i] condense to a supericosahedron. The crystal structure can be regarded as a model structure for the crystallisation of intermetallic phases and shows a remarkable self similarity.
TL;DR: The first € price and the £ and $ price are net prices, subject to local VAT as mentioned in this paper, and prices and other details are subject to change without notice. All errors and omissions excepted.
Abstract: The first € price and the £ and $ price are net prices, subject to local VAT. Prices indicated with * include VAT for books; the €(D) includes 7% for Germany, the €(A) includes 10% for Austria. Prices indicated with ** include VAT for electronic products; 19% for Germany, 20% for Austria. All prices exclusive of carriage charges. Prices and other details are subject to change without notice. All errors and omissions excepted. B. Bhushan (Ed.) Micro/Nanotribology and Its Applications
TL;DR: In this paper, the crystal structure of the high-symmetry γ-phase stable above 899 K has been investigated by high temperature neutron diffraction of samples prepared by solid state reactions.
Abstract: The compound Bi 2 Sn 2 O 7 shows three polymorphic forms between room temperature and 1400 K As a first step towards the structural characterization of this material in the present study the crystal structure of the high-symmetry γ-phase stable above 899 K has been investigated by high temperature neutron diffraction of samples prepared by solid state reactions At 996 K this modification possesses the following parameters: cubic, space group Fd3m, a = 107225(7)A, V = 12328(2)A, Z = 8, D calc = 827 g cm -3 A Rietveld analysis was performed using the structural model of pyrochlore for this phase, where the Bi 3+ and Sn 4+ cations occupy the Wyckoff-positions 16(c) and 16(d), respectively The oxygen atoms O1 and O2 are distributed to the positions 48(f) and 8(a) The refinement calculations confirm this approach The octahedral SnO1 6 -network shows the lowest degree of distortion within the group of A 2 Sn 2 O 7 pyrochlores It is suggested that the comparatively high isotropic displacement factor of the Bi 3+ cation indicates that the phase transition in this compound occurring at 899 K is primarily due to displacements of the bismuth atoms
TL;DR: In this article, the kinetics of the α-quartz to coesite transition have been studied in the pressure range of 3.2 GPa to 5.2GPa and in the temperature range 950 K-1175 K by in situ X-ray diffraction using a MAX80 cubic anvil highpressure apparatus at the Hamburger Synchrotronstrahlungslabor (HASYLAB).
Abstract: The kinetics of the α-quartz to coesite transition have been studied in the pressure range of 3.2 GPa to 5.2GPa and in the temperature range 950 K-1175 K by in situ X-ray diffraction using a MAX80 cubic anvil highpressure apparatus at the Hamburger Synchrotronstrahlungslabor (HASYLAB). During the phase transition, X-ray diffraction patterns were collected at intervals of 60 s. The transformed volume fraction has been calculated from the diffracted intensities of the respective phases as a function of time. By fitting a fundamental rate equation for grain boundary nucleation and interface-controlled growth to the transformation-time data, rates of nucleation and growth have been calculated. A discrepancy between the experimental determined and theoretical calculated phase boundary is discussed. After quenching the samples, the reaction products could be investigated by TEM images. A specific feature of the synthesised coesite crystals was a concentrated microtwinning. Its influence on the crystal structure and thus on the thermodynamic behaviour of the α-quartz-coesite phase transition is illustrated.