Crystallographic Shear Structures Research Articles

High-resolution electron microscopy of microsyntactic intergrowth in V nO 2 n−1

Structural features of the Magnéli phases V n O 2 n−1 were studied by means of high-resolution transmission electron microscopy. Microsyntactic intergrowth on a unit cell scale of several phases with neighboring n were frequently observed in arc-melted specimens, and this was ascribed to the structural compensation of the local composition fluctuation. Also observed were periodic microsyntaxy, where two neighboring phases intergrow periodically, and the crystallographic shear ( CS) structure with laterally displaced CS planes. The origin of this microsyntactic intergrowth is discussed from the standpoint of the CS plane interaction.

A new monoclinic phase in the Fe 2O 3TiO 2 system. II. The defect structure

Defects in monoclinic Fe 2TiO 5 were studied by electron microscopy and diffraction. The monoclinic structure found, which is isostructural with V 3O 5, can be derived in a simple way from a hexagonal close-packed structure and can be considered as a member of the family of crystallographic shear structures, derived from rutile. Two different slips corresponding to two hexagonal basal ones, were found to cause stacking faults, antiphase boundaries, and two different crystallographic shear planes.

Microstructural study for a homologous series of Cr 2Ti n−2O 2n−1 with (12̄1) r crystallographic shear structure

Electron microscopy and X-ray diffraction studies revealed that the homologous series of Cr 2Ti n−2O 2n−1 with n = 6, 7, and 8 were the members of (12̄1)r crystallographic shear (CS) family derived from rutile-type structure. The highest end member of (12̄1)r CS family below 1420°C appeared to be n = 8 instead of n = 9 which was previously regarded as a discrete phase by X-ray diffraction technique. All the members belonged to the triclinic system with almost constant cell parameters except for the c-axis which increased according to the increasing values of n. Their microstructures showed well ordered features with equally spaced shear planes parallel to (12̄1)r. The adjacent members could be detected in different crystal fragments and sometimes observed to be coherently intergrown within one fragment with compositions of intermediate values of n.

Diffusion de l'oxyge`ne dans les oxydes de transition. II. Autodiffusion de l'oxyge`ne dans Nb 28O 70− x, sous oxyde le plus proche de la stoechiome´trie de Nb 2O 5

To take advantage of a new approach to nonstoichiometry in the oxide Nb2O5, for which there are several phases in the region of nonstoichiometry showing crystallographic shear structures, a study of oxygen self-diffusion by isotopic solid-gas exchange reaction has been undertaken. The self-diffusion coefficient of oxygen in Nb2O5 has been measured in the range 900–1200°C for partial pressures of oxygen varying from 0.3 to 3.3 × 10−2 atmosphere, a region corresponding to existence of the single-phase su☐ide Nb28O70, the closest to Nb2O5 stoichiometry. Results obtained at 1046, 1100, and 1200°C show thatD follows a Po−1/42 dependence in the composition range close to stoichiometry and can be interpreted as arising from singly ionized oxygen vacancies. Pour tenir compte de l'approche nouvelle de la non-stoechiome´trie de l'oxyde Nb2O5 pour lequel il existe plusieurs phases dans le domaine de non-stoechiome´trie pre´sentant des structuresa`plan de cisaillement cristallographique, unee´tude de l'autodiffusion de l'oxyge`ne par re´action d'e´change isotopique solide-gaz ae´te´entreprise. Le coefficient d'autodiffusion de l'oxyge`ne dans Nb2O5 ae´te´mesure´dans le domaine de tempe´rature 900–1200°C et pour des pressions partielles d'oxyge`ne variant de 0.3a`3.3 × 10−2 atmosphe`re, domaine correspondant au domaine monophase´d'existence du sous oxyde Nb28O70 le plus proche de la stoechiome´trie de Nb2O5. Les re´sultats obtenusa`1046, 1100 et 1200°C montrent queD suit une loi en (Po2)−1/4 dans le domaine de composition proche de la stoechiome´trie, re´sultats qui peuventeˆtre interpre´te´s comme re´sultant de l'existence de lacunes ponctuelles d'oxyge`ne ionise´es une fois.

Structural relations in the Nb[sbnd]W[sbnd]O and Ta[sbnd]W[sbnd]O systems for the phase region near WO3

Phase relationships in the two systems NbWO and TaWO have been determined for metal to oxygen ratios between MO3.0 and MO2.72 at a temperature of 1373°K, using a combination of X-ray diffraction, electron microscopy, and optical microscopy. The two systems are found to be very similar. An extensive series of crystallographic shear (CS) phases existing over a wide phase field did not form and all CS structures found had compositions close to the binary WO line. These CS phases were based upon {102} and {103} CS planes and no evidence for CS upon any other planes was found. Within the phase region examined the majority of the niobium or tantalum combines to form ternary tungsten oxides of the tetragonal tungsten bronze type which are often disordered and which coexist either with the CS phases or with W18O49. In addition, the high-temperature forms of WO3 were often found to be stabilized to room temperature in slightly reduced samples. The results are summarized in phase diagrams.

High temperature rutile-derived crystallographic shear structures. I. (020) r CS structures : L. A. Bursill, Physics Department, University of Melbourne. I. E. Grey, CSIRO Division of Mineral Chemistry, P.O. Box 124, Port Melbourne. and D. J. Lloyd, Chemistry Department, University of Melbourne, Parkville, Victoria 3052, Australia

High temperature rutile-derived crystallographic shear structures. I. (020) r CS structures : L. A. Bursill, Physics Department, University of Melbourne. I. E. Grey, CSIRO Division of Mineral Chemistry, P.O. Box 124, Port Melbourne. and D. J. Lloyd, Chemistry Department, University of Melbourne, Parkville, Victoria 3052, Australia

High-resolution electron microscopy of crystallographic shear structures in tungsten oxides

Crystallographic shear (CS) structure in reduced crystals of W0 3 has been imaged at a resolution of 3–4A˚by a high-resolution electron microscope.A˚large distortion of the W0 6 octahedra sharing their edges at the CS planes has been directly recognized in electron micrographs. Crystallographic shear occurs preferentially in a particular crystallographic orientation. The preference may be explained by a different degree of distortion along the principal axes in the W0 6 octahedra of the pseudocubic structure of W03 crystal. A model for growth and ordering of the CS planes is discussed.

High resolution electron microscopy of crystallographic shear structures in tungsten oxides: S. Iijima. Department of Physics, Arizona State University, Tempe, Arizona 85281

High resolution electron microscopy of crystallographic shear structures in tungsten oxides: S. Iijima. Department of Physics, Arizona State University, Tempe, Arizona 85281

Anion coordination geometry as a determining factor in crystallographic shear

ATTENTION has been drawn to some of the consequences of anion polarisation in one crystallographic shear (CS) system, the reduced titanium oxides1; particularly to the effect on crystal energy and cation coordination geometry. The existence of polarisation in typical CS structures is indicated by the values of certain common properties—the dielectric constant and refractive index are both high—by the more direct, experimental determination of net ion charges in some double CS (column, or so-called ‘block’) structures2, and by the characteristic persistence of the cation coordination geometry3, which is invariably octahedral.

Continuous structure variation and rotating reciprocal lattices in the titanium–chromium oxides

High-index, rutile-derived, crystallographic shear (CS) structures occupy the composition range (Ti, Cr)Ox, 1.875 ≤ x ≤ 1.93. The CS plane rotates continuously from (121), to (132), about the [1{\bar 1}1]r zone axis, producing a continuous series of intermediate CS planes with (hkl)r = p. (121)r + q. (011)r. In the reciprocal lattice a fine net of superstructure reflexions rotates continuously relative to a coarse net of rutile subcell reflexions. Analogous reciprocal-lattice behaviour should be a general crystallographic phenomena and may appear in any system where superstructures occur. It is shown how the intermediate diffraction patterns may be indexed by deriving a single transformation matrix relating the unit cells of the high-index superstructures to that of the basic subcell.

Crystallographic shear structures derived from CrO 2: An electron microscopic study

We have established by electron microscopy and electron diffraction the existence of several crystallographic shear phases in the family of chromium oxides Cr n O 2 n−1 , where n = 4 and 6. As in the systems Ti n O 2 n−1 and V n O 2 n−1 , the phases are based on crystallographic shear on the planes {121}. A further series of phases found in this study show a superlattice along g ̄ 210 and bear similarities to the M m O 2 m−2 phases found by Gibb and Anderson in the Ga 2O 3TiO 2 system.

The thermodynamics of the titanium + oxygen system: an isothermal gravimetric study of the composition range Ti3O5 to TiO2 at 1304 K

The compositions of titanium oxide samples at 1304 K have been studied by, as far as possible, equilibrating them with (H2 + H aO) gas buffer mixtures of various compositions. The high precision with which the mass, and therefore the sample’s composition, was determined reveals that complete equilibrium is rarely attained, even after several days in a constant ambient: at most compositions there is gross hysteresis. Two composite hysteresis loops exist in the regions occupied by the (121) and (132) crystallographic shear structure families. Between these loops the oxidation and reduction paths are very nearly coincident: this is the region in which a change in composition can be accommodated by varying only the orientation and not the number of crystallographic shear planes. A third hysteresis loop is believed to exist between the (132) loop and the composition of slightly reduced rutile. A plausible explanation of the behaviour is developed. All the observations appear to be consistent with those from parallel electron microscope studies of reduced ‘rutile’. Nucleation of the product phase appears to be the most important factor affecting the behaviour in the region Ti30 5 to Ti40 7, as would be expected from a consideration of their structures. The differences between the behaviour observed by us and that reported by other workers is believed to be only apparent: it results from (a) the higher precision of our measurements, and (b) the much higher density and number of data points we have acquired. Various thermodynamic data are calculated for the many higher titanium oxides on the basis of a reasonable assumption for deducing approximate equilibrium oxygen potentials from the observed values. Most of these have not previously been available. By incorporating Zador’s (1967) data for T i0 199 to T i0 1999 we deduce for ^Ti30 5+ i 0 2 = TiOa (rutile), AG® (1304 K) = — 78.3 kj mol-1 compared with — 79.4 kj mol-1 in the JANAF tables. The stabilities of the Tiw0 2w_1 phases with respect to mixtures of the (n— 1) and (w+1) neighbours are seen to be extremely small ( —104 J/mol of Ti70 13, i.e. — 5.2 J/mol of atoms), and to decrease with increasing n. 1

Relations between theDO9(ReO3) structure type and some `bronze' and `tunnel' structures

A number of the 'bronze' and 'tunnel' structures are derived from the DO9 type by a simple geometrical operation: examples given are the tetragonal tungsten bronze and related structures, and Mo5O14 and LiNb6O15F. They are seen to form families of crystallographic shear structures of a new type.

Defect control in oxidation catalysts

The crystal structure of a complex molybdenum oxide Ag1/8Pr5/8MoO4 is reported. The Ag1/8Pr5/8MoO4 sub-structure can be described on the base of the scheelite (CaWO4) structure. Transmission electron microscopy reveals that the real structure is better described in a (3+1)D formalism. According to electron diffraction study the new scheelite-type complex oxide Ag1/8Pr5/8MoO4 crystallizes in the B2/b(αβ0)00 (3+1)D superspace group with unit cell parameters a≈7.50Å, b≈5.30Å, c≈11.74Å and γ≈135° (Z=4) and modulation vector q=0.56a*+0.59b*. The structure of Ag1/8Pr5/8MoO4 is refined from X-ray powder data in the scheelite setting I2/b(αβ0) with a=b=5.3013(4)Å, c=11.7407(11)Å, q=1.14690(14)a*+0.58921(12)b* with fixed γ=90° angle (Rp=0.033, R=0.033, Rm=0.029, R1=0.047, S=1.36). Displacement modulations apply for all atoms. The occupancy modulation shows that one-fourth of the Ag/Pr atoms are absent. The structure can be considered as a crystallographic shear structure with incommensurate ordering of vacancies and displacement modulations for all atoms. The arrangement of Ag/Pr atoms and vacancies is at the origin of the incommensurate modulation in the cation-deficient Ag1/8Pr5/8MoO4 phase.

On infinitely adaptive structures

A class of crystalline substances is recognized in which changes of chemical composition are accompanied neither by forming a step-wise succession of phases nor by forming nonstoicheiometric or solid solution phases with disordered, defect structures. Instead, every composition orders into a superlattice and the diffraction patterns invariably show only a single set of superstructure lines which is, however, different at every composition. The true superlattice multiplicity is that of a large unit cell, built up by the ordered repetition of a set of sub-units, which differ in compositions but are structurally closely related and readily interconvertible with a minimum of adjustment of atomic positions. Infinitesimal changes of total composition can then be accommodated in a fully ordered structure by a change in the relative numbers of these sub-units and a consequential change in total multiplicity. The consequences of this adaptive ordering are discussed in relation to phases derived from L-Ta2O5 and the crystallographic shear structures, and it is shown that the observed complexities in the chemistry of a wider range of solid-state systems can be interpreted consistently on the same basis.

Crystallographic shear and ordered reduction of UO2

Abstract Electron microscopy was used to study the surfaces (with replicas) and the near surface regions (with transmission) of UO2 annealed under reducing conditions. Parallel striations were observed with both techniques following isothermal or pulse annealing to temperatures between 1480° and 1950°c, and simultaneously a splitting of the spots in the diffraction patterns occurred. These observations are consistent with the formation of planar faults in a crystallographic shear structure of partly reduced UO2. A reduction to UO1·98 is indicated for regions near the surface on annealing UO2 to 1500°c in H2. The surface structure is probably due to additional etching to minimize the misfit. Implications of the ordered reduction on physical measurements sensitive to surface conditions (especially of uranium self-diffusion), and on the understanding of the defect structure of UO2, are discussed.

The system TiO 2Cr 2O 3: Electron microscopy of solid solutions and crystallographic shear structures

The TiO 2-rich end of the system TiO 2CrO 1.5 has been studied by electron diffraction and electron microscopy, to examine the effects of controlled anion deficiency upon rutile-based structures. Materials equilibrated at 1330 K to 1570 K fall into three ranges. With > 15 mol % CrO 1.5, compounds of the known series Cr 2Ti n−2 O 2 n−1 , with (121) crystallographic shear planes are formed. With < 5 mol % CrO 1.5, rutile structure-defect solid solutions are found, with no shear planes. It appears that the anion deficiency is accommodated by point defects or small defect clusters. The limiting composition of this solid-solution range is strongly temperature dependent, and the rutile solid solutions are readily reduced, with the introduction of shear planes on (132), as for TiO 2 itself. Between these limiting ranges, well-ordered shear phases are formed, in which the slabs of rutile structure (the normal spacing between recurrent shear planes) remain constant but the orientation of the shear planes pivots round progressively from (121) to (132) as the anion : cation ratio is changed.

Crystallographic shear in WO 3· xNb 2O 5 ( x = 0.03–0.09)

Tungsten trioxide samples doped with 3–9 mole% of niboium pentoxide have been examined by electron diffraction and microscopy. The effect of the dopant is to introduce crystallographic shear (CS) planes, which lie parallel to {100} R, {410} R (R = ReO 3-type parent lattice), depending upon the composition. Idealized models for these CS structures are suggested, and the results are discussed in terms of the relative stability of various possible structures, and the formation and diffusion of CS surfaces.