Crystallographic Shear and Diffusion Paths in Certain Higher Oxides of Niobium, Tungsten, Molybdenum and Titanium

Defect-Mediated Growth of Crystallographic Shear Plane

As representative extended planar defects, crystallographic shear (CS) planes, namely Wadsley defects, play an important role in modifying the physical and chemical properties of metal oxides. Although these special structures have been intensively investigated for high-rate anode materials and catalysts, it is still experimentally unclear how the CS planes form and propagate at the atomic scale. Here, the CS plane evolution in monoclinic WO3 is directly imaged via in situ scanning transmission electron microscope. It is found that the CS planes nucleate preferentially at the edge step defects and proceed by the cooperative migration of WO6 octahedrons along particular crystallographic orientations, passing through a series of intermediate states. The local reconstruction of atomic columns tends to form (102) CS planes featured with four edge-sharing octahedrons in preference to the (103) planes, which matches well with the theoretical calculations. Associated with the structure evolution, the sample undergoes a semiconductor-to-metal transition. In addition, the controlled growth of CS planes and V-shaped CS structures can be achieved by artificial defects for the first time. These findings enable an atomic-scale understanding of CS structure evolution dynamics.

In situ studies on defect formation dynamics in flash-sintered TiO2

Flash-sintered (FS) ceramics have shown promising mechanical deformability at room temperature compared to conventional sintered ceramics. One major contributing factor to plasticity is high-density defects, such as dislocations, stacking faults and point defects, resulted presumably from the high electrical field during flash sintering. However, such direct experiemtnal evidence for defect formation and evolution under the electric field remains lacking. Here we performed in situ biasing experiments in FS and conventionally sintered (CS) polycrystalline TiO2 in a transmission electron microscope (TEM) to compare the defect evolution dynamics. In situ TEM studies revealed the coalescence of point defects under the electrical field in both FS and CS TiO2 and the subsequent formation of stacking faults, which are often referred to as Wadsley defects. Surprisingly, under the electrical field, the average fault growth rate in the FS samples is 10 times as much as that in the CS TiO2. Furthermore, the Magnéli phase, a 3D oxygen-deficient phase formed by the aggregation of Wadsley defects, is observed in the FS samples, but not in the CS samples. The present study provides new insights into defect dynamics in FS ceramics.

Structural and electrochemical insights into novel Wadsley Roth Nb7Ti1.5Mo1.5O25 and Ta7Ti1.5Mo1.5O25 anodes for Li-ion battery application

Niobium based anodes are gaining increasing popularity for application in high-power lithium-ion batteries, due to their high theoretical capacities, inherent safety at high current densities, and long-term stability. Here, we report the discovery and characterisation of a new Wadsley Roth niobate system, Nb7Ti1.5Mo1.5O25, showing that it is isostructural with known systems: Nb9PO25 and Nb9VO25. To evaluate the material's electrochemical performance, including performance at high current densities (for potential high power applications), and long-term stability, Li half-coin cells were prepared. The material showed an initial capacity of 268(9) mA h g-1 at 0.01 A g-1 (voltage range of 2.5-1.0 V). However, in subsequent cycles, some of this initial capacity is lost, which is attributed to Li trapping associated with the presence of reducible MoO4 units, similar to the situation observed for isostructural Nb9VO25. After this initial irreversible capacity loss, the material showed good performance at high current density rates, such that at 2 A g-1 and 4 A g-1 respective capacities of 132(10) mA h g-1 and 115(14) mA g-1 were delivered. Moreover, the material showed respectable capacity retention (97%) after being cycled for 100 cycles at 0.2 A g-1. In order to identify the different Nb, Ti, Mo redox couples involved in this system, a Ta analogue was also synthesized (Ta7Ti1.5Mo1.5O25) and the electrochemical performance for this phase is also reported. This phase shows a lower initial capacity at 0.01 A g-1 (140(3) mA h g-1) than the Nb analogue in the same voltage range, which can be increased (225 mA h g-1) if a lower cutoff voltage (0.5 V) is applied. The capacity retention for this Ta system after 100 cycles at 0.2 A g-1 is similar to the Nb analogue (97%). Further work has explored whether the Nb-Ti-Mo contents could be varied, and these results showed that single phase Nb10-2xTixMoxO25 samples could be prepared for 1.5 ≤ x ≤ 1.75, and electrochemical testing results for the x = 1.75 endmember are also reported. Overall, this research highlights the synthesis and electrochemical characterisation of two new Wadsley Roth phases, and further highlights the challenges associated with the presence of reducible cations in tetrahedral sites in such structures with respect to minimising initial irreversible capacity loss.

Metal-Metal Bonding as an Electrode Design Principle in the Low-Strain Cluster Compound LiScMo3O8

Electrode materials for Li⁺-ion batteries require optimization along several disparate axes related to cost, performance, and sustainability. One of the important performance axes is the ability to retain structural integrity though cycles of charge/discharge. Metal-metal bonding is a distinct feature of some refractory metal oxides that has been largely underutilized in electrochemical energy storage, but that could potentially impact structural integrity. Here LiScMo₃O₈, a compound containing triangular clusters of metal-metal bonded Mo atoms, is studied as a potential anode material in Li⁺-ion batteries. Electrons inserted though lithiation are localized across rigid Mo₃ triangles (rather than on individual metal ions), resulting in minimal structural change as suggested by operando diffraction. The unusual chemical bonding allows this compound to be cycled with Mo atoms below a formally +4 valence state, resulting in an acceptable voltage regime that is appropriate for an anode material. Several characterization methods including potentiometric entropy measurements indicate two-phase regions, which are attributed through extensive first-principles modeling to Li⁺ ordering. This study of LiScMo₃O₈ provides valuable insights for design principles for structural motifs that stably and reversibly permit Li⁺ (de)insertion.

Wadsley-Roth Crystallographic Shear Structure Niobium-Based Oxides: Promising Anode Materials for High-Safety Lithium-Ion Batteries

Wadsley-Roth crystallographic shear structure niobium-based oxides are of great interest in fast Li+ storage due to their unique 3D open tunnel structures that offer facile Li+ diffusion paths. Their moderate lithiation potential and reversible redox couples hold the great promise in the development of next-generation lithium-ion batteries (LIBs) that are characterized by high power density, long lifespan, and high safety. Despite these outstanding merits, there is still extensive advancement space for further enhancing their electrochemical kinetics. And the industrial feasibility of Wadsley-Roth crystallographic shear structure niobium-based oxides as anode materials for LIBs requires more systematic research. In this review, recent progress in this field is summarized with the aim of realizing the practical applications of Wadsley-Roth phase anode materials in commercial LIBs. The review focuses on research toward the crystalline structure analyses, electrochemical reaction mechanisms, modification strategies, and full cell performance. In addition to highlighting the current research advances, the outlook and perspective on Wadsley-Roth anode materials is also concisely provided.

Formation and growth mechanism for niobium oxide nanoparticles: atomistic insight from in situ X-ray total scattering

Understanding the mechanisms for nanoparticle nucleation and growth is crucial for the development of tailormade nanomaterials. Here, we use X-ray total scattering and Pair Distribution Function analysis to follow the formation and growth of niobium oxide nanoparticles. We study the solvothermal synthesis from niobium chloride in benzyl alcohol, and through investigations of the influence of reaction temperature, a formation pathway can be suggested. Upon dissolution of niobium chloride in benzyl alcohol, octahedral [NbCl6-xOx] complexes form through exchange of chloride ligands. Heating of the solution results in polymerization, where larger clusters built from multiple edge-sharing [NbCl6-xOx] octahedra assemble. This leads to the formation of a nucleation cluster with the ReO3 type structure, which grows to form nanoparticles of the Wadsley-Roth type H-Nb2O5 structure, which in the bulk phase usually only forms at high temperature. Upon further growth, structural defects appear, and the presence of shear-planes in the structure appears highly dependent on nanoparticle size.

Local crystallographic shear structures in a[201] extended mixed dislocations of SrTiO3 unraveled by atomic-scale imaging using transmission electron microscopy and spectroscopy

Recently, extended mixed dislocations were observed at a [001]/(100) low-angle tilt grain boundary of a SrTiO3 bicrystal because of a slight twist between the two crystal parts. The b = a[201]/(100) mixed dislocations at the grain boundary dissociate into three dislocations with Burgers vector b of a/2[101], a[100], and a/2[101], respectively. A structure model has been proposed in particular for the dislocation cores of the two partials with b = a/2[101] based on the high-angle annular dark-field (HAADF) images acquired by scanning transmission electron microscopy (STEM). However, the details of the atomic structure and the chemical composition of the dislocation cores remain unexplored, especially for the b = a[100] dislocation that is evidently disassociated into two b = a/2[101] partial dislocations. In this work, we study the further atomic details of the extended mixed dislocations, in particular the local chemistry, in a SrTiO3 bicrystal using STEM, electron energy loss spectroscopy (EELS), and energy dispersive X-ray (EDX) spectroscopy techniques. By these atomic-scale imaging techniques, we reveal a unique feature for the atomic structure of the b = a[201]/(100) extended mixed dislocation, which we named as local crystallographic shear (LCS) structures. In addition, we identify a rock salt FCC-type TiOx (x = 0.66-1.24) phase at the locations of the extended mixed dislocations. In contrast to the insulating TiO2 phases, the TiOx phase is known to exhibit very low electrical resistivity of only several μΩ cm. In this regard, the extended mixed dislocations of SrTiO3 comprising the FCC TiOx phase may function as the conducting filament in resistive switching processes by completion and disruption of the TiOx phase along the dislocation cores through electrically stimulated redox reactions.

Spark Plasma Sintering (SPS)-Assisted Synthesis and Thermoelectric Characterization of Magnéli Phase V6O11

The Magnéli phase V₆O₁₁ was synthesized in gram amounts from a powder mixture of V₆O₁₁/V₇O₁₃ and vanadium metal, using the spark plasma sintering (SPS) technique. Its structure was determined with synchrotron X-ray powder diffraction data from a phase-pure sample synthesized by conventional solid-state synthesis. A special feature of Magnéli-type oxides is a combination of crystallographic shear and intrinsic disorder that leads to relatively low lattice thermal conductivities. SPS prepared V₆O₁₁ has a relatively low thermal conductivity of κ = 2.72 ± 0.06 W (m K)^-1 while being a n-type conductor with an electrical conductivity of σ = 0.039 ± 0.005 (μΩ m)^-1, a Seebeck coefficient of α = -(35 ± 2) μV K^-1, which leads to a power factor of PF = 4.9 ± 0.8 × 10^-5W (m K²)^-1 at ∼600 K. Advances in the application of Magnéli phases are mostly hindered by synthetic and processing challenges, especially when metastable and nanostructured materials such as V₆O₁₁ are involved. This study gives insight into the complications of SPS-assisted synthesis of complex oxide materials, provides new information about the thermal and electrical properties of vanadium oxides at high temperatures, and supports the concept of reducing the thermal conductivity of materials with structural building blocks such as crystallographic shear (CS) planes.

Transformation of the Chromium Coordination Environment in LaCrAsO Induced by Hydride Doping: Formation of La2Cr2As2OyHx

We report the synthesis of La₂Cr₂As₂O_yH_x (0.1 < y < 1.6) oxyhydride solid solutions using a solid-state reaction under high pressure with a solid-state hydrogen source and exhibit an example of how H^- doping can also promote structural changes: H^- doping in LaCrAsO results in the formation of La₂Cr₂As₂O_yH_x with the La₂Fe₂Se₂O₃-type layered structure. Remarkably, this transformation includes a change of the coordination number of Cr from 4 to 6, with the some of the H^- being accommodated in new sites within the CrAs layers. In this way, H^- not only serves as a conventional electron dopant by the substitution of O^2- but also makes new bonds to the transition metals.

Manipulation of native point defect behavior in rutile TiO2 via surfaces and extended defects

Semiconductor surfaces offer efficient pathways for exchanging native point defects with the underlying bulk. For rutile TiO₂(1 1 0), isotopic self-diffusion studies of oxygen have suggested that the surface may act as a source for O_i while simultaneously acting as a sink for titanium interstitials Ti_i. Through self-diffusion measurements with labeled Ti as well as O, the present work develops a more complete picture of the diffusion-reaction network involving O_i and Ti_i, complete with the surface acting as a source for whichever elements are available from the gas phase and a sink for elements that are not. The picture points to the importance of extended defects such as platelets and crystallographic shear planes as reservoirs of O_i and Ti_i, acting as net sources or sinks of these species depending upon specific conditions. The results exemplify the combined roles of surfaces and extended defects in regulating point defect behavior even in macroscopic metal oxide crystals, and point to specific strategies for manipulating that behavior intentionally.

Structural Stability from Crystallographic Shear in TiO2-Nb2O5 Phases: Cation Ordering and Lithiation Behavior of TiNb24O62

The host structure and reversible lithium insertion and extraction of an intercalation compound, TiNb₂₄O₆₂, are described. Neutron diffraction, applied for the first time to TiNb₂₄O₆₂, allowed an accurate refinement of the complex block superstructure, particularly with respect to the oxygen sublattice. Analysis of the transition-metal sites revealed significant cation ordering in the mixed-metal oxide. Electrochemical analysis demonstrated highly reversible lithium intercalation with ca. 190 mA·h·g^-1 after 100 cycles (C/10 rate, 3 months). The effect of the potential window on the capacity, polarization, and reversibility was carefully examined; a minimum voltage limit of 1.1-1.2 V is critical for efficient and reversible cycling. The galvanostatic intermittent titration technique revealed three solid-solution regions, with different lithium diffusivities, in addition to the two-phase plateau that was clearly observed in the V versus Q discharge/charge profile. Lithium-ion diffusion decreases by over 3 orders of magnitude from the dilute lithium limit early in the discharge to the lithium-stuffed phase Li_37.5(1.0)TiNb₂₄O₆₂. Nevertheless, prior to lithium stuffing, TiNb₂₄O₆₂ possesses intrinsically rapid lithium-ion kinetics, as demonstrated by the high-rate performance in thick films of ca. 10 μm particles when interfaced with a carbon-coated aluminum foil substrate. The TiO₂·Nb₂O₅ phase diagram is examined and electrochemical results are compared to related superstructures of crystallographically sheared blocks of octahedra in the TiO₂·Nb₂O₅ homologous series including the H-Nb₂O₅ end member.

Heterogeneous catalytic hydrogenation of CO2by metal oxides: defect engineering – perfecting imperfection

In this review, we discuss how metal oxides with designed defects can be synthesized and engineered, to enable heterogeneous catalytic hydrogenation of gaseous carbon dioxide to chemicals and fuels.

Atomic structure of defects in anion-deficient perovskite-based ferrites with a crystallographic shear structure

Crystallographic shear (CS) planes provide a new structure-generation mechanism in the anion-deficient perovskites containing lone-pair cations. Pb2Sr2Bi2Fe6O16, a new n = 6 representative of the A(n)B(n)O(3n-2) homologous series of the perovskite-based ferrites with the CS structure, has been synthesized using the solid-state technique. The structure is built of perovskite blocks with a thickness of four FeO6 octahedra spaced by double columns of FeO5 edge-sharing distorted tetragonal pyramids, forming 1/2[110](101)p CS planes (space group Pnma, a = 5.6690(2) Å, b = 3.9108(1) Å, c = 32.643(1) Å). Pb2Sr2Bi2Fe6O16 features a wealth of microstructural phenomena caused by the flexibility of the CS planes due to the variable ratio and length of the constituting fragments with {101}p and {001}p orientation. This leads to the formation of "waves", "hairpins", "Γ-shaped" defects, and inclusions of the hitherto unknown layered anion-deficient perovskites Bi2(Sr,Pb)Fe3O8.5 and Bi3(Sr,Pb)Fe4O11.5. Using a combination of diffraction, imaging, and spectroscopic transmission electron microscopy techniques this complex microstructure was fully characterized, including direct determination of positions, chemical composition, and coordination number of individual atomic species. The complex defect structure makes these perovskites particularly similar to the CS structures in ReO3-type oxides. The flexibility of the CS planes appears to be a specific feature of the Sr-based system, related to the geometric match between the SrO perovskite layers and the {100}p segments of the CS planes.

Exploiting dimensionality and defect mitigation to create tunable microwave dielectrics

The miniaturization and integration of frequency-agile microwave circuits--relevant to electronically tunable filters, antennas, resonators and phase shifters--with microelectronics offers tantalizing device possibilities, yet requires thin films whose dielectric constant at gigahertz frequencies can be tuned by applying a quasi-static electric field. Appropriate systems such as BaxSr1-xTiO3 have a paraelectric-ferroelectric transition just below ambient temperature, providing high tunability. Unfortunately, such films suffer significant losses arising from defects. Recognizing that progress is stymied by dielectric loss, we start with a system with exceptionally low loss--Srn+1TinO3n+1 phases--in which (SrO)2 crystallographic shear planes provide an alternative to the formation of point defects for accommodating non-stoichiometry. Here we report the experimental realization of a highly tunable ground state arising from the emergence of a local ferroelectric instability in biaxially strained Srn+1TinO3n+1 phases with n ≥ 3 at frequencies up to 125 GHz. In contrast to traditional methods of modifying ferroelectrics-doping or strain-in this unique system an increase in the separation between the (SrO)2 planes, which can be achieved by changing n, bolsters the local ferroelectric instability. This new control parameter, n, can be exploited to achieve a figure of merit at room temperature that rivals all known tunable microwave dielectrics.

Suppression of metal-insulator transition in VO2 by electric field-induced oxygen vacancy formation

Electrolyte gating with ionic liquids is a powerful tool for inducing novel conducting phases in correlated insulators. An archetypal correlated material is vanadium dioxide (VO(2)), which is insulating only at temperatures below a characteristic phase transition temperature. We show that electrolyte gating of epitaxial thin films of VO(2) suppresses the metal-to-insulator transition and stabilizes the metallic phase to temperatures below 5 kelvin, even after the ionic liquid is completely removed. We found that electrolyte gating of VO(2) leads not to electrostatically induced carriers but instead to the electric field-induced creation of oxygen vacancies, with consequent migration of oxygen from the oxide film into the ionic liquid. This mechanism should be taken into account in the interpretation of ionic liquid gating experiments.

Coulomb-potential-dependent decohesion of Magnéli phases

Transition metal oxide Magnéli phases are traditionally described as crystallographic shear structures. Some of these phases feature a layered crystal structure based on deformed metal-oxygen octahedra. Here, we describe the correlation between structure, decohesion energies and elastic properties of WO(3), ReO(3), MoO(2), VO(2), V(2)O(5) and TiO(2) using ab initio calculations. Decohesion energies and elastic constants C(44) are inversely proportional to the original distance between the cleaved layers and correspond to the screened Coulomb potential. This can be understood based on the electronic structure. The bond strength decreases rapidly as the distance is increased, resulting in weak coupling between the layers, which in turn causes the formation of easily plastically deformable structures.

Solid-gas reactions of complex oxides inside an environmental high-resolution transmission electron microscope

In a gas reaction cell (GRC), installed in a high-resolution transmission electron microscope (HRTEM) (JEOL 4000EX), samples can be manipulated in an ambient atmosphere (p<50mbar). This experimental setup permits not only the observation of solid-gas reactions in situ at close to the atomic level but also the induction of structural modifications under the influence of a plasma, generated by the ionization of gas particles by an intense electron beam. Solid state reactions of non-stoichiometric niobium oxides and niobium tungsten oxides with different gases (O2, H2 and He) have been carried out inside this controlled environment transmission electron microscope (CETEM), and this has led to reaction products with novel structures which are not accessible by conventional solid state synthesis methods. Monoclinic and orthorhombic Nb(12)O(29) crystallize in block structures comprising [3x4] blocks. The oxidation of the monoclinic phase occurs via a three step mechanism: firstly, a lamellar defect of composition Nb(11)O(27) is formed. Empty rectangular channels in this defect provide the diffusion paths in the subsequent oxidation. In the second step, microdomains of the Nb(22)O(54) phase are generated as an intermediate state of the oxidation process. The structure of the final product Nb(10)O(25), which consists of [3x3] blocks and tetrahedral coordinated sites, is isostructural to PNb(9)O(25). Microdomains of this apparently metastable phase appear as a product of the Nb(22)O(54) oxidation. The oxidation reaction of Nb(12)O(29) was found to be a reversible process: the reduction of the oxidation product with H(2) results in the formation of the starting Nb(12)O(29) structure. On the other hand, the block structure of Nb(12)O(29) has been destroyed by a direct treatment of the sample with H(2) while NbO in a cubic rock salt structure is produced. This in situ technique has also been applied to niobium tungsten oxides which constitute the solid solution series Nb(8-n)W9(+n)O47 with 0< or =n< or =4. All of these phases crystallize in the threefold tetragonal tungsten bronze (TTB) superstructure of Nb(8)W(9)O(47) (n=0). In the main reaction, these phases decompose in a gas plasma (O2, H2 or He, p=20mbar) into WO(3-x), which evaporates and solidifies again near the irradiated crystallite, and (Nb,W)(24)O(64), which crystallizes in a 2a superstructure of the TTB type observed here for the first time in the system Nb-W-O. Nb(8)W(9)O(47), Nb(7)W(10)O(47) and Nb(6)W(11)O(47) always react in this way, independent of the applied gas. On the other hand, the treatment of Nb(5)W(12)O(47) (n=3) and Nb(4)W(13)O(47) (n=4) in an oxygen atmosphere often caused a different reaction: these phases have been oxidized and a heavily disordered bronze-type structure has been formed. The oxygen excess in these products is largely accommodated in segregated domains of WO(3).

Homogeneous solid state transformations in niobium oxides

In homogeneous solid state reactions, transformation takes place within the crystals, and structural continuity is preserved; reactant and product structures are coherently intergrown. If the transformation starts independently at numerous loci, the incompletely reacted state of the crystals is inherently disordered. Crystallographic methods that average the structure over the whole crystal cannot fully characterize the actual, real-space structure, as it evolves during reaction, but that structure can, in favourable cases, be sampled by high resolution electron microscopy. Homogeneous reactions in the niobium oxide ‘block structure’ type crystals lend themselves to this technique. This study relates to the oxidation of the lower niobium oxides, Nb 22 O 54 , Nb 25 O 62 and Nb 53 O 132 , to niobium pentoxide. In each case, complete oxidation takes place at very low temperatures; the first products are reported to differ little in structure from the parent oxides, and to transform continuously into the stable polymorphic form, H -Nb 2 O 5 , over a wide temperature range and via several intermediate structures. By using lattice imaging methods, the structure elements present at each stage in the reactions have been identified, and their rearrangement has been followed. The initial oxidation step is completely non-reconstructive, and produces cation-deficient modifications of the original structures; all subsequent changes take place at constant composition, Nb 2 O 5 . In oxidized Nb 22 O 54 , a drastic rearrangement sets in at 350-400 °C, involving ( a ) an exchange of places and an apparent diffusion of the columnar structure elements (or blocks) and ( b ) a disproportionation that creates blocks of different cross sections. By a suitable mapping method, it is possible to infer exactly where, and how many, niobium atoms have undergone displacement into new sites, and to show that a single mechanism, proposed by Andersson and Wadsley, effects the observed changes of structure. Only a few atoms at a time, at the edge of one block, need to shift. Passing through a highly disordered state, blocks (3x3) octahedra in cross section are progressively segregated into microdomains of a new polymorphic structure of Nb 2 O 5 , Nb 10 O 25 , while blocks (5 x 3) octahedra in cross section are formed by disproportionation and built into microdomains of H -Nb 2 O 5 . These changes are initiated at random centres, 8-10 nm apart, starting at the edge of a single block; the displacements are propagated rapidly up the crystal b -axis and relatively slowly from block to block along rows and up files of blocks. Extended defects and compromise structures are formed where effects spreading from different initiating centres impinge; these faults serve as sinks for eliminating cation vacancies. The final result, stable up to 950 °C, is a domain structure, with nearly continuous walls of Nb 10 O 25 enclosing slabs of H -Nb 2 O 5 structure, and intersected by nearly continuous walls of fault structure. In oxidized Nb 25 O 62 , the first stage detected is the formation of triangular nuclei; one side of the triangle is formed of H -Nb 2 O 5 structure, the other side of Nb 10 O 25 structure, with an abrupt boundary to unchanged Nb 25 O 62 in each case. As these nuclei expand, by propagation of atom row displacements along rows and files of blocks, they merge to create relatively extensive domains of H -Nb 2 O 5 structure, separated by narrow walls of Nb 10 O 25 and linked by faults. Transformation of oxidized Nb 53 O 132 is similar. Long annealing of all the oxides at 900 °G gives products with essentially similar domain structures, but differing in the relative proportions of H -Nb 2 O 5 and Nb 10 O 25 domains. They are stable up to 950 °C; at higher temperatures, transformation into H -Nb 2 O 5 appears to be a catastrophic, and probably a heterogeneous process, for after heating at 950-1100 °C, any one crystal is either fully transformed into recrystallized H -Nb 2 O 5 or retains its domain structure with no significant change. There is some evidence that the domain structure is mobile at high temperatures.

Energy calculations for planar faults in reduced rutile (TiO 2 )

Calculations have been made of the energetics of reduced rutile (TiO 2 ) crystals containing ( a ) an isolated {132} c. s. plane, ( b ) an ordered array of {132} c. s. planes corresponding to an oxide of composition Ti 20 O 39 , ( c ) a vacancy disc lying on {132} planes and ( d ) an ordered array of vacancy discs lying on {132} planes, which gives the crystal a composition of Ti 20 O 39 . The calculations were made by using the polarizable point ion shell model, and by taking the electronic polarizabilities of the ions involved into account. The results show that the c. s. planes are preferred to vacancy discs, and that this is largely due to the electronic polarization energy terms. The enthalpy of formation of an isolated {132} c. s. plane in rutile was calculated. A comparison with an estimate of the enthalpy of formation of the vacancy disc suggests that vacancy discs will be converted into c. s. planes in real crystals and will not exist as discrete defects. The change of the energy terms as the polarizabilities of the ions varied was also calculated to quantify the relation between preferred defect structure and dielectric constant. It was found that a change in dielectric constant did not have a significant effect upon the stability of vacancy discs, but did have a large effect on the energetics of c. s. planes, which suggests that c. s. planes are only favoured in crystals of high dielectric constant. It is shown that electronic polarizability is of importance in stabilizing {132} c. s. planes and that the polarizability of the O 2 ─ ions, in particular, is of great significance.

Precipitation phenomena in non-stoichiometric oxides I. Pairs of crystallographic shear planes in reduced rutiles

High-resolution electron microscopy, in conjunction with controlled specimen preparation procedures, has enabled the nature and structure of the small defects existing within the nonstoichiometric phase TiO 2-x (0 ≤ x ≤ 0.01) to be investigated. In particular, the precipitation of pairs of crystallographic shear planes (c. s. p.) and their subsequent separation has been observed. The atomic mechanisms involved in c. s. p. nucleation and growth and the eventual evolution to an ordered c. s. structure have been elucidated by using new linear cationic interstitial defect structural models. The results emphasize the decisive influence that cooling history has on the microstructures observed at room temperature and suggest that similar new phenomena will be discovered in observations of other non-stoichiometric chemical systems.