Crystallographic Shear Planes Research Articles

Modulated crystallographic shear structure in titanium–chromium oxides: their structure and phonon-transport properties

Advancements in phonon engineering have propelled the study of heat conduction within nanostructures, focusing on the wave nature of phonons for thermal conductivity manipulation. This work investigates the annealing-induced structural transformation of titanium–chromium oxide crystals, highlighting a role in modulating thermal conductivity through the regularization of crystallographic shear (CS) plane spacing. Utilizing high-angle annular dark-field scanning transmission electron microscopy and selected-area electron diffraction, a transformation from disordered to ordered arrangements of CS planes was observed through annealing at high temperatures. The thermal conductivity increased following annealing. The variability observed in the spacing of CS planes before annealing implies that phonon Anderson localization might play a role in the changes in thermal conductivity.

Elastic and breaking properties of epitaxial face‐centered crystals in neutron star crusts and white dwarf cores

AbstractCrystallization of dense matter in neutron star crusts and white dwarf cores may be similar to epitaxial crystal growth in terrestrial laboratories. However, in stellar crystals, the spacing between horizontal planes has to gradually increase with the outward movement of the crystallization front, tracing decrease of the electron density. This process produces Coulomb crystals with stretched rather than cubic elementary cells. We extend the analysis of the elastic and breaking properties of such crystals to the face‐centered (fc) lattice. Shear deformations orthogonal to the stretch direction have been studied for 22 crystallographic shear planes. A common property for all these planes is the reduction and eventual nulling of the breaking shear strain with deviation from the unstretched configuration. The effective shear moduli for deformations orthogonal to the stretch direction have been calculated. It is possible that the epitaxial crystallization in compact stars results in a formation of large‐scale crystallites or, at least, in growth of the whole crystallization front perpendicular to particular crystallographic planes. For fc structure growth orthogonal to the planes, we expect that, at any density, () of crystallite height is occupied by layers one (two) orders of magnitude weaker than the bulk of the crystallite. This may be important for realistic modeling of crustquakes on neutron stars.

Regulating the local coordination model of homologous and heterogeneous niobium tungsten oxides toward ultrafast lithium storage

Intercalation-type niobium tungsten oxide anodes show potential for use in high-power-density lithium-ion batteries (LIBs). Lithium activation barriers are vital factors determining the rate performance of niobium tungsten oxides. Lithium activation barriers located in the crystallographic shear plane are the highest in the entire block structure, as the rate-limiting factor for Li+ diffusion in the Wadsley–Roth crystallographic shear structure of niobium tungsten oxides. Nb(W)O6 octahedrons are distorted by introducing Nb18W8O69 (Nb18) into Nb16W5O55 (Nb16) to form heterogeneous composite structures. This distortion induces metal–oxygen bond lengthening in Nb16/Nb18, increasing the interspace between the edge-sharing octahedrons and thereby reducing the highest lithium activation barriers in the Nb16/Nb18 crystallographic shear plane and mitigating volume expansion. Nb16/Nb18 has an extraordinary rate capacity of 85 mA h g−1 at 100 C and structural stability with 86.3% capacity retention of its highest value at 100 C after 1000 cycles, with a high volumetric energy density of 647 W h L−1 at 134930 W L−1. Nb16/Nb18//LiCoO2 have a high energy density of 158 W h kg−1 at 12480 W kg−1. Thus, a new approach is proposed to improve the rate performance of niobium-based oxides with Wadsley–Roth crystallographic shear structure by employing homologous and heterogeneous composites.

Lithium Intercalation Mechanisms in Wadsley-Roth Phases

Wadsley-Roth crystallographic shear phases are complex inorganic compounds that intercalate alkali ions at a rapid rate, allowing for fast charging lithium-ion battery electrodes with high power densities. Wadsley-Roth crystallographic shear phases consist of mxn size blocks of corner-sharing octahedra. These mxn blocks of octahedra share edges with other blocks of corner sharing octahedra, thereby forming what are referred to as crystallographic shear planes. A variety of Wadsley-Roth phases have been synthesized with differing structural features including the size of the blocks of corner-sharing transition metal octahedra, transition metal chemistry, and disorder among the transition metal sites. Using first-principles statistical mechanics methods, we have predicted the electrochemical properties of PNb9O25, TiNb2O7, and Nb2O5 (Figure 1), three important Wadsley-Roth shear phases, to explore the interplay between crystal structure, transition metal chemistry, Li site preferences, and the voltage profile. The computational approach relied on a triptych of ab initio density functional theory calculations, cluster expansion methods and finite temperature Monte Carlo methods. The calculations provide insights about lithium site preferences, strain evolution[1], and electronic structure [2, 3] as a function of state of charge in representative Wadsley-Roth phases.For the PNb9O25 compound, we find that lithium prefers to initially fill pyramidal sites at the crystallographic shear planes. It is not until higher lithium concentrations that Li begins to fill window sites at the center of the 3 by 3 octahedral blocks. This inversion in Li-Va ordering is paired with an increase of crystallographic strain and a decrease in octahedral distortions. In contrast to PNb9O25, the TiNb2O7 compound, a commercialized Wadsley-Roth phase, is characterized by Ti-Nb disorder on the transition metal sublattice. We find that the Nb, due to its higher oxidation state, prefers octahedral sites that share fewer edges with neighboring octahedra, relegating the lower oxidation state Ti cations to octahedra at the block edges. First-principles calculations and Monte Carlo simulations predict that the lithium site stability is paired to the local transition metal chemistry and ordering, with Li preferring to fill sites coordinated by TiO6 octahedra. With the use of symmetry adapted collective displacement order parameters we uncover changes in octahedral distortions that differ between the TiO6 and NbO6 octahedra upon intercalation of lithium. Furthermore, we find a similar evolution in strain upon intercalation as is predicted for PNb9O25. To study the effect of octahedral block size on electrochemical properties of Wadsley-Roth phases, we performed an additional in-depth study on Nb2O5, a Wadsley-Roth structure with 4 by 4 corner-sharing octahedral blocks. Predictions for this phase indicate that the dilute limit Li site preference changes when increasing block sizes. Unlike TiNb2O7 and PNb9O25, which have 3 by 3 octahedral block structures, lithium window sites at the center of the block are stable in Nb2O5 at the dilute limit. We attribute this to differences in octahedral distortions in the 4 by 4 and 3 by 3 blocks. Our results indicate that there are a wide variety of structural and chemical levers with which to modify and further optimize the electrochemical properties of Wadsley-Roth phases. References Saber, M., Preefer, M.B., Kolli, S.K., Zhang, W., Laurita, G., Dunn, B., Seshadri, R. and Van der Ven, A., 2021. Role of Electronic Structure in Li Ordering and Chemical Strain in the Fast Charging Wadsley–Roth Phase PNb9O25. Chemistry of Materials, 33(19), pp.7755-7766.Preefer, M.B., Saber, M., Wei, Q., Bashian, N.H., Bocarsly, J.D., Zhang, W., Lee, G., Milam-Guerrero, J., Howard, E.S., Vincent, R.C. and Melot, B.C., 2020. Multielectron redox and insulator-to-metal transition upon lithium insertion in the fast-charging, Wadsley-Roth phase PNb9O25. Chemistry of Materials, 32(11), pp.4553-4563.Baek, S.W., Preefer, M.B., Saber, M., Zhai, K., Frajnkovič, M., Zhou, Y., Dunn, B.S., Van der Ven, A., Seshadri, R. and Pilon, L., 2022. Potentiometric entropy and operando calorimetric measurements reveal fast charging mechanisms in PNb9O25. Journal of Power Sources, 520, p.230776. Figure 1

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.

Impact of mechanical activation on the crystal structure and dielectric properties of ceramics based on bismuth ferrite

The solid solutions of the (1-x)BiFeO3 – x/2PbFe1/2Nb1/2O3 -x/2PbFe2/3W1/3O3 system (x = 0.05, x = 0.50) were produced by conventional solid state technology using mechanical activation (MA). It is discovered that particle destruction occurs along planar defects (crystallographic shear planes, CSP) during MA. The MA has a different impact on the microstructure formation of the solid solutions (1-x)BiFeO3 – x/2PbFe1/2Nb1/2O3 – x/2PbFe2/3W1/3O3 system, depending on their localization on the phase diagram. It is shown that MA leads to an increase in the temperature stability of the dielectric properties in the solid solutions of the studied system.

High Power Factor Nb-Doped TiO2 Thermoelectric Thick Films: Toward Atomic Scale Defect Engineering of Crystallographic Shear Structures.

Donor-doped TiO2-based materials are promising thermoelectrics (TEs) due to their low cost and high stability at elevated temperatures. Herein, high-performance Nb-doped TiO2 thick films are fabricated by facile and scalable screen-printing techniques. Enhanced TE performance has been achieved by forming high-density crystallographic shear (CS) structures. All films exhibit the same matrix rutile structure but contain different nano-sized defect structures. Typically, in films with low Nb content, high concentrations of oxygen-deficient {121} CS planes are formed, while in films with high Nb content, a high density of twin boundaries are found. Through the use of strongly reducing atmospheres, a novel Al-segregated {210} CS structure is formed in films with higher Nb content. By advanced aberration-corrected scanning transmission electron microscopy techniques, we reveal the nature of the {210} CS structure at the nano-scale. These CS structures contain abundant oxygen vacancies and are believed to enable energy-filtering effects, leading to simultaneous enhancement of both the electrical conductivity and Seebeck coefficients. The optimized films exhibit a maximum power factor of 4.3 × 10-4 W m-1 K-2 at 673 K, the highest value for TiO2-based TE films at elevated temperatures. Our modulation strategy based on microstructure modification provides a novel route for atomic-level defect engineering which should guide the development of other TE materials.

Thermal conduction in titanium-chromium oxide natural superlattices with an ordered arrangement of nearly pristine interfaces

Thermal conduction was investigated in titanium-chromium oxide natural superlattices with an ordered arrangement of crystallographic shear planes, which were nearly pristine interfaces for thermal phonons. Thermal conductivity exhibited a clear maximum value at around 50 K, which indicates interface roughness as small as 20 pm based on the calculation of thermal conductivity by a modified Debye-Callaway model. With an ordered arrangement of nearly pristine interfaces at nanoscale, the wave contribution to phonon transport was estimated to be predominant at lower temperatures (less than 100 K) and as much as 30 % even at room temperature. We found that the interface roughness greatly influences the thermal conduction in natural superlattice systems, which may lead to variations in the reported value of thermal conductivity. We have demonstrated that the wave contribution of thermal conductivity become apparent in bulk crystal having nano scale periodic structure with pristine interfaces.

Crossover from incoherent to coherent thermal conduction in bulk titanium oxide natural superlattices

We have investigated thermal conduction in bulk titanium oxide natural superlattices with crystallographic shear (CS) structures, in which dense planar faults are introduced with different periodicities, prepared by reductive annealing of rutile TiO2 and crystal growth by the floating zone method. High-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) revealed that (132)rutile and (121)rutile CS planes with interspacings of 2.7 and 1.0 nm were introduced in the mother rutile structure. Time-domain thermoreflectance (TDTR) revealed that the thermal conductivity decreased by the introduction of CS planes, but that the decrease is not monotonic with increasing density of CS planes. Calculation of the thermal conductivity and the mean free path for phonons revealed that a crossover from incoherent to coherent thermal conduction took place, and coherent interfaces with nanoscale periodicity were formed as thermodynamically stable phases in bulk titanium oxide natural superlattices.

Synthesis and Characterization of Tungsten Suboxide WnO3n-1 Nanotiles.

WnO3n−1 nanotiles, with multiple stoichiometries within one nanotile, were synthesized via the chemical vapour transport method. They grow along the [010] crystallographic axis, with the thickness ranging from a few tens to a few hundreds of nm, with the lateral size up to several µm. Distinct surface corrugations, up to a few 10 nm deep appear during growth. The {102}r crystallographic shear planes indicate the WnO3n−1 stoichiometries. Within a single nanotile, six stoichiometries were detected, namely W16O47 (WO2.938), W15O44 (WO2.933), W14O41 (WO2.928), W13O38 (WO2.923), W12O35 (WO2.917), and W11O32 (WO2.909), with the last three never being reported before. The existence of oxygen vacancies within the crystallographic shear planes resulted in the observed non-zero density of states at the Fermi energy.

Local Atomic Structures for Tunable Ordered Arrangements of Crystallographic Shear Planes in Titanium-Chromium Oxide Natural Superlattices

Local Atomic Structures for Tunable Ordered Arrangements of Crystallographic Shear Planes in Titanium-Chromium Oxide Natural Superlattices

Shear Pleasure: The Structure, Formation, and Thermodynamics of Crystallographic Shear Phases

A renaissance of interest in crystallographic shear structures and our recent work in this remarkable class of materials inspired this review. We first summarize the geometrical aspects of shear plane formation and possible transformations in ReO3, rutile, and perovskite-based structures. Then we provide a mechanistic overview of crystallographic shear formation, plane ordering, and propagation. Next we describe the energetics of planar defect formation and interaction, equilibria between point and extended defect structures, and thermodynamic stability of shear compounds. Finally, we emphasize the remaining challenges and propose future directions in this exciting area.

Electrical properties and charge compensation mechanisms of Cr-doped rutile, TiO2.

Cr-doped rutile, Ti1-xCrxO2-x/2-δ, powders and ceramics with 0 ≤ x ≤ 0.05 were prepared by solid state reaction and sintered at 1350 °C. Cr distribution is homogeneous with no evidence of either segregation or crystallographic shear plane formation. For high x compositions, >∼0.01, Cr substitution is charge-compensated ionically by oxygen vacancies with two Cr3+ ions for each vacancy and the materials are electronically insulating. For low x compositions, the materials are semiconducting. This is attributed to a new charge compensation mechanism involving Ti3+ ions created in response to the local electroneutrality requirement for two trivalent cations to be in close proximity to each oxygen vacancy. At very low dopant concentrations, ≪0.01, the dopants are well-separated and instead, some Ti3+ ions act as a second dopant to preserve local electroneutrality. For intermediate x compositions, a core-shell structure is proposed consisting of semiconducting grain interiors containing Ti3+ ions surrounded by a more insulating shell with Cr3+ ions as the only acceptor dopant. Lattice parameters show unusual, non-linear Vegard's law behaviour characterised by a maximum in cell volume at intermediate x ∼ 0.005, that is attributed to the composition-dependent presence of Ti3+ ions.

Unveiling Oxygen Vacancy Superstructures in Reduced Anatase Thin Films.

Oxygen vacancies are known to play a crucial role in tuning the physical properties and technological applications of titanium dioxide TiO2. Over the last decades, defects in substoichiometric TiO2 have been commonly associated with the formation of TinO2n-x Magnéli phases, which are extended planar defects originating from crystallographic shear planes. By combining advanced transmission electron microscopy techniques, electron energy-loss spectroscopy and atomistic simulations, we reach new understanding of the oxygen vacancy induced structural modulations in anatase, ruling out the earlier shear-plane model. Structural modulations are instead shown to be due to the formation of oxygen vacancy superstructures that extend periodically inside the films, preserving the crystalline order of anatase. Elucidating the structure of oxygen defects in anatase is a crucial step for improving the functionalities of such material system and to engineer devices with targeted properties.

Entropy Stabilization of TiO2–Nb2O5 Wadsley–Roth Shear Phases and Their Prospects for Lithium-Ion Battery Anode Materials

Wadsley–Roth phases accommodate variable cation charge by crystallographic shear planes delineating blocks of the parent ReO3 structure. The homologous series TiNbxO2+2.5x provides possible new ano...

Lithium Diffusion in Niobium Tungsten Oxide Shear Structures.

Niobium tungsten oxides with crystallographic shear structures form a promising class of high-rate Li-ion anode materials. Lithium diffusion within these materials is studied in this work using density functional theory calculations, specifically nudged elastic band calculations and ab initio molecular dynamics simulations. Lithium diffusion is found to occur through jumps between 4-fold coordinated window sites with low activation barriers (80–300 meV) and is constrained to be effectively one-dimensional by the crystallographic shear planes of the structures. We identify a number of other processes, including rattling motions with barriers on the order of the thermal energy at room temperature, and intermediate barrier hops between 4-fold and 5-fold coordinated lithium sites. We demonstrate differences regarding diffusion pathways between different cavity types; within the ReO3-like block units of the structures, cavities at the corners and edges host more isolated diffusion tunnels than those in the interior. Diffusion coefficients are found to be in the range of 10–12 to 10–11 m2 s–1 for lithium concentrations of 0.5 Li/TM. Overall, the results provide a complete picture of the diffusion mechanism in niobium tungsten oxide shear structures, and the structure–property relationships identified in this work can be generalized to the entire family of crystallographic shear phases.

High mobility approaching the intrinsic limit in Ta-doped SnO2 films epitaxially grown on TiO2 (001) substrates

Achieving high mobility in SnO2, which is a typical wide gap oxide semiconductor, has been pursued extensively for device applications such as field effect transistors, gas sensors, and transparent electrodes. In this study, we investigated the transport properties of lightly Ta-doped SnO2 (Sn1−xTaxO2, TTO) thin films epitaxially grown on TiO2 (001) substrates by pulsed laser deposition. The carrier density (ne) of the TTO films was systematically controlled by x. Optimized TTO (x = 3 × 10−3) films with ne ~ 1 × 1020 cm−3 exhibited a very high Hall mobility (μH) of 130 cm2V−1s−1 at room temperature, which is the highest among SnO2 films thus far reported. The μH value coincided well with the intrinsic limit of μH calculated on the assumption that only phonon and ionized impurities contribute to the carrier scattering. The suppressed grain-boundary scattering might be explained by the reduced density of the {101} crystallographic shear planes.

Signatures of Filamentary Superconductivity up to 94 K in Tungsten Oxide WO2.90

We report results of the search for possible superconducting state in tungsten oxides WO3−x with various oxygen deficiency 0 < x < 1. In samples with one particular composition WO2.9, the signatures of superconductivity with the same transition temperature Tc = 80 K were registered by means of magnetization measurements. By lithium intercalation, the Tc was further increased to 94 K. The observed small superconducting fraction and the absence of clear transition in resistivity measurements indicate that the superconductivity is localized in small regions which do not percolate. Electron Paramagnetic Resonance experiments showed the presence of W5+ − W5+ electron bipolarons in reduced tungsten oxide samples. It is proposed that such bipolarons form and cluster within crystallographic shear planes which exist in the Magneli phase of WO2.9 (W20O58) and represent charge-carrier rich quasi-1D stripes or puddles. When decreasing temperature, superconducting state can be established locally in such regions similar to cuprates. The obtained results demonstrate that the Magneli-type tungsten oxides are promising materials to explore high-temperature superconductivity above liquid nitrogen temperature.

Block Structure and Macroresponces of Perovskite Media in a Sodium–Calcium Niobate System

The block structure of Na1 – xCaxNbO3 (0.0 ≤ x ≤ 0.20) solid solutions formed by the crystallographic shear planes is studied via powder X-ray diffraction. A correlation between the block sizes and the dielectric properties of the samples is established as well.

Microstructure and broadband dielectric properties of Zn2SiO4 ceramics with nano-sized TiO2 addition

Abstract Zn2SiO4 ceramics with nano-sized TiO2 addition (ZST) were synthesized by conventional solid state method. The association between the new composite's microstructures and dielectric properties reveals that reduced pores, increased density and average grain sizes with increasing sintering temperatures, have contributed to the increased permittivities at kHz and microwave bands; the decrease of the permittivities at 1275 °C is due to the form of twin planes. At the terahertz band, the competition of generating oxygen vacancies and forming them into twin crystallographic shear planes dominates the change of permittivities: the crystallographic shear planes decrease the permittivity at the sintering temperature 1225 °C and 1250 °C, and the high-rate generation of oxygen vacancies at 1275 °C increases the permittivities. The ZST ceramics demonstrate stable permittivity and low dielectric losses (