Crystallographic Shear Planes Research Articles

Oxygen stoichiometry, chemical expansion or contraction, and electrical properties of rutile, TiO2±δ ceramics

AbstractRutile, TiO2 is increasingly oxygen‐deficient on heating in air above ~700°C. The weight loss is generally too small for accurate measurement, but the electrical properties of quenched samples provide a sensitive qualitative indicator of oxygen content since their conductivity can vary by many orders of magnitude. The oxygen lost at high temperature is fully recovered if samples are cooled slowly. With rapid quenching, by dropping samples into liquid N2, the oxygen stoichiometry at high temperature is preserved to ambient and the resulting materials are kinetically stable but thermodynamically metastable. The lattice parameters of quenched samples showed an unusual dependence on quench temperature and, by implication, on oxygen stoichiometry. Lattice parameters increased with a small oxygen loss, δ; chemical expansion of the lattice occurred and is attributed to reduction in average Ti oxidation state and increase in Ti–O bond lengths. At higher δ, lattice parameters started to decrease giving a chemical contraction effect attributed to partial collapse of columns of edge‐sharing TiO6 octahedra in the rutile structure and elimination of oxygen vacancies by crystallographic shear plane formation. Oxygen‐deficient samples quenched from above 700°C were n‐type, as were samples annealed and measured at 650 and 700°C. Samples measured at 450‐500°C were p‐type and believed to be slightly oxygen‐rich; it is suggested that holes located on oxide ions at or near the sample surface arose from redox electron transfer between underbonded surface oxide ions and adsorbed O2 molecules. Samples annealed between 550 and 600°C showed cross‐over between n‐ and p‐type behavior.

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.

Formation and stability of reduced TiOx layers on anatase TiO2(101) : Identification of a novel Ti2O3 phase

We use density functional theory (DFT) calculations to investigate structural models consisting of anatase TiO2(101) slabs covered by reduced overlayers formed by (101) crystallographic shear planes (CSPs). Ab initio thermodynamics supports the stability of these structures under a wide range of experimental conditions. The overlayers are found to have Ti2O3 stoichiometry with a crystal structure different from the known corundum-like Ti2O3 (here denoted {\alpha}-Ti2O3) phase. DFT calculations predict this new "csp-Ti2O3" phase to be energetically close to {\alpha}-Ti2O3 and to have also a similar band gap. These results suggest a possible role of the csp-Ti2O3 phase in the properties of black TiO2, a promising photocatalytic material made of nanoparticles with a crystalline TiO2 core and a highly reduced TiOx shell that is capable of absorbing the whole spectrum of visible light.

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 Oi while simultaneously acting as a sink for titanium interstitials Tii. 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 Oi and Tii, 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 Oi and Tii, 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.

Preparation of monophasic titanium sub-oxides of Magnéli phase with enhanced thermoelectric performance

Titanium sub-oxides of Magnéli phase are considered as earth-abundant, non-toxic materials for thermoelectric application at high temperature, but very limited investigation has been reported mainly owing to the difficulty lying in obtaining monophasic material. Herein we describe a facile strategy to prepare monophasic Ti9O17 and Ti4O7 powder, in which the single Ti9O17 phase is obtained for the first time. After spark plasma sintering, the thermoelectric properties of monophasic Ti9O17 and Ti4O7 ceramics are studied comparatively. Despite the one magnitude lower electrical conductivity, Ti9O17 shows higher power factor compared with Ti4O7 due to the large Seebeck coefficient. Moreover, with the low electronic thermal conductivity, sub-micron scaled grain size and nano-scaled crystallographic shear plane, the Ti9O17 ceramic exhibits even lower thermal conductivity compared to Ti4O7, leading to a ZT of 0.16 at 764K, which is the best thermoelectric performance among reported Magnéli phases.

Spark Plasma Sintering of Tungsten Oxides WOx (2.50 ≤ x ≤ 3): Phase Analysis and Thermoelectric Properties

The solid-state reaction of WO3 with W was studied in order to clarify the phase formation in the binary system W-O around the composition WOx (2.50 ≤ x ≤ 3) during spark plasma sintering (SPS). A new phase “WO2.82” is observed in the range 2.72 ≤ x ≤ 2.90 which might have the composition W12O34. The influence of the composition on the thermoelectric properties was investigated for 2.72 ≤ x ≤ 3. The Seebeck coefficient, electrical conductivity and electronic thermal conductivity are continuously tunable with the oxygen-to-tungsten ratio. The phase formation mainly affects the lattice thermal conductivity κlat which is significantly reduced until 700 K for the sample with the composition x = 2.84, which contains the phases W18O49 and “WO2.82”. In single-phase WO2.90 and multi-phase WOx materials (2.90 ≤ x ≤ 3), which contain crystallographic shear plane phases, a similar reduced κlat is observed only below 560 K and 550 K, respectively. Therefore, the composition range x < 2.90 in which the pentagonal column structural motif is formed might be more suitable for decreasing the lattice thermal conductivity at high temperatures.

Metal-Organic Nanosheets Formed via Defect-Mediated Transformation of a Hafnium Metal-Organic Framework.

We report a hafnium-containing MOF, hcp UiO-67(Hf), which is a ligand-deficient layered analogue of the face-centered cubic fcu UiO-67(Hf). hcp UiO-67 accommodates its lower ligand:metal ratio compared to fcu UiO-67 through a new structural mechanism: the formation of a condensed “double cluster” (Hf12O8(OH)14), analogous to the condensation of coordination polyhedra in oxide frameworks. In oxide frameworks, variable stoichiometry can lead to more complex defect structures, e.g., crystallographic shear planes or modules with differing compositions, which can be the source of further chemical reactivity; likewise, the layered hcp UiO-67 can react further to reversibly form a two-dimensional metal–organic framework, hxl UiO-67. Both three-dimensional hcp UiO-67 and two-dimensional hxl UiO-67 can be delaminated to form metal–organic nanosheets. Delamination of hcp UiO-67 occurs through the cleavage of strong hafnium-carboxylate bonds and is effected under mild conditions, suggesting that defect-ordered MOFs could be a productive route to porous two-dimensional materials.

Coupling of bias-induced crystallographic shear planes with charged domain walls in ferroelectric oxide thin films

Polar discontinuity at interfaces plays deterministic roles in charge transport, magnetism, and even superconductivity of functional oxides. To date, most polar discontinuity problems have been explored in hetero-interfaces between two dissimilar materials. Here, we show that charged domain walls (CDWs) in epitaxial thin films of ferroelectric PbZr0.2Ti0.8O3 are strongly coupled to polar interfaces through the formation of ½ {h0l} type crystallographic shear planes (CSPs). Using atomic resolution imaging and spectroscopy we illustrate that the CSPs consist of both conservative and nonconservative segments when coupled to the CDWs, where necessary compensating charges for stabilizing the CDWs are associated with vacancies at the CSPs. Lasly, the CDW/CSP coupling yields an atomically narrow domain walls, consisting of a single atomic layer of oxygen. This study shows that the CDW/CSP coupling is a fascinating venue to develop emergent material properties.

Unusual Relaxor Ferroelectric Behavior in Stairlike Aurivillius Phases.

New ferroelectric layered materials were found in the pseudobinary system Bi5Nb3O15-ABi2Nb2O9 (A= Ba, Sr and Pb). Preliminary observations made by transmission electron microscopy indicate that these compounds exhibit a complex incommensurately modulated structure. A (3 + 1)D structural model is obtained using ab initio phasing by charge flipping based on the analysis of precession electron diffraction tomography data. The (3 + 1)D structure is further validated by a refinement against neutron powder diffraction. These materials possess a layered structure with discontinuous [Bi2O2] slabs and perovskite blocks. While these structural units are characteristics of Aurivillius phases, the existence of periodic crystallographic shear planes offers strong similarities with collapsed or stairlike structures known in high-Tc superconductors and related compounds. Using dielectric spectroscopy, we study the phase transitions of these new layered materials. For A = Ba and Sr, a Vögel-Fulcher-like behavior characteristic of the so-called relaxor ferroelectrics is observed and compared to "canonical" relaxors. For A = Sr, the absence of a Burns temperature separated from the freezing temperature appears as a rather unusual behavior.

A chemists view: Metal oxides with adaptive structures for thermoelectric applications

Thermoelectric devices can help to tackle future challenges in the energy sector through the conversion of waste heat directly into usable electric energy. For a wide applicability low‐cost materials with reasonable thermoelectric performances and cost‐efficient preparation techniques are required. In this context metal oxides are an interesting class of materials because of their inherent high‐temperature stability and relative high sustainability. Their thermoelectric performance, however, needs to be improved for wide application. Compounds with adaptive structures are a very interesting class of materials. A slight reduction of early transition metal oxides generates electrons as charge carriers and crystallographic shear planes as structure motif. The crystallographic shear planes lead to a reduction of intrinsic thermal conductivity. At the same time, the electronic transport properties can be tuned by the degree of reduction. So far only a few transition metal oxides with adaptive structures have been investigated with respect to their thermoelectric properties, leaving much room for improvement. This review gives an overview of thermoelectric oxides, highlights the structural aspects of the crystallographic shear planes and the resulting thermoelectric properties.

Oxygen deficiency in MoO3 polycrystalline nanowires and nanotubes

We report on the synthesis of polycrystalline molybdenum oxide (MoO3) nanowires via oxidation of molybdenum-sulfur-iodine (Mo6S2I8) nanowires. This unique synthesis route results in an interesting morphology comprising porous nanowires and nanotubes. We found the nanowires to have the orthorhombic MoO3 structure. The structure is slightly oxygen deficient which results in the appearance of a new resonant Raman band (1004 cm−1) and paramagnetic defects (Mo5+) of both the point and crystallographic shear plane nature.

Trapping of Oxygen Vacancies at Crystallographic Shear Planes in Acceptor-Doped Pb-Based Ferroelectrics.

The defect chemistry of the ferroelectric material PbTiO3 after doping with Fe(III) acceptor ions is reported. Using advanced transmission electron microscopy and powder X-ray and neutron diffraction, we demonstrate that even at concentrations as low as circa 1.7% (material composition approximately ABO2.95), the oxygen vacancies are trapped into extended planar defects, specifically crystallographic shear planes. We investigate the evolution of these defects upon doping and unravel their detailed atomic structure using the formalism of superspace crystallography, thus unveiling their role in nonstoichiometry in the Pb-based perovskites.

Trapping of Oxygen Vacancies at Crystallographic Shear Planes in Acceptor‐Doped Pb‐Based Ferroelectrics

AbstractThe defect chemistry of the ferroelectric material PbTiO3 after doping with FeIII acceptor ions is reported. Using advanced transmission electron microscopy and powder X‐ray and neutron diffraction, we demonstrate that even at concentrations as low as circa 1.7 % (material composition approximately ABO2.95), the oxygen vacancies are trapped into extended planar defects, specifically crystallographic shear planes. We investigate the evolution of these defects upon doping and unravel their detailed atomic structure using the formalism of superspace crystallography, thus unveiling their role in nonstoichiometry in the Pb‐based perovskites.

Synergy between transmission electron microscopy and powder diffraction: application to modulated structures.

The crystal structure solution of modulated compounds is often very challenging, even using the well established methodology of single-crystal X-ray crystallography. This task becomes even more difficult for materials that cannot be prepared in a single-crystal form, so that only polycrystalline powders are available. This paper illustrates that the combined application of transmission electron microscopy (TEM) and powder diffraction is a possible solution to the problem. Using examples of anion-deficient perovskites modulated by periodic crystallographic shear planes, it is demonstrated what kind of local structural information can be obtained using various TEM techniques and how this information can be implemented in the crystal structure refinement against the powder diffraction data. The following TEM methods are discussed: electron diffraction (selected area electron diffraction, precession electron diffraction), imaging (conventional high-resolution TEM imaging, high-angle annular dark-field and annular bright-field scanning transmission electron microscopy) and state-of-the-art spectroscopic techniques (atomic resolution mapping using energy-dispersive X-ray analysis and electron energy loss spectroscopy).

{110}-Layered B-cation ordering in the anion-deficient perovskite Pb2.4Ba2.6Fe2Sc2TiO13 with the crystallographic shear structure.

A novel anion-deficient perovskite-based compound, Pb(2.4)Ba(2.6)Fe(2)Sc(2)TiO(13), was synthesized via the citrate-based route. This compound is an n = 5 member of the AnBnO(3n-2) homologous series with unit-cell parameters related to the perovskite subcell a(p)≈ 4.0 Å as a(p)√2 ×a(p)× 5a(p)√2. The crystal structure of Pb(2.4)Ba(2.6)Fe(2)Sc(2)TiO(13) consists of quasi-2D perovskite blocks with a thickness of three octahedral layers separated by the 1/2[110](1[combining macron]01)(p) crystallographic shear (CS) planes, which are parallel to the {110} plane of the perovskite subcell. The CS planes transform the corner-sharing octahedra into chains of edge-sharing distorted tetragonal pyramids. Using a combination of neutron powder diffraction, (57)Fe Mössbauer spectroscopy and atomic resolution electron energy-loss spectroscopy we demonstrate that the B-cations in Pb(2.4)Ba(2.6)Fe(2)Sc(2)TiO(13) are ordered along the {110} perovskite layers with Fe(3+) in distorted tetragonal pyramids along the CS planes, Ti(4+) preferentially in the central octahedra of the perovskite blocks and Sc(3+) in the outer octahedra of the perovskite blocks. Magnetic susceptibility and Mössbauer spectroscopy indicate a broadened magnetic transition around T(N)∼ 45 K and the onset of local magnetic fields at low temperatures. The magnetic order is probably reminiscent of that in other AnBnO(3n-2) homologues, where G-type AFM order within the perovskite blocks has been observed.

Effect of Lone Pair Cations on the Orientation of Crystallographic Shear Planes in Anion-Deficient Perovskites

Anion deficiency in perovskite-based ferrites containing lone pair cations Pb2+ and Bi3+ can be accommodated by the formation of crystallographic shear (CS) planes. In this contribution, using (Pb1-zSrz)1-xFe1+xO3-y as a model system, we demonstrate that the lone pair cations are not only responsible for the formation of the CS planes, but also control their orientation. The isovalent substitution of Pb2+ by stereochemically inactive Sr2+ highlights the influence of the A-cation electronic structure owing to very close ionic radii of these cations. Employing a combination of transmission electron microscopy and X-ray and neutron powder diffraction, we have systematically investigated the structure evolution of the compounds with changing P/Sr ratio. Two compositional ranges with distinct orientations of the CS planes have been identified in the system: 0.05 ≤ z ≤ 0.2, where the CS planes orientation systematically varies but stays close to (203)p, and 0.3 ≤ z ≤ 0.45 with the (101)p CS planes. The incommensurate structure of Pb0.792Sr0.168Fe1.040O2.529, a representative compound of the first range, has been refined from the neutron powder diffraction data (S.G. X2/m(α0γ), X = [1/2,1/2,1/2,1/2], a = 3.9512(1)Å, b = 3.9483(1)Å, c = 3.9165(1)Å, β = 93.268(2)0, q = 0.0879(1)a* + 0.1276(1)c*, RF = 0.023, RP = 0.029, T = 900K). Comparing the CS structures in both compositional ranges, we demonstrate that the orientation of the CS planes is primarily governed by a specific bonding configuration of the lone pair cations with the anions.

New modular oxide structures using lone pair cations as "chemical scissors"

"It is known that lone pair cations, such as Bi3+ or Pb2+ have a flexible coordination environment that enables them to operate as ""chemical scissors"". Their flexibility reduces the strain that would otherwise be present at the interfaces separating structure modules. We have found that in complex oxides it allows many variants of interfaces, for example crystallographic shear planes or (non)conservative twin planes in structures, enabling the synthesis of new structural families. A common characteristic for all these new compounds is the presence of magnetical frustration. As a first example, this concept allowed to introduce crystallographic shear planes into the perovskite structure, a feat that was considered highly unlikely before. This allowed to generate a new anion deficient perovskite based homologous series AnBnO3n-2 (n = 4 - 6). There is magnetic frustration at the crystallographic shear plane separating the perovskite blocks, due to competing FM and AFM interactions. Also incommensurately modulated perovskites can be obtained, for example (Pb,Bi)1-xFe1+xO3-y. These arise by replacing Bi3+ with Pb2+, which introduces an oxygen deficiency, which is then accommodated by periodically spaced CS planes to reduce the coordination of the A-cations at the interface. The flexible coordination environment of Bi3+ and Pb2+ makes them ideally suited for these A cation positions. Other possibilities were encountered in BiMnFe2O6 and Bi4Fe5O13F. In BiMnFe2O6 the Bi3+ induces the existence of a non-conservative twin plane. The result is a new structure type with hcp structured modules. In Bi4Fe5O13F, the Bi3+-cations separate layers with magnetically frustrated Cairo lattices."

Combining powder diffraction with TEM for solving modulated structures

In many materials competing interactions of different nature may give rise to incommensurate modulations causing extreme structure complexity. Ab initio solution of the modulated structures even with using high quality synchrotron X-ray and/or neutron powder diffraction data appears to be a very challenging problem due to weakness of the satellite reflections, ambiguity in the determination of the modulation vector(s) and superspace symmetry and difficulties in building the initial model for further Rietveld refinement. These problems can be resolved or, at least, mitigated if the diffraction, imaging and spectroscopic advanced transmission electron microscopy techniques are combined with the analysis of powder diffraction data. Complete reconstruction of the reciprocal space, structure solution using quasi-kinematical electron diffraction data, mapping projected scattering density in the unit cell, visualization of the light atoms, displacive and occupational ordering, mapping chemical composition and coordination number can be utilized to reveal the nature of incommensurate modulations and construct the reliable model for the refinement from powder diffraction data. The benefit of the strategy of combining the powder diffraction data with the reciprocal and real space information obtained using aberration-corrected scanning transmission electron microscopy will be illustrated on the examples of the transition metal oxides: Li3xNd2/3-xTiO3 perovskites with frustrated incommensurately modulated octahedral tilting pattern [1]; perovskites (Bi,Pb)1-xFe1+xO3-y, modulated by crystallographic shear planes [2]; CaGd2(1-x)Eu2x(MoO4)4(1-y)(WO4)4y scheelites with incommensurately modulated ordering of cation vacancies [3].

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.

Solar water oxidation in sputter-deposited nanocrystalline WO3 photoanodes via tuning of Ar:O2 flow rate combinations

Thin film WO3 photoanodes were prepared by reactive sputtering in Ar and O2 gas mixtures of various flow rate combinations. Furnace annealed films were nanocrystalline monoclinic WO3 with (002), (020) and (200) plane orientations. Water oxidation in 0.33 M H2SO4 electrolyte under simulated solar illumination showed that photoanodes deposited in highest Ar and O2 flow rate combinations exhibited highest photocurrent of 4.1 mA cm−2 (at 1.3 V vs Ag/AgCl) compared to 3–3.8 mA cm−2 for photoanodes deposited in lower flow rate combinations. The higher photocurrents were ascribed to lower bulk resistivity and charge transfer resistance at the WO3/electrolyte interface. These photoanodes consisted of randomly oriented (002), (020) and (200) planes in contrast to the preferentially orientated (002) and (200) planes of photoanodes which were highly resistive with poorer photocurrent responses. These results were interpreted in terms of the effects of Ar:O2 flow rate combinations on the distribution of oxygen vacancies and formation of crystallographic shear planes in the sputtered films.