Al-excessive feldspars: a hypothetical model with octahedrally coordinated Al in planar defects

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.

Polaron interaction energies in reduced tungsten trioxide

Strain energy between parallel {001} crystallographic shear planes in the tungsten trioxide structure

Extended vacuum annealing of rutile ${\mathrm{TiO}}_{2}(110)$ produces macroscopic and nanoscopic changes in the surface morphology. These result from the ordering of oxygen ${\mathrm{v}\mathrm{a}\mathrm{c}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{s}/\mathrm{T}\mathrm{i}}^{n+}$ interstitials in bulk ${\mathrm{TiO}}_{2}$ into crystallographic shear (CS) planes. We have employed low-energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) to identify the surface termination of CS planes produced in the bulk. The directions of the planar defects along the surface indicate the {132} series of shear planes dominate the STM images, which is also supported by the LEED results. The spacing between planes gradually varies across the surface suggesting that a series of Magn\'eli phases are formed. LEED patterns confirm the coexistence of both relatively large areas of a single phase and a continuum of separations between planes. Formation of CS planes also leads to extensive facetting of the crystal, which is visible to the naked eye as a rippled surface finish. Some Ca segregation is observed during CS plane formation; however, when removed the CS planes remain, indicating they are not significantly stabilized by the Ca.

Factors affecting the structure and orientation of the crystallographic shear (CS) planes in anion-deficient perovskites were investigated using the (Pb(1-z)Sr(z))(1-x)Fe(1+x)O(3-y) perovskites as a model system. The isovalent substitution of Sr(2+) for Pb(2+) highlights the influence of the A cation electronic structure because these cations exhibit very close ionic radii. Two compositional ranges have been identified in the system: 0.05 ≤ z ≤ 0.2, where the CS plane orientation gradually varies but stays close to (203)p, and 0.3 ≤ z ≤ 0.45 with (101)p CS planes. The incommensurately modulated structure of Pb0.792Sr0.168Fe1.040O2.529 was refined from neutron powder diffraction data using the (3 + 1)D approach (space group 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)°, q = 0.0879(1)a* + 0.1276(1)c*, RF = 0.023, RP = 0.029, and T = 900 K). A comparison of the compounds with different CS planes indicates that the orientation of the CS planes is governed mainly by the stereochemical activity of the lone-electron-pair cations inside the perovskite blocks.

Because of the technological importance of perovskite-based oxides, numerous pathways towards new structures have been developed, including ordering at the A and/or B cation sites of the perovskite (ABO3) structure, vacancy ordering at the anion site, the intergrowth of perovskite and related structures, and the formation of hexagonal perovskite polytypes. However, it has been generally accepted that crystallographic shear (CS) structures cannot be realized in perovskites. CS planes commonly occur in anion-deficient oxides derived from the ReO3 or rutile (TiO2) structures, which are based on metal-centered octahedra. Arrangement of the CS planes at different periodicities generates a homologous series. Some of the well-known examples of CS structures are MnO3n 1, MnO3n 2 (M=Mo, W), TinO2n 1, and the block structures of Nb2O5 and its Ti-containing derivatives. [2–6] The shear operation is such that along the CS plane the octahedra share edges or faces, instead of corners or edges as in the basic structure. The ReO3 structure is based on the same threedimensional framework of corner-sharing BO6 octahedra found in the perovskite structure, but the 12-coordinate A cation sites of the perovskite structure are empty in ReO3. Usually, the presence of A cations in perovskites favors point defects in the anion sites (in a random or ordered arrangement), leading to a lower coordination number for the B cations, and prevents the elimination of anion point defects by a shear operation. Herein, we report the occurrence of CS structures in perovskite-based “Pb2Fe2O5” for the first time. We also describe the general crystallographic mechanism of CS-plane formation in a perovskite framework with the A sites filled by cations bearing a lone 6s electron pair, which involves the elimination of O vacancies by the shear operation and the relaxation of the structure at the CS plane. We discuss the role of the lone electron pair in directing structure formation, as well as the relationship between the crystallographic orientation of the CS plane and the chemical composition. The discovery of this mechanism opens up new possibilities for the design of novel perovskite-based compounds containing A cations with a lone 6s electron pair. “Pb2Fe2O5” (or “PbFeO2.5”) is expected to be an aniondeficient perovskite of the brownmillerite (Ca2AlFeO5) type; however, its structure has not yet been determined. Because of the complexity of the structure and the prevalence of domain fragmentation, the only road to solving the structure is by transmission electron microscopy (TEM). The brighter reflections in the electron diffraction (ED) patterns of “Pb2Fe2O5” (Figure 1a) reveal a perovskite sublattice with a parameter ap 3.9 < (p denotes the perovskite subcell). Linear arrays of satellite reflections associated with each basic reciprocal lattice node are typical for periodic CS planes. The displacement vector for the CS planes was deduced from the ED patterns as R= 1/2 [110]p+ 1/ 3 [001]p by measuring the fractional shifts hR for the satellite reflections (Figure 1b). 11][*] Closer inspection of the [010]* ED pattern reveals the incommensurate character of the modulation. The monoclinic unit-cell parameters (denoted by

Fe(II) only occupies octahedral sites in phyllosilicates, whereas Fe(III) can occupy both octahedral and tetrahedral sites. The controls on Fe(III) distribution between tetrahedral and octahedral sites have been a matter of great interest to understand the interplay between formation environment (Fe abundance, redox conditions) and crystal-chemical factors (stability of the crystal lattice) during crystallization of Fe-phyllosilicates. Here, for the first time, we present a model of Fe(III) distribution in 2:1 phyllosilicates. We investigated 21 samples of 2:1 phyllosilicates of submarine hydrothermal origin using XRD, chemical analysis, and Mössbauer spectroscopy (and other supporting techniques not presented here). An additional data set of 49 analyses of 2:1 phyllosilicates from the literature was also used. Overall, the data cover a wide range of dioctahedral and trioctahedral phyllosilicates, including end-member minerals and interstratified phases. Dioctahedral phyllosilicates have a steric control whereby tetrahedral Fe(III) is only allowed if at least five out of six octahedral atoms are larger than Al (typically Fe[III], Fe[II], Mg) that produces an expanded structure where tetrahedral sites can accommodate Fe(III). After this threshold, further Fe(III) atoms occupy tetrahedral sites preferentially (~73% of further Fe[III] atoms) over octahedral sites. In trioctahedral 2:1 phyllosilicates there is no steric hindrance to tetrahedral Fe(III) because the crystal dimensions are such that tetrahedral sites can accommodate Fe(III). On average, Fe(III) enters tetrahedral and octahedral sites in similar proportion, and the only apparent control on tetrahedral Fe(III) abundance is Fe(III) availability during crystallization. This model allows to predict Fe(III) distribution between structural sites, provides an avenue for further exploration of the thermodynamic stability of phyllosilicates using cationic size, and provides a tool to better describe stability/reactivity of Fe-rich phyllosilicates, the most reactive of phyllosilicates and very relevant in geochemical and biological processes.

Crystal structures of V nO 2 n−1 (2≤ n≤7)

Interplay between bulk and intrinsic pinning and commensurability effects in high- Tc superconductors

The thermoelectric properties of Magnèli phase titanium oxides TinO2n−1 (n=2,3,…) have been investigated, paying special attention to how the thermoelectric performance can be altered by changing the microstructure. Dense polycrystalline specimens with nominal composition of TiO2−x (x=0.05, 0.10, 0.15, and 0.20) prepared by conventional hot-pressing are all identified to be one of the Magnèli phases, in which crystallographic shear planes are regularly introduced according to the oxygen deficiency. Electrical conduction is n-type for all specimens and the carrier concentration increases with the increase in the oxygen deficiency. The values of lattice thermal conductivity, on the other hand, decrease with the increase in the oxygen deficiency, which can be attributed to phonon scattering at the crystallographic shear plane. The largest value of thermoelectric figure of merit Z, 1.6×10−4 K−1 was obtained at 773 K for the hot-pressed specimen of TiO1.90.

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.

The Mössbauer effect has been used to study the microscopic magnetic properties of the cubic microwave ferrite (Zn0.41Ni0.59)Fe2O4 at 300 K. The magnetically split spectrum is broad and unresolved due to the overlap of the hyperfine magnetic fields at the tetrahedral and octahedral sites. This is caused by a distribution of magnetic fields at each site. The distribution is especially large and asymmetric at the octahedral site and is due to the fact that the iron atoms at this site are influenced by the presence of both iron and zinc at the tetrahedral site. The octahedral site is more sensitive to changes in the number of zinc neighbors than is the tetrahedral site. Assuming a random distribution of the zinc at the tetrahedral site, and knowing the fraction of zinc in the ferrite, the binomial distribution has been used to calculate the expected magnetic-field distribution at the octahedral site as a function of the number of zinc neighbors. These figures have been compared with the magnetic-field distribution profile determined from the fit to the Mössbauer spectrum. Sufficiently good agreement has been found to allow us to isolate the magnetic-field contribution due to most of the different numbers, between zero and six, of neighboring zinc atoms.

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.

A near infrared (NIR) spectroscopy approach was undertaken to collect information on the Al cationic distribution in tetrahedral and octahedral sites in natural chlorite (clinochlore) samples. Structural formulae were established using electron microprobe and Mössbauer spectroscopy. A band located near 7115 cm–1was attributed to the overtone of OH fundamental stretching mode of Mg2AlOH and increases with the total Al amount. Good correlation was obtained between the (SiAl)O–OH vibration band area and tetrahedral Al content, making it easy to partition Al (and thus Fe3+) between octahedral and tetrahedral sites.

Cation distribution in magnetoplumbite and β′′-alumina structures