Crystallographic Shear Structures Research Articles

Crystallographic Shear Structures as a Route to Anion-Deficient Perovskites

Crystallographic Shear Structures as a Route to Anion-Deficient Perovskites

Crystallographic Shear Structures as a Route to Anion‐Deficient Perovskites

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

Superspace description of the homologous series Ga4Ti m−4O2m−2 with the crystallographic shear structure based on that of rutile

A generalized approach is proposed for the structure description of the homologous series Ga(4)Ti(m-4)O(2m-2) with crystallographic shear (CS) structures based on that of rutile. A (3 + 1)-dimensional model for an ideal CS structure is built up in connection with the CS operation in three-dimensional space. Its structural parameters are determined by m, the shear plane and the shear vector. The Rietveld fitting of the X-ray diffraction profile of Ga(4)Ti(13)O(32) (m = 17) was successfully carried out using the ideal CS structure as an initial model, where modulation functions of atomic positions are inherently discontinuous and sawtooth-like. The deviation of a real structure from the ideal one was described by a few additional Fourier terms. This method was confirmed to be efficient for reducing the number of structural parameters required in the refinement in comparison to the conventional three-dimensional description.

Ag 1/8Pr 5/8MoO 4: An incommensurately modulated scheelite-type structure

The crystal structure of a complex molybdenum oxide Ag 1/8Pr 5/8MoO 4 is reported. The Ag 1/8Pr 5/8MoO 4 sub-structure can be described on the base of the scheelite (CaWO 4) structure. Transmission electron microscopy reveals that the real structure is better described in a (3+1)D formalism. According to electron diffraction study the new scheelite-type complex oxide Ag 1/8Pr 5/8MoO 4 crystallizes in the B2/ b( αβ0)00 (3+1)D superspace group with unit cell parameters a ≈ 7.5 0 Å , b ≈ 5.3 0 Å , c ≈ 1 1.7 4 Å and γ ≈ 1 3 5 ° ( Z = 4 ) and modulation vector q = 0.5 6 a * + 0.5 9 b * . The structure of Ag 1/8Pr 5/8MoO 4 is refined from X-ray powder data in the scheelite setting I2/ b( αβ0) with a = b = 5.3 0 1 3 ( 4 ) Å , c = 1 1.7 4 0 7 ( 1 1 ) Å , q = 1.1 4 6 9 0 ( 1 4 ) a * + 0.5 8 9 2 1 ( 1 2 ) b * with fixed γ = 9 0 ° angle ( R p = 0.0 3 3 , R = 0.0 3 3 , R m = 0.0 2 9 , R 1 = 0.0 4 7 , S = 1.3 6 ). Displacement modulations apply for all atoms. The occupancy modulation shows that one-fourth of the Ag/Pr atoms are absent. The structure can be considered as a crystallographic shear structure with incommensurate ordering of vacancies and displacement modulations for all atoms. The arrangement of Ag/Pr atoms and vacancies is at the origin of the incommensurate modulation in the cation-deficient Ag 1/8Pr 5/8MoO 4 phase.

不規則性をもつ結晶，超イオン導電体，非晶質，ガラス，融体の構造解析―不規則系・ランダム系の結晶学―１．周期系における不規則性の解析 インコメンシュレート相の構造解析

Four-dimensional approach for incommensurate structures was briefly reviewed. Recent developments of the method using discontinuous modulation functions were explained, which enabled us to get the 4-dimensional description of structures forming the homologous series such as intergrowth compounds and crystallographic shear (CS) structures. An example of the incommensurate CS structure was given, in which domain boundaries were aperiodically inserted in contrast to the usual CS structures.

Incommensurate crystallographic shear structure of BaxBi2-2xTi4-xO11-4x(x= 0.275)

The title compound generates diffraction patterns which are indexable within the framework of the higher-dimensional description of incommensurate structures. However, it is difficult to discriminate the main reflections from the satellite ones. This paper has clarified that the structure can be treated as a strongly modulated structure with sawtooth-like modulation functions and is classified as an incommensurate crystallographic shear (CS) structure. The structure consists of domains isostructural to β-Bi2Ti4O11 and domain boundaries composed of TiO6 octahedra. Ba and Bi ions are accommodated in the cavities between TiO6 octahedra in the domain. Domain boundaries are aperiodically inserted, in contrast to the usual CS structures, forming an incommensurate structure.

Pyrogenic Iron(III)-Doped TiO2 Nanopowders Synthesized in RF Thermal Plasma: Phase Formation, Defect Structure, Band Gap, and Magnetic Properties

Iron(III)-doped TiO(2) nanopowders, with controlled iron to titanium atomic ratios (R(Fe/Ti)) ranging from nominal 0 to 20%, were synthesized using oxidative pyrolysis of liquid-feed metallorganic precursors in a radiation-frequency (RF) thermal plasma. The valence of iron doped in the TiO(2), phase formation, defect structures, band gaps, and magnetic properties of the resultant nanopowders were systematically investigated using Mössbauer spectroscopy, XRD, Raman spectroscopy, TEM/HRTEM, UV-vis spectroscopy, and measurements of magnetic properties. The iron doped in TiO(2) was trivalent (3+) in a high-spin state as determined by the isomer shift and quadrupole splitting from the Mössbauer spectra. No other phases except anatase and rutile TiO(2) were identified in the resultant nanopowders. Interestingly, thermodynamically metastable anatase predominated in the undoped TiO(2) nanopowders, which can be explained from a kinetic point of view based on classical homogeneous nucleation theory. With iron doping, the formation of rutile was strongly promoted because rutile is more tolerant than anatase to the defects such as oxygen vacancies resulting from the substitution of Fe(3+) for Ti(4+) in TiO(2). The concentration of oxygen vacancies reached a maximum at R(Fe/Ti) = 2% above which excessive oxygen vacancies tended to concentrate. As a result of this concentration, an extended defect like crystallographic shear (CS) structure was established. With iron doping, red shift of the absorption edges occurred in addition to the d-d electron transition of iron in the visible light region. The as-prepared iron-doped TiO(2) nanopowders were paramagnetic in nature at room temperature.

Incommensurate crystallographic shear structure of Ba x Bi2 − 2x Ti4 − x O11 − 4x (x = 0.275)

The title compound generates diffraction patterns which are indexable within the framework of the higher-dimensional description of incommensurate structures. However, it is difficult to discriminate the main reflections from the satellite ones. This paper has clarified that the structure can be treated as a strongly modulated structure with sawtooth-like modulation functions and is classified as an incommensurate crystallographic shear (CS) structure. The structure consists of domains isostructural to beta-Bi(2)Ti(4)O(11) and domain boundaries composed of TiO(6) octahedra. Ba and Bi ions are accommodated in the cavities between TiO(6) octahedra in the domain. Domain boundaries are aperiodically inserted, in contrast to the usual CS structures, forming an incommensurate structure.

PbMnO 2.75—a high-pressure phase having a new type of crystallographic shear structure derived from perovskite

PbMnO 2.75, prepared at 7.8 GPa and 880°C, has monoclinic symmetry, A2/ m, with cell parameters a=32.232(5)) Å, b=3.831(1)) Å, c=35.671(5) Å, β=130.00(1)°. Electron diffraction patterns of the phase show features characteristic of ReO 3-based crystallographic shear (CS) structures, and a model for the structure was developed on this basis which gave a good fit to powder X-ray diffraction and neutron diffraction patterns. The structure can be formally derived from the perovskite structure by periodic shear on (704) p planes, giving blocks of perovskite that are displaced relative to one another by the CS vector of 1 2 [110] p . Ordered oxygen vacancies occur at the CS planes which convert the MnO 6 octahedra to MnO 5 square pyramids, resulting in the formation of pseudo-hexagonal channels along [010]. Pb atoms occupy these channels and form pyramidal coordination to oxygen. The PbO 4 pyramids edge-share with the MnO 5 square pyramids to form elements with the topology of tetragonal PbO. Pb atoms also occupy the A-cation sites in the perovskite blocks. This is the first example of a structure based on CS of the ReO 3 structure where the A-cation sites are occupied, thus giving a perovskite CS phase.

Phase Relations in the System Fe2O3–Cr2O3–TiO2 between 1000 and 1300°C and the Stability of (Cr,Fe)2Tin−2O2n−1 Crystallographic Shear Structure Compounds

Phase relations and the stability of crystallographic shear (CS) structure compounds (Cr,Fe)2Tin−2O2n−1 in the system Fe2O3–Cr2O3–TiO2 were investigated between 1000 and 1300°C. The ternary comprises five major solid solution series. These are as follows: an M2O3 series; an M3O5 series made up of two separate solid solution series—the first an orthorhombic pseudobrookite M3O5 solid solution and the second a monoclinic M3O5 series based on the V3O5 structure type; an M4O7 series; and an M5O9 series. These latter three series represent lower homologues (n=3, 4, and 5) of the (Cr,Fe)2Tin−2O2n−1CS compound series. Between adjacent M3O5 and M4O7 and M4O7 and M5O9 solid solutions, ordered intergrowths may occur. The stability and compositional limits of the solid solution series and intergrowth phases are dependent upon the temperature and Fe:Cr ratio. At high-TiO2 contents, assemblages may contain either members of the Andersson phase series Cr2Tin−2O2n−1, a continuous CS structure series extending into the ternary, or a rutile-based solid solution. A comparison of results from this study with previously published phase relations has led to a revised version of the Fe2O3–Cr2O3–TiO2 phase diagram.

Effect of SR irradiation on crystallization of amorphous tin oxide film

In order to see the effect of SR irradiation on crystal growth, crystallization of tin oxide films has been performed in vacuum under SR irradiation. A thin amorphous tin oxide film 50 nm thick was prepared on the carbon substrate by vacuum evaporation of SnO 2 power. A SnO crystal appeared between 450–500°C upon vacuum heating, with a preferred orientation of (0 0 1). By SR irradiation using a cylindrical mirror for 20 s, the SnO crystal appeared with the preferred orientation of (1 1 1). The crystal with the crystallographic shear structure was grown by SR irradiation. This growth under a SR beam is discussed in terms of SR beam excitation of lone-pair electrons seen in the SnO crystal structure.

Tribological properties of thermal-sprayed Magnéli-type coatings with different stoichiometries (Ti nO 2 n−1 )

This paper summarizes results on the possibility of coating aluminium with titania-based Magnéli-type coatings by thermal spraying, as well as the tribological properties of these coatings. For this purpose, a spray powder with a defined phase composition was applied on AlMg substrates by atmospheric plasma spraying. Crystalline Magnéli phases of the type Ti n O 2 n−1 with a crystallographic shear structure could be identified after spraying in the coating by means of X-ray diffraction. Dry sliding experiments were conducted on a high-temperature pin-on-disc test rig against polycrystalline alumina at 23 and 400°C up to a sliding distance of 2000 m at 1 m/s. The coefficient of friction (COF) remains high, at between 0.7 and 0.8 for all tests. The best wear results are achieved for the Ti 6O 11 coatings, showing wear rates down to 1.2×10 −6 mm 3/N m. The tribological behaviour of the sprayed coatings indicates strong anti-wear properties under dry sliding conditions. Under mixed/boundary-lubricated conditions, the interaction of a low-additive polyalkyleneglycol (PAG100) with Magnéli-type phases results in very low friction and wear coefficients in the range of 10 −10 mm 3/N m.

Dislocation Loop Formation in Nonstoichiometric (Ba,Ca)TiO3and BaTiO3Ceramics

Dislocation loops observed in nonstoichiometric and stoichiometric (Ba,Ca)TiO3, and in stoichiometric BaTiO3 sintered in a reducing atmosphere, were characterized by conventional transmission electron microscopy (TEM) under two-beam conditions and high-resolution TEM atomic structure analysis. Dislocation loops mostly lay on {100} planes with Burgers vectors of type 〈100〉. The dynamic behavior of these dislocation loops during the electron beam irradiation (EBI), however, was classified into two different types of dislocation loops: in A-site-excess (Ba,Ca)TiO3, contrasts of dislocation loops faded completely away; in BaTiO3 and B-site-excess (Ba,Ca)TiO3, fine-line contrasts remained. Dislocation loops with Burgers vectors of type 1/2〈100〉 and the resultant crystallographic shear (CS) structure with a displacement vector of type 1/2〈110〉 after EBI were proposed to interpret residual line images. Disappearance of these line images in A-site-excess (Ba,Ca)TiO3 strongly suggests preferential Ca ion site occupancy at the CS structure.

High-Resolution Transmission Electron Microscopy and Computer Simulation of Defect Structures in Electronic Perovskite Ceramics.

Defect chemistry of electronic perovskite ceramics was quite advanced by recent studies using high-resolution transmission electron microscopy (TEM) and computer simulation techniques. Selective site occupation of alkali-earth metal, Ca at Ruddlesden-Popper (RP) faults, causing a break of conduction pathway within a framework of corner-sharing TiO 6 octahedra and improved leakage current properties, were unquestionable clarified in A-site-excess SrTiO 3 . Existence of meta-stable RP faults in BaTiO 3 processed under thermodynamically non-equilibrium condition was first empirically and then theoretically proved. Crystallographic shear (CS) structure TiO 2 double layer and BaO double layer were first observed and precisely characterized in perovskite ceramics. The obtained new information about defect structure can trigger new ideas about standing reliability issues and should open a challenge in atomic design, based upon computer structural energy calculation in electronic perovskite materials.

Characterization of indium tin oxide film and practical ITO film by electron microscopy

In order to clarify the structure of indium oxide film containing tin and tin oxides, various In 2O 3 based films prepared by vacuum evaporation were studied using high-resolution electron microscope (HREM). Indium tin oxide (ITO) film was composed of In 2O 3 and SnO. SnO crystal also contained (110) or (101) crystallographic shear (CS) structures that indicate excess amounts of tin. The CS structure was also found in a commercial ITO film having the resistivity of 2×10 −4 Ω cm.

Defect structure in homoepitaxial non-stoichiometric strontium titanate thin films

Abstract The defect structure of non-stoichiometric strontium titanate thin films epitaxially grown on SrTiO3(100) substrates by pulsed-laser deposition was studied using transmission electron microscopy and X-ray diffraction. The A-site-excess SrTiO3 thin film adopted a completely unrelaxed pseudomorphic perovskite structure with an enlarged lattice parameter normal to the surface. Excess SrO was accommodated as Ruddlesden-Popper planar faults showing a peculiar three-dimensional mosaic structure, without forming secondary phases or dislocations due to non-stoichiometry. Biaxial compressive stress induced by lattice mismatch between the film and substrate was estimated at 9,7 GPa. Such a highly strained film is attributable to planar fault formation, which increases the cell volume and suppresses misfit dislocations. The B-site-excess SrTiO3 thin-film microstructure consisted of crystalline SrTiO3 and amorphous TiO2-rich phases. The slight increase in the lattice parameter normal to the surface and crystallographic shear structure with a TiO2 double layer suggest the low solubility of excess TiO2.

Solid Solution of Epitaxially Grown (Sr, Mg)TiO3 Thin Films

Microstructures of (Sr1-xMgx)TiO3 thin films epitaxially grown on (001) SrTiO3 substrates were investigated by X-ray diffraction and high-resolution transmission electron microscopy to discuss the solid-solution of the SrTiO3–MgTiO3 system under pulsed laser deposition processes. In cases of x=0.15 and x=0.3, magnesium ions were mostly accommodated at A-sites of the perovskite structure of general formula ABO3, to form single-phase perovskite thin films without ilmenite secondary phases, and the possibility of partial B-site occupation of magnesium ions was inferred to interpret the formation of (100) and (010) planar defects with a crystallographic shear structure. The (Sr0.5Mg0.5)TiO3 thin film exhibits a phase-separated microstructure consisting of an epitaxial crystalline perovskite phase and an amorphous phase, which could be derived from the limitation of the magnesium solubility in the SrTiO3 perovskite lattice.

Wear Engineering Oxides/Antiwear Oxides©

The tribological behavior of self-mated ceramic composites, like SiC-TiC, Si3N4-TiN and (Ti, Mo) (C, N)+Ni, were characterized under dry-sliding friction. The coefficient of friction still remains unchanged on a level known from SiC and Si3N4 ceramics. The addition of titanium compounds to SiC and Si3N4 ceramics reduced the wear rate compared to titanium-free ceramics. Increasing sliding speed and temperature enhances this trend. The (Ti, Mo) (C, N) materials exhibited a wear behavior nearly independent from sliding speed and temperature with wear rates lower than 10−7 mm3/Nm up to 800°C. The present study utilized small-spot X-ray diffraction, laser raman spectroscopy (LRS) and transmission electron microscopy (TEM) TiO2 and TinO2n·1,-type crystalline Magneli-phases with a crystallographic shear structure as well as some double oxides like NiTiO3, Mo0.975Ti0.025O2 and β-NiMoO4, were detected on different ceramic substrates formed by tribo-oxidation during tests at 400°C and 800°C. The conditions during tribological operation are sufficient to generate crystalline Magneli-phases by tribo-oxidation, like γ -Ti3O5, Ti5O9 and Ti9O17.

Wear engineering oxides/anti-wear oxides

The tribological behaviour of self-mated ceramic composites, like SiCTiC, Si 3N 4TiN and (Ti,Mo)(C,N) + Ni, were characterized under dry sliding friction. The coefficient of friction still remain unchanged on a level known from SiC and Si 3N 4 ceramics. The addition of titanium compounds to SiC and Si 3N 4 ceramics reduced the wear rate compared to titanium-free ceramics. Increasing sliding speed and temperature enhances this trend. The (Ti,Mo)(C,N) materials exhibited a wear behaviour nearly independent from sliding speed and temperature with wear rates lower than 10 7mm 3/N m up to 800°C. The present study has by means of Small-Spot X-ray diffraction. Laser Raman Spectroscopy (LRS) and Transmission Electron Microscopy (TEM) demonstrated, that TiO 2 and crystalline Magnéli-phases of Ti n O 2 n type with a crystallographic shear structure as well as some double oxides like NiTiO 3, Mo 0.075Ti 0.025O 2 and β-NiMoO 4 were detected on different ceramic substrates formed by tribo-oxidation during tests at 400°C and 800°C. The conditions during tribological operation are sufficient to generate crystalline Magnèli-type phases by tribo-oxidation, like γ-Ti 3O 5, Ti 5O 9 and Ti 9O 17.

New Phases Formed in the Li–Ti–O System under Reducing Conditions

Phase equilibria in the low-lithia (<30 mole% Li2O) part of the system Li2O–TiO2–Ti2O3have been studied under reducing conditions established by H2/CO2mixtures. At 1100° C and a H2/CO2ratio of 70 (fO2=10−16.7atm), the Li2O–TiO2−Xjoin comprises alternating one- and two-phase regions in the range 30–6.5 mole% Li2O. Single-phase regions occur at compositions centered at ∼24 mole% Li2O (ramsdellite-type, Li2.2Ti3.5O8) and ∼13 mole% Li2O (1:1 ramsdellite:rutile ordered intergrowth, Li0.9Ti2.94O). By varying the temperature and H2/CO2ratio, the intergrowth phase was prepared as a single phase in the range 12.8–14.5 mole% Li2O. The intergrowth phase reacts readily with air, undergoing both oxidation of Ti3+to Ti4+, and hydroxylation/hydration of lithium in the structure to form LiOH·H2O. The reactions with air were quantified using weight gain experiments and Ti3+analyses. The composition region from 6.5 to 0.7 mole% Li2O is spanned by series of rutile-derived crystallographic shear (CS) structures. The CS plane orientation changes continuously with decreasing lithium content from (051)rto (021)r. At less reducing conditions, the (0kl)rCS phases become unstable relative to a high-temperature, lithium-stabilized anatase phase containing ∼4 mole% Li2O. Powder X-ray, neutron, and electron diffraction techniques have been applied to determine and refine the structures of the different phases.