Disproportionation of Co2+ in the Topochemically Reduced Oxide LaSrCoRuO5

Complex transition-metal oxides exhibit a wide variety of chemical and physical properties which are a strong function the local electronic states of the transition-metal centres, as determined by a combination of metal oxidation state and local coordination environment. Topochemical reduction of the double perovskite oxide, LaSrCoRuO6 , using Zr, yields LaSrCoRuO5 . This reduced phase contains an ordered array of apex-linked square-based pyramidal Ru3+ O5 , square-planar Co1+ O4 and octahedral Co3+ O6 units, consistent with the coordination-geometry driven disproportionation of Co2+ . Coordination-geometry driven disproportionation of d7 transition-metal cations (e.g. Rh2+ , Pd3+ , Pt3+ ) is common in complex oxides containing 4d and 5d metals. However, the weak ligand field experienced by a 3d transition-metal such as cobalt leads to the expectation that d7+ Co2+ should be stable to disproportionation in oxide environments, so the presence of Co1+ O4 and Co3+ O6 units in LaSrCoRuO5 is surprising. Low-temperature measurements indicate LaSrCoRuO5 adopts a ferromagnetically ordered state below 120 K due to couplings between S=1 /2 Ru3+ and S=1 Co1+ .

Solution-gel-based surface modification of LiNi0.5Mn1.5O4-δ with amorphous Li-Ti-O coating

LNMO (LiNi0.5Mn1.5O4-δ) is a high-energy density positive electrode material for lithium ion batteries. Unfortunately, it suffers from capacity loss and impedance rise during cycling due to electrolyte oxidation and electrode/electrolyte interface instabilities at high operating voltages. Here, a solution-gel synthesis route was used to coat 0.5-2.5 μm LNMO particles with amorphous Li-Ti-O (LTO) for improved Li conduction, surface structural stability and cyclability. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) analysis coupled with energy dispersive X-ray (EDX) showed Ti-rich amorphous coatings/islands or Ti-rich spinel layers on many of the LTO-modified LNMO facets, with a thickness varying from about 1 to 10 nm. The surface modification in the form of amorphous islands was mostly possible on high-energy crystal facets. Physicochemical observations were used to propose a molecular mechanism for the surface modification, combining insights from metalorganic chemistry with the crystallographic properties of LNMO. The improvements in functional properties were investigated in half cells. The cell impedance increased faster for the bare LNMO compared to amorphous LTO modified LNMO, resulting in Rct values as high as 1247 Ω (after 1000 cycles) for bare LNMO, against 216 Ω for the modified material. At 10C, the modified material boosted a 15% increase in average discharge capacity. The improvements in electrochemical performance were attributed to the increase in electrochemically active surface area, as well as to improved HF-scavenging, resulting in the formation of protective byproducts, generating a more stable interface during prolonged cycling.

Impact of Anionic Ordering on the Iron Site Distribution and Valence States in Oxyfluoride Sr2FeO3+xF1-x (x = 0.08, 0.2) with a Layered Perovskite Network

Sr₂FeO₃F, an oxyfluoride compound with an n = 1 Ruddlesden-Popper structure, was identified as a potential interesting mixed ionic and electronic conductor (MIEC). The phase can be synthesized under a range of different pO₂ atmospheres, leading to various degrees of fluorine for oxygen substitution and Fe⁴⁺ content. A structural investigation and thorough comparison of both argon- and air-synthesized compounds were performed by combining high-resolution X-ray and electron diffraction, high-resolution scanning transmission electron microscopy, Mössbauer spectroscopy, and DFT calculations. While the argon-synthesized phase shows a well-behaved O/F ordered structure, this study revealed that oxidation leads to averaged large-scale anionic disorder on the apical site. In the more oxidized Sr₂FeO_3.2F_0.8 oxyfluoride, containing 20% of Fe⁴⁺, two different Fe positions can be identified with a 32%/68% occupancy (P4/nmm space group). This originates due to the presence of antiphase boundaries between ordered domains within the grains. Relations between site distortion and valence states as well as stability of apical anionic sites (O vs F) are discussed. This study paves the way for further studies on both ionic and electronic transport properties of Sr₂FeO_3.2F_0.8 and its use in MIEC-based devices, such as solid oxide fuel cells.

Anion redox as a means to derive layered manganese oxychalcogenides with exotic intergrowth structures

Topochemistry enables step-by-step conversions of solid-state materials often leading to metastable structures that retain initial structural motifs. Recent advances in this field revealed many examples where relatively bulky anionic constituents were actively involved in redox reactions during (de)intercalation processes. Such reactions are often accompanied by anion-anion bond formation, which heralds possibilities to design novel structure types disparate from known precursors, in a controlled manner. Here we present the multistep conversion of layered oxychalcogenides Sr₂MnO₂Cu_1.5Ch₂ (Ch = S, Se) into Cu-deintercalated phases where antifluorite type [Cu_1.5Ch₂]^2.5- slabs collapsed into two-dimensional arrays of chalcogen dimers. The collapse of the chalcogenide layers on deintercalation led to various stacking types of Sr₂MnO₂Ch₂ slabs, which formed polychalcogenide structures unattainable by conventional high-temperature syntheses. Anion-redox topochemistry is demonstrated to be of interest not only for electrochemical applications but also as a means to design complex layered architectures.

Nanoscale phase separation in the oxide layer at GeTe (111) surfaces

As a semiconductor ferroelectric, GeTe has become a focus of renewed attention due to the recent discovery of giant Rashba splitting. It already has a wide range of applications, from thermoelectricity to data storage. Its stability in ambient air, as well as the structure and properties of an oxide layer, define the processing media for device production and operation. Here, we studied a reaction between the GeTe (111) surface and molecular oxygen for crystals having solely inversion domains. We evaluated the reaction kinetics both ex situ and in situ using NAP XPS. The structure of the oxide layer is extensively discussed, where, according to HAADF-STEM and STEM-EDX, nanoscale phase separation of GeO₂ and Te is observed, which is unusual for semiconductors. We believe that such behaviour is closely related to the ferroelectric properties and the domain structure of GeTe.

Structures and Magnetic Ordering in Layered Cr Oxide Arsenides Sr2CrO2Cr2OAs2 and Sr2CrO3CrAs

Two novel chromium oxide arsenide materials have been synthesized, Sr₂CrO₂Cr₂OAs₂ (i.e., Sr₂Cr₃As₂O₃) and Sr₂CrO₃CrAs (i.e., Sr₂Cr₂AsO₃), both of which contain chromium ions in two distinct layers. Sr₂CrO₂Cr₂OAs₂ was targeted following electron microscopy measurements on a related phase. It crystallizes in the space group P4/mmm and accommodates distorted CrO₄As₂ octahedra containing Cr²⁺ and distorted CrO₂As₄ octahedra containing Cr³⁺. In contrast, Sr₂CrO₃CrAs incorporates Cr³⁺ in CrO₅ square-pyramidal coordination in [Sr₂CrO₃]⁺ layers and Cr²⁺ ions in CrAs₄ tetrahedra in [CrAs]⁻ layers and crystallizes in the space group P4/nmm. Powder neutron diffraction data reveal antiferromagnetic ordering in both compounds. In Sr₂CrO₃CrAs the Cr²⁺ moments in the [CrAs]⁻ layers exhibit long-range ordering, while the Cr³⁺ moments in the [Sr₂CrO₃]⁺ layers only exhibit short-range ordering. However, in Sr₂CrO₂Cr₂OAs₂, both the Cr²⁺ moments in the CrO₄As₂ environments and the Cr³⁺ moments in the CrO₂As₄ polyhedra are long-range-ordered below 530(10) K. Above this temperature, only the Cr³⁺ moments are ordered with a Néel temperature slightly in excess of 600 K. A subtle structural change is evident in Sr₂CrO₂Cr₂OAs₂ below the magnetic ordering transitions.

Sr₂CrO₂Cr₂OAs₂ and Sr₂CrO₃CrAs are both mixed-anion materials containing chromium ions in two unique layers. In Sr₂CrO₂Cr₂OAs₂, Cr³⁺ ions in CrO₂As₄ environments order antiferromagnetically at around 600 K and Cr²⁺ ions in CrO₄As₂ environments also order antiferromagnetically at a lower temperature of 530(10) K. In contrast, only the Cr²⁺ moments in the [CrAs]⁻ layers exhibit long-range ordering in Sr₂CrO₃CrAs as the Cr³⁺ moments in the [Sr₂CrO₃]⁺ layers only exhibit short-range ordering.

Chemistry, Local Molybdenum Clustering, and Electrochemistry in the Li2+xMo1-xO3 Solid Solutions

A broad range of cationic nonstoichiometry has been demonstrated for the Li-rich layered rock-salt-type oxide Li₂MoO₃, which has generally been considered as a phase with a well-defined chemical composition. Li_2+xMo_1-xO₃ (-0.037 ≤ x ≤ 0.124) solid solutions were synthesized via hydrogen reduction of Li₂MoO₄ in the temperature range of 650-1100 °C, with x decreasing with the increase of the reduction temperature. The solid solutions adopt a monoclinically distorted O3-type layered average structure and demonstrate a robust local ordering of the Li cations and Mo₃ triangular clusters within the mixed Li/Mo cationic layers. The local structure was scrutinized in detail by electron diffraction and aberration-corrected scanning transmission electron microcopy (STEM), resulting in an ordering model comprising a uniform distribution of the Mo₃ clusters compatible with local electroneutrality and chemical composition. The geometry of the triangular clusters with their oxygen environment (Mo₃O₁₃ groups) has been directly visualized using differential phase contrast STEM imaging. The established local structure was used as input for density functional theory (DFT)-based calculations; they support the proposed atomic arrangement and provide a plausible explanation for the staircase galvanostatic charge profiles upon electrochemical Li⁺ extraction from Li_2+xMo_1-xO₃ in Li cells. According to DFT, all electrochemical capacity in Li_2+xMo_1-xO₃ solely originates from the cationic Mo redox process, which proceeds via oxidation of the Mo₃ triangular clusters into bent Mo₃ chains where the electronic capacity of the clusters depends on the initial chemical composition and Mo oxidation state defining the width of the first charge low-voltage plateau. Further oxidation at the high-voltage plateau proceeds through decomposition of the Mo₃ chains into Mo₂ dimers and further into individual Mo⁶⁺ cations.

The crystal and defect structures of polar KBiNb2O7

KBiNb₂O₇ was prepared from RbBiNb₂O₇ by a sequence of cation exchange reactions which first convert RbBiNb₂O₇ to LiBiNb₂O₇, before KBiNb₂O₇ is formed by a further K-for-Li cation exchange. A combination of neutron, synchrotron X-ray and electron diffraction data reveal that KBiNb₂O₇ adopts a polar, layered, perovskite structure (space group A11m) in which the BiNb₂O₇ layers are stacked in a (0, ½, z) arrangement, with the K⁺ cations located in half of the available 10-coordinate interlayer cation sites. The inversion symmetry of the phase is broken by a large displacement of the Bi³⁺ cations parallel to the y-axis. HAADF-STEM images reveal that KBiNb₂O₇ exhibits frequent stacking faults which convert the (0, ½, z) layer stacking to (½, 0, z) stacking and vice versa, essentially switching the x- and y-axes of the material. By fitting the complex diffraction peak shape of the SXRD data collected from KBiNb₂O₇ it is estimated that each layer has approximately a 9% chance of being defective - a high level which is attributed to the lack of cooperative NbO₆ tilting in the material, which limits the lattice strain associated with each fault.

The influence of the 6s2 configuration of Bi3+ on the structures of A'BiNb2O7 (A' = Rb, Na, Li) layered perovskite oxides

Solid state compounds which exhibit non-centrosymmetric crystal structures are of great interest due to the physical properties they can exhibit. The 'hybrid improper' mechanism - in which two non-polar distortion modes couple to, and stabilize, a further polar distortion mode, yielding an acentric crystal structure - offers opportunities to prepare a range of novel non-centrosymmetric solids, but examples of compounds exhibiting acentric crystal structures stabilized by this mechanism are still relatively rare. Here we describe a series of bismuth-containing layered perovskite oxide phases, RbBiNb₂O₇, LiBiNb₂O₇ and NaBiNb₂O₇, which have structural frameworks compatible with hybrid-improper ferroelectricity, but also contain Bi³⁺ cations which are often observed to stabilize acentric crystal structures due to their 6s² electronic configurations. Neutron powder diffraction analysis reveals that RbBiNb₂O₇ and LiBiNb₂O₇ adopt polar crystal structures (space groups I2cm and B2cm respectively), compatible with stabilization by a trilinear coupling of non-polar and polar modes. The Bi³⁺ cations present are observed to enhance the magnitude of the polar distortions of these phases, but are not the primary driver for the acentric structure, as evidenced by the observation that replacing the Bi³⁺ cations with Nd³⁺ cations does not change the structural symmetry of the compounds. In contrast the non-centrosymmetric, but non-polar structure of NaBiNb₂O₇ (space group P2₁2₁2₁) differs significantly from the centrosymmetric structure of NaNdNb₂O₇, which is attributed to a second-order Jahn-Teller distortion associated with the presence of the Bi³⁺ cations.

Topochemical Deintercalation of Li from Layered LiNiB: toward 2D MBene

The pursuit of two-dimensional (2D) borides, MBenes, has proven to be challenging, not the least because of the lack of a suitable precursor prone to the deintercalation. Here, we studied room-temperature topochemical deintercalation of lithium from the layered polymorphs of the LiNiB compound with a considerable amount of Li stored in between [NiB] layers (33 at. % Li). Deintercalation of Li leads to novel metastable borides (Li_∼0.5NiB) with unique crystal structures. Partial removal of Li is accomplished by exposing the parent phases to air, water, or dilute HCl under ambient conditions. Scanning transmission electron microscopy and solid-state ⁷Li and ¹¹B NMR spectroscopy, combined with X-ray pair distribution function (PDF) analysis and DFT calculations, were utilized to elucidate the novel structures of Li_∼0.5NiB and the mechanism of Li-deintercalation. We have shown that the deintercalation of Li proceeds via a "zip-lock" mechanism, leading to the condensation of single [NiB] layers into double or triple layers bound via covalent bonds, resulting in structural fragments with Li[NiB]₂ and Li[NiB]₃ compositions. The crystal structure of Li_∼0.5NiB is best described as an intergrowth of the ordered single [NiB], double [NiB]₂, or triple [NiB]₃ layers alternating with single Li layers; this explains its structural complexity. The formation of double or triple [NiB] layers induces a change in the magnetic behavior from temperature-independent paramagnets in the parent LiNiB compounds to the spin-glassiness in the deintercalated Li_∼0.5NiB counterparts. LiNiB compounds showcase the potential to access a plethora of unique materials, including 2D MBenes (NiB).

Magnetic Ordering in the Layered Cr(II) Oxide Arsenides Sr2CrO2Cr2As2 and Ba2CrO2Cr2As2

Sr₂CrO₂Cr₂As₂ and Ba₂CrO₂Cr₂As₂ with Cr²⁺ ions in CrO₂ sheets and in CrAs layers crystallize with the Sr₂Mn₃Sb₂O₂ structure (space group I4/mmm, Z = 2) and lattice parameters a = 4.00800(2) Å, c = 18.8214(1) Å (Sr₂CrO₂Cr₂As₂) and a = 4.05506(2) Å, c = 20.5637(1) Å (Ba₂CrO₂Cr₂As₂) at room temperature. Powder neutron diffraction reveals checkerboard-type antiferromagnetic ordering of the Cr²⁺ ions in the arsenide layers below T_{N1_Sr}, of 600(10) K (Sr₂CrO₂Cr₂As₂) and T_{N1_Ba} 465(5) K (Ba₂CrO₂Cr₂As₂) with the moments initially directed perpendicular to the layers in both compounds. Checkerboard-type antiferromagnetic ordering of the Cr²⁺ ions in the oxide layer below 230(5) K for Ba₂CrO₂Cr₂As₂ occurs with these moments also perpendicular to the layers, consistent with the orientation preferences of d⁴ moments in the two layers. In contrast, below 330(5) K in Sr₂CrO₂Cr₂As₂, the oxide layer Cr²⁺ moments are initially oriented in the CrO₂ plane; but on further cooling, these moments rotate to become perpendicular to the CrO₂ planes, while the moments in the arsenide layers rotate by 90° with the moments on the two sublattices remaining orthogonal throughout [behavior recently reported independently by Liu et al. [Liu et al. Phys. Rev. B 2018, 98, 134416]]. In Sr₂CrO₂Cr₂As₂, electron diffraction and high resolution powder X-ray diffraction data show no evidence for a structural distortion that would allow the two Cr²⁺ sublattices to couple, but high resolution neutron powder diffraction data suggest a small incommensurability between the magnetic structure and the crystal structure, which may account for the coupling of the two sublattices and the observed spin reorientation. The saturation values of the Cr²⁺ moments in the CrO₂ layers (3.34(1) μ_B (for Sr₂CrO₂Cr₂As₂) and 3.30(1) μ_B (for Ba₂CrO₂Cr₂As₂)) are larger than those in the CrAs layers (2.68(1) μ_B for Sr₂CrO₂Cr₂As₂ and 2.298(8) μ_B for Ba₂CrO₂Cr₂As₂) reflecting greater covalency in the arsenide layers.

Recent Advances in Transmission Electron Microscopy for Materials Science at the EMAT Lab of the University of Antwerp

The rapid progress in materials science that enables the design of materials down to the nanoscale also demands characterization techniques able to analyze the materials down to the same scale, such as transmission electron microscopy. As Belgium's foremost electron microscopy group, among the largest in the world, EMAT is continuously contributing to the development of TEM techniques, such as high-resolution imaging, diffraction, electron tomography, and spectroscopies, with an emphasis on quantification and reproducibility, as well as employing TEM methodology at the highest level to solve real-world materials science problems. The lab's recent contributions are presented here together with specific case studies in order to highlight the usefulness of TEM to the advancement of materials science.

Grain-Boundary Engineering for Aging and Slow-Crack-Growth Resistant Zirconia

Ceramic materials are prone to slow crack growth, resulting in strength degradation over time. Although yttria-stabilized zirconia (Y-TZP) ceramics have higher crack resistance than other dental ceramics, their aging susceptibility threatens their long-term performance in aqueous environments such as the oral cavity. Unfortunately, increasing the aging resistance of Y-TZP ceramics normally reduces their crack resistance. Our recently conducted systematic study of doping 3Y-TZP with various trivalent cations revealed that lanthanum oxide (La₂O₃) and aluminum oxide (Al₂O₃) have the most potent effect to retard the aging kinetics of 3Y-TZP. In this study, the crack-propagation behavior of La₂O₃ and Al₂O₃ co-doped 3Y-TZP ceramics was investigated by double-torsion methods. The grain boundaries were examined using scanning transmission electron microscopy and energy-dispersive spectroscopy (STEM-EDS). Correlating these analytic data with hydrothermal aging studies using different doping systems, a strategy to strongly bind the segregated dopant cations with the oxygen vacancies at the zirconia-grain boundary was found to improve effectively the aging resistance of Y-TZP ceramics without affecting the resistance to crack propagation.

Crystal Structure, Defects, Magnetic and Dielectric Properties of the Layered Bi3n+1Ti7Fe3n-3O9n+11 Perovskite-Anatase Intergrowths

The Bi_3n+1Ti₇Fe_3n-3O_9n+11 materials are built of (001)_p plane-parallel perovskite blocks with a thickness of n (Ti,Fe)O₆ octahedra, separated by periodic translational interfaces. The interfaces are based on anatase-like chains of edge-sharing (Ti,Fe)O₆ octahedra. Together with the octahedra of the perovskite blocks, they create S-shaped tunnels stabilized by lone pair Bi³⁺ cations. In this work, the structure of the n = 4-6 Bi_3n+1Ti₇Fe_3n-3O_9n+11 homologues is analyzed in detail using advanced transmission electron microscopy, powder X-ray diffraction, and Mössbauer spectroscopy. The connectivity of the anatase-like chains to the perovskite blocks results in a 3a_p periodicity along the interfaces, so that they can be located either on top of each other or with shifts of ±a_p along [100]_p. The ordered arrangement of the interfaces gives rise to orthorhombic Immm and monoclinic A2/m polymorphs with the unit cell parameters a = 3a_p, b = b_p, c = 2(n + 1)c_p and a = 3a_p, b = b_p, c = 2(n + 1)c_p - a_p, respectively. While the n = 3 compound is orthorhombic, the monoclinic modification is more favorable in higher homologues. The Bi_3n+1Ti₇Fe_3n-3O_9n+11 structures demonstrate intricate patterns of atomic displacements in the perovskite blocks, which are supported by the stereochemical activity of the Bi³⁺ cations. These patterns are coupled to the cationic coordination of the oxygen atoms in the (Ti,Fe)O₂ layers at the border of the perovskite blocks. The coupling is strong in the n = 3, 4 homologues, but gradually reduces with the increasing thickness of the perovskite blocks, so that, in the n = 6 compound, the dominant mode of atomic displacements is aligned along the interface planes. The displacements in the adjacent perovskite blocks tend to order antiparallel, resulting in an overall antipolar structure. The Bi_3n+1Ti₇Fe_3n-3O_9n+11 materials demonstrate an unusual diversity of structure defects. The n = 4-6 homologues are robust antiferromagnets below T_N = 135, 220, and 295 K, respectively. They show a high dielectric constant that weakly increases with temperature and is relatively insensitive to the Ti/Fe ratio.

Complex Microstructure and Magnetism in Polymorphic CaFeSeO

The structural complexity of the antiferromagnetic oxide selenide CaFeSeO is described. The compound contains puckered FeSeO layers composed of FeSe₂O₂ tetrahedra sharing all their vertexes. Two polymorphs coexist that can be derived from an archetype BaZnSO structure by cooperative tilting of the FeSe₂O₂ tetrahedra. The polymorphs differ in the relative arrangement of the puckered layers of vertex-linked FeSe₂O₂ tetrahedra. In a noncentrosymmetric Cmc2₁ polymorph (a = 3.89684(2) Å, b = 13.22054(8) Å, c = 5.93625(2) Å) the layers are related by the C-centering translation, while in a centrosymmetric Pmcn polymorph, with a similar cell metric (a = 3.89557(6) Å, b = 13.2237(6) Å, c = 5.9363(3) Å), the layers are related by inversion. The compound shows long-range antiferromagnetic order below a Neél temperature of 159(1) K with both polymorphs showing antiferromagnetic coupling via Fe-O-Fe linkages and ferromagnetic coupling via Fe-Se-Fe linkages within the FeSeO layers. The magnetic susceptibility also shows evidence for weak ferromagnetism which is modeled in the refinements of the magnetic structure as arising from an uncompensated spin canting in the noncentrosymmetric polymorph. There is also a spin glass component to the magnetism which likely arises from the disordered regions of the structure evident in the transmission electron microscopy.

Strength, toughness and aging stability of highly-translucent Y-TZP ceramics for dental restorations

OBJECTIVE

The aim was to evaluate the optical properties, mechanical properties and aging stability of yttria-stabilized zirconia with different compositions, highlighting the influence of the alumina addition, Y₂O₃ content and La₂O₃ doping on the translucency.

METHODS

Five different Y-TZP zirconia powders (3 commercially available and 2 experimentally modified) were sintered under the same conditions and characterized by X-ray diffraction with Rietveld analysis and scanning electron microscopy (SEM). Translucency (n=6/group) was measured with a color meter, allowing to calculate the translucency parameter (TP) and the contrast ratio (CR). Mechanical properties were appraised with four-point bending strength (n=10), single edge V-notched beam (SEVNB) fracture toughness (n=8) and Vickers hardness (n=10). The aging stability was evaluated by measuring the tetragonal to monoclinic transformation (n=3) after accelerated hydrothermal aging in steam at 134°C, and the transformation curves were fitted by the Mehl-Avrami-Johnson (MAJ) equation. Data were analyzed by one-way ANOVA, followed by Tukey's HSD test (α=0.05).

RESULTS

Lowering the alumina content below 0.25wt.% avoided the formation of alumina particles and therefore increased the translucency of 3Y-TZP ceramics, but the hydrothermal aging stability was reduced. A higher yttria content (5mol%) introduced about 50% cubic zirconia phase and gave rise to the most translucent and aging-resistant Y-TZP ceramics, but the fracture toughness and strength were considerably sacrificed. 0.2mol% La₂O₃ doping of 3Y-TZP tailored the grain boundary chemistry and significantly improved the aging resistance and translucency. Although the translucency improvement by La₂O₃ doping was less effective than for introducing a substantial amount of cubic zirconia, this strategy was able to maintain the mechanical properties of typical 3Y-TZP ceramics.

SIGNIFICANCE

Three different approaches were compared to improve the translucency of 3Y-TZP ceramics.

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.

KCN Chemical Etch for Interface Engineering in Cu2ZnSnSe4 Solar Cells

The removal of secondary phases from the surface of the kesterite crystals is one of the major challenges to improve the performances of Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells. In this contribution, the KCN/KOH chemical etching approach, originally developed for the removal of CuxSe phases in Cu(In,Ga)(S,Se)2 thin films, is applied to CZTSe absorbers exhibiting various chemical compositions. Two distinct electrical behaviors were observed on CZTSe/CdS solar cells after treatment: (i) the improvement of the fill factor (FF) after 30 s of etching for the CZTSe absorbers showing initially a distortion of the electrical characteristic; (ii) the progressive degradation of the FF after long treatment time for all Cu-poor CZTSe solar cell samples. The first effect can be attributed to the action of KCN on the absorber, that is found to clean the absorber free surface from most of the secondary phases surrounding the kesterite grains (e.g., Se0, CuxSe, SnSex, SnO2, Cu2SnSe3 phases, excepting the ZnSe-based phases). The second observation was identified as a consequence of the preferential etching of Se, Sn, and Zn from the CZTSe surface by the KOH solution, combined with the modification of the alkali content of the absorber. The formation of a Cu-rich shell at the absorber/buffer layer interface, leading to the increase of the recombination rate at the interface, and the increase in the doping of the absorber layer after etching are found to be at the origin of the deterioration of the FF of the solar cells.

{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.

Highly-translucent, strong and aging-resistant 3Y-TZP ceramics for dental restoration by grain boundary segregation

Latest trends in dental restorative ceramics involve the development of full-contour 3Y-TZP ceramics which can avoid chipping of veneering porcelains. Among the challenges are the low translucency and the hydrothermal stability of 3Y-TZP ceramics. In this work, different trivalent oxides (Al2O3, Sc2O3, Nd2O3 and La2O3) were selected to dope 3Y-TZP ceramics. Results show that dopant segregation was a key factor to design hydrothermally stable and high-translucent 3Y-TZP ceramics and the cation dopant radius could be used as a controlling parameter. A large trivalent dopant, oversized as compared to Zr(4+), exhibiting strong segregation at the ZrO2 grain boundary was preferred. The introduction of 0.2 mol% La2O3 in conventional 0.1-0.25 wt.% Al2O3-doped 3Y-TZP resulted in an excellent combination of high translucency and superior hydrothermal stability, while retaining excellent mechanical properties.

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).

Synthesis and cation distribution in the new bismuth oxyhalides with the Sillén–Aurivillius intergrowth structures

Extensive synthetic and structural exploration of a new family of Sillen–Aurivillius intergrowth bismuth oxyhalides reveals clear size preferences for the alkaline earth cations and suggest a new stability criterion for complex intergrowth structures.

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."

Crystal and magnetic structure of new perovskite-based lead iron oxychlorides

The hematophanite Pb4Fe3O8Cl crystal structure is built of incomplete perovskite Pb4Fe3O8 blocks separated by layers of chlorine atoms [1,2]. Each perovskite block consists of a corner-sharing FeO6 octahedral layer sandwiched between the sheets of the FeO5 square pyramids. We have proven that the thickness of the perovskite block in the hematophanite structure can be extended to two and even three octahedral layers forming homologous series with the general formula An+1BnO3n–1Cl (where hematophanite is the n=3 member). The n=4 members with composition Pb4BiFe4O11Cl and Pb5Fe3TiO11Cl have been synthesized. We were also able to introduce Aurivillius-type PbBiO2 blocks between the hematophanite blocks forming another new homologous series [PbBiO2]An+1BnO3n-1Cl2. Two successive members with n=3 (Pb5BiFe3O10Cl2) and n=4 (Pb5Bi2Fe4O13Cl2 and isostructural Pb5BiFe3TiO13Cl2) have been obtained. The crystal and magnetic structure has been determined and refined in a wide temperature range (1.5 – 700 K) using a combination of neutron powder diffraction (NPD) and electron microscopy techniques (electron diffraction, high angle annular dark field scanning transmission electron microscopy (STEM), atomic resolution STEM-EDX). Using NPD and STEM-EDX data we demonstrated that Ti4+ cations occupy both octahedral and square-pyramidal sites. This makes these structural types rare examples of Ti4+ in five-fold oxygen coordination environment. Pb4BiFe4O11Cl and Pb5Fe3TiO11Cl are antiferromagnetically (AFM) ordered below 600(10) and 450(10) K, respectively. Pb5BiFe3O10Cl2, Pb5Bi2Fe4O13Cl2 and Pb5BiFe3TiO13Cl2 demonstrate signs of local magnetic ordering below ~600, ~600 and ~400 K, respectively. However, the long range magnetic ordering does not set in and the magnetic reflections appear enormously broadened merging into a halo. Presumably, AFM ordering establishes within the perovskite blocks but is disrupted along the c-axis, because of a high thickness of the non-magnetic modules.

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.

Structural and magnetic phase transitions in the A(n)B(n)O(3n-2) anion-deficient perovskites Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16

Novel anion-deficient perovskite-based ferrites Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 were synthesized by solid-state reaction in air. Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 belong to the perovskite-based A(n)B(n)O(3n-2) homologous series with n = 5 and 6, respectively, with a unit cell related to the perovskite subcell a(p) as a(p)√2 × a(p) × na(p)√2. Their structures are derived from the perovskite one by slicing it with 1/2[110]p(101)p crystallographic shear (CS) planes. The CS operation results in (101)p-shaped perovskite blocks with a thickness of (n - 2) FeO6 octahedra connected to each other through double chains of edge-sharing FeO5 distorted tetragonal pyramids which can adopt two distinct mirror-related configurations. Ordering of chains with a different configuration provides an extra level of structure complexity. Above T ≈ 750 K for Pb2Ba2BiFe5O13 and T ≈ 400 K for Pb(1.5)Ba(2.5)Bi2Fe6O16 the chains have a disordered arrangement. On cooling, a second-order structural phase transition to the ordered state occurs in both compounds. Symmetry changes upon phase transition are analyzed using a combination of superspace crystallography and group theory approach. Correlations between the chain ordering pattern and octahedral tilting in the perovskite blocks are discussed. Pb2Ba2BiFe5O13 and Pb(1.5)Ba(2.5)Bi2Fe6O16 undergo a transition into an antiferromagnetically (AFM) ordered state, which is characterized by a G-type AFM ordering of the Fe magnetic moments within the perovskite blocks. The AFM perovskite blocks are stacked along the CS planes producing alternating FM and AFM-aligned Fe-Fe pairs. In spite of the apparent frustration of the magnetic coupling between the perovskite blocks, all n = 4, 5, 6 A(n)Fe(n)O(3n-2) (A = Pb, Bi, Ba) feature robust antiferromagnetism with similar Néel temperatures of 623-632 K.

Artificial construction of the layered Ruddlesden-Popper manganite La2Sr2Mn3O10 by reflection high energy electron diffraction monitored pulsed laser deposition

Pulsed laser deposition has been used to artificially construct the n = 3 Ruddlesden-Popper structure La(2)Sr(2)Mn(3)O(10) in epitaxial thin film form by sequentially layering La(1-x)Sr(x)MnO(3) and SrO unit cells aided by in situ reflection high energy electron diffraction monitoring. The interval deposition technique was used to promote two-dimensional SrO growth. X-ray diffraction and cross-sectional transmission electron microscopy indicated that the trilayer structure had been formed. A site ordering was found to differ from that expected thermodynamically, with the smaller Sr(2+) predominantly on the R site due to kinetic trapping of the deposited cation sequence. A dependence of the out-of-plane lattice parameter on growth pressure was interpreted as changing the oxygen content of the films. Magnetic and transport measurements on fully oxygenated films indicated a frustrated magnetic ground state characterized as a spin glass-like magnetic phase with the glass temperature T(g) ≈ 34 K. The magnetic frustration has a clear in-plane (ab) magnetic anisotropy, which is maintained up to temperatures of 150 K. Density functional theory calculations suggest competing antiferromagnetic and ferromagnetic long-range orders, which are proposed as the origin of the low-temperature glassy state.