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.

Chemical Activity of the Peroxide/Oxide Redox Couple: Case Study of Ba5Ru2O11 in Aqueous and Organic Solvents.

The finding that triggering the redox activity of oxygen ions within the lattice of transition metal oxides can boost the performances of materials used in energy storage and conversion devices such as Li-ion batteries or oxygen evolution electrocatalysts has recently spurred intensive and innovative research in the field of energy. While experimental and theoretical efforts have been critical in understanding the role of oxygen nonbonding states in the redox activity of oxygen ions, a clear picture of the redox chemistry of the oxygen species formed upon this oxidation process is still missing. This can be, in part, explained by the complexity in stabilizing and studying these species once electrochemically formed. In this work, we alleviate this difficulty by studying the phase Ba5Ru2O11, which contains peroxide O22- groups, as oxygen evolution reaction electrocatalyst and Li-ion battery material. Combining physical characterization and electrochemical measurements, we demonstrate that peroxide groups can easily be oxidized at relatively low potential, leading to the formation of gaseous dioxygen and to the instability of the oxide. Furthermore, we demonstrate that, owing to the stabilization at high energy of peroxide, the high-lying energy of the empty σ* antibonding O-O states limits the reversibility of the electrochemical reactions when the O22-/O2- redox couple is used as redox center for Li-ion battery materials or as OER redox active sites. Overall, this work suggests that the formation of true peroxide O22- states are detrimental for transition metal oxides used as OER catalysts and Li-ion battery materials. Rather, oxygen species with O-O bond order lower than 1 would be preferred for these applications.

Electrochemical behavior of Bi4B2O9 towards lithium-reversible conversion reactions without nanosizing.

Conversion type materials, in particular metal fluorides, have emerged as attractive candidates for positive electrodes in next generation Li-ion batteries (LIBs). However, their practical use is being hindered by issues related to reversibility and large polarization. To minimize these issues, a few approaches enlisting the anionic network have been considered. We herein report the electrochemical properties of bismuth oxyborate Bi4B2O9 and show that this compound reacts with lithium via a conversion reaction leading to a sustained capacity of 140 mA h g-1 when cycled between 1.7 and 3.5 V vs. Li+/Li0 while having a surprisingly small polarization (∼300 mV) in the presence of solely 5% in weight of a carbon additive. These observations are rationalized in terms of charge transfer kinetics via complementary XRD, HRTEM and NMR measurements. This finding demonstrates that borate based conversion type materials display rapid charge transfer with limited carbon additives, hence offering a new strategy to improve their overall cycling efficiency.

Phosphate Ion Functionalization of Perovskite Surfaces for Enhanced Oxygen Evolution Reaction.

Recent findings revealed that surface oxygen can participate in the oxygen evolution reaction (OER) for the most active catalysts, which eventually triggers a new mechanism for which the deprotonation of surface intermediates limits the OER activity. We propose in this work a "dual strategy" in which tuning the electronic properties of the oxide, such as La1-xSrxCoO3-δ, can be dissociated from the use of surface functionalization with phosphate ion groups (Pi) that enhances the interfacial proton transfer. Results show that the Pi functionalized La0.5Sr0.5CoO3-δ gives rise to a significant enhancement of the OER activity when compared to La0.5Sr0.5CoO3-δ and LaCoO3. We further demonstrate that the Pi surface functionalization selectivity enhances the activity when the OER kinetics is limited by the proton transfer. Finally, this work suggests that tuning the catalytic activity by such a "dual approach" may be a new and largely unexplored avenue for the design of novel high-performance catalysts.

Evidence for anionic redox activity in a tridimensional-ordered Li-rich positive electrode ß-Li2IrO3.

Lithium-ion battery cathode materials have relied on cationic redox reactions until the recent discovery of anionic redox activity in Li-rich layered compounds which enables capacities as high as 300 mAh g-1. In the quest for new high-capacity electrodes with anionic redox, a still unanswered question was remaining regarding the importance of the structural dimensionality. The present manuscript provides an answer. We herein report on a ß-Li2IrO3 phase which, in spite of having the Ir arranged in a tridimensional (3D) framework instead of the typical two-dimensional (2D) layers seen in other Li-rich oxides, can reversibly exchange 2.5 e- per Ir, the highest value ever reported for any insertion reaction involving d-metals. We show that such a large activity results from joint reversible cationic (Mn+) and anionic (O2)n- redox processes, the latter being visualized via complementary transmission electron microscopy and neutron diffraction experiments, and confirmed by density functional theory calculations. Moreover, ß-Li2IrO3 presents a good cycling behaviour while showing neither cationic migration nor shearing of atomic layers as seen in 2D-layered Li-rich materials. Remarkably, the anionic redox process occurs jointly with the oxidation of Ir4+ at potentials as low as 3.4 V versus Li+/Li0, as equivalently observed in the layered α-Li2IrO3 polymorph. Theoretical calculations elucidate the electrochemical similarities and differences of the 3D versus 2D polymorphs in terms of structural, electronic and mechanical descriptors. Our findings free the structural dimensionality constraint and broaden the possibilities in designing high-energy-density electrodes for the next generation of Li-ion batteries.

Synthesis, Structure, and Electrochemical Properties of K-Based Sulfates K2M2(SO4)3 with M = Fe and Cu.

Stabilizing new host structures through potassium extraction from K-based polyanionic materials has been proven to be an interesting approach to develop new Li+/Na+ insertion materials. Pursuing the same trend, we here report the feasibility of preparing langbeinite "Fe2(SO4)3" via electrochemical and chemical oxidation of K2Fe2(SO4)3. Additionally, we succeeded in stabilizing a new K2Cu2(SO4)3 phase via a solid-state synthesis approach. This novel compound crystallizes in a complex orthorhombic structure that differs from that of langbeinite as deduced from synchrotron X-ray and neutron powder diffraction. Electrochemically, the performance of this new phase is limited, which we explain in terms of sluggish diffusion kinetics. We further show that K2Cu2(SO4)3 decomposes into K2Cu3O(SO4)3 on heating, and we report for the first time the synthesis of fedotovite K2Cu3O(SO4)3. Finally, the fundamental attractiveness of these S = 1/2 systems for physicists is examined by neutron magnetic diffraction, which reveals the absence of a long-range ordering of Cu2+ magnetic moments down to 1.5 K.

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

The Bi3n+1Ti7Fe3n-3O9n+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 Bi3n+1Ti7Fe3n-3O9n+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 3ap periodicity along the interfaces, so that they can be located either on top of each other or with shifts of ±ap 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 = 3ap, b = bp, c = 2(n + 1)cp and a = 3ap, b = bp, c = 2(n + 1)cp - ap, respectively. While the n = 3 compound is orthorhombic, the monoclinic modification is more favorable in higher homologues. The Bi3n+1Ti7Fe3n-3O9n+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 Bi3n+1Ti7Fe3n-3O9n+11 materials demonstrate an unusual diversity of structure defects. The n = 4-6 homologues are robust antiferromagnets below TN = 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.

Room Temperature Magnetically Ordered Polar Corundum GaFeO3 Displaying Magnetoelectric Coupling.

The polar corundum structure type offers a route to new room temperature multiferroic materials, as the partial LiNbO3-type cation ordering that breaks inversion symmetry may be combined with long-range magnetic ordering of high spin d5 cations above room temperature in the AFeO3 system. We report the synthesis of a polar corundum GaFeO3 by a high-pressure, high-temperature route and demonstrate that its polarity arises from partial LiNbO3-type cation ordering by complementary use of neutron, X-ray, and electron diffraction methods. In situ neutron diffraction shows that the polar corundum forms directly from AlFeO3-type GaFeO3 under the synthesis conditions. The A3+/Fe3+ cations are shown to be more ordered in polar corundum GaFeO3 than in isostructural ScFeO3. This is explained by DFT calculations which indicate that the extent of ordering is dependent on the configurational entropy available to each system at the very different synthesis temperatures required to form their corundum structures. Polar corundum GaFeO3 exhibits weak ferromagnetism at room temperature that arises from its Fe2O3-like magnetic ordering, which persists to a temperature of 408 K. We demonstrate that the polarity and magnetization are coupled in this system with a measured linear magnetoelectric coupling coefficient of 0.057 ps/m. Such coupling is a prerequisite for potential applications of polar corundum materials in multiferroic/magnetoelectric devices.

ZnTaO2N: Stabilized High-Temperature LiNbO3-type Structure.

By using a high-pressure reaction, we prepared a new oxynitride ZnTaO2N that crystallizes in a centrosymmetric (R3̅c) high-temperature LiNbO3-type structure (HTLN-type). The stabilization of the HTLN-type structure down to low temperatures (at least 20 K) makes it possible to investigate not only the stability of this phase, but also the phase transition to a noncentrosymmetric (R3c) LiNbO3-type structure (LN-type) which is yet to be clarified. Synchrotron and neutron diffraction studies in combination with transmission electron microscopy show that Zn is located at a disordered 12c site instead of 6a, implying an order-disorder mechanism of the phase transition. It is found that the closed d-shell of Zn2+, as well as the high-valent Ta5+ ion, is responsible for the stabilization of the HTLN-type structure, affording a novel quasitriangular ZnO2N coordination. Interestingly, only 3% Zn substitution for MnTaO2N induces a phase transition from LN- to HTLN-type structure, implying the proximity in energy between the two structural types, which is supported by the first-principles calculations.

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.

Layered-to-Tunnel Structure Transformation and Oxygen Redox Chemistry in LiRhO2 upon Li Extraction and Insertion.

Layered Li(M,Li)O2 (where M is a transition metal) ordered rock-salt-type structures are used in advanced metal-ion batteries as one of the best hosts for the reversible intercalation of Li ions. Besides the conventional redox reaction involving oxidation/reduction of the M cation upon Li extraction/insertion, creating oxygen-located holes because of the partial oxygen oxidation increases capacity while maintaining the oxidized oxygen species in the lattice through high covalency of the M-O bonding. Typical degradation mechanism of the Li(M,Li)O2 electrodes involves partially irreversible M cation migration toward the Li positions, resulting in gradual capacity/voltage fade. Here, using LiRhO2 as a model system (isostructural and isoelectronic to LiCoO2), for the first time, we demonstrate an intimate coupling between the oxygen redox and M cation migration. A formation of the oxidized oxygen species upon electrochemical Li extraction coincides with transformation of the layered Li1-xRhO2 structure into the γ-MnO2-type rutile-ramsdellite intergrowth LiyRh3O6 structure with rutile-like [1 × 1] channels along with bigger ramsdellite-like [2 × 1] tunnels through massive and concerted Rh migration toward the empty positions in the Li layers. The oxidized oxygen dimers with the O-O distances as short as 2.26 Å are stabilized in this structure via the local Rh-O configuration reminiscent to that in the µ-peroxo-µ-hydroxo Rh complexes. The LiyRh3O6 structure is remarkably stable upon electrochemical cycling illustrating that proper structural implementation of the oxidized oxygen species can open a pathway toward deliberate employment of the anion redox chemistry in high-capacity/high-voltage positive electrodes for metal-ion batteries.

Topochemical Nitridation with Anion Vacancy-Assisted N(3-)/O(2-) Exchange.

We present how the introduction of anion vacancies in oxyhydrides enables a route to access new oxynitrides, by conducting ammonolysis of perovskite oxyhydride EuTiO3-xHx (x ∼ 0.18). At 400 °C, similar to our studies on BaTiO3-xHx, hydride lability enables a low temperature direct ammonolysis of EuTi(3.82+)O2.82H0.18, leading to the N(3-)/H(-)-exchanged product EuTi(4+)O2.82N0.12□0.06. When the ammonolysis temperature was increased up to 800 °C, we observed a further nitridation involving N(3-)/O(2-) exchange, yielding a fully oxidized Eu(3+)Ti(4+)O2N with the GdFeO3-type distortion (Pnma) as a metastable phase, instead of pyrochlore structure. Interestingly, the same reactions using the oxide EuTiO3 proceeded through a 1:1 exchange of N(3-) with O(2-) only above 600 °C and resulted in incomplete nitridation to EuTiO2.25N0.75, indicating that anion vacancies created during the initial nitridation process of EuTiO2.82H0.18 play a crucial role in promoting anion (N(3-)/O(2-)) exchange at high temperatures. Hence, by using (hydride-induced) anion-deficient precursors, we should be able to expand the accessible anion composition of perovskite oxynitrides.

Bi(3n+1)Ti7Fe(3n-3)O(9n+11) Homologous Series: Slicing Perovskite Structure with Planar Interfaces Containing Anatase-like Chains.

The n = 3-6 members of a new perovskite-based homologous series Bi(3n+1)Ti7Fe(3n-3)O(9n+11) are reported. The crystal structure of the n = 3 Bi10Ti7Fe6O38 member is refined using a combination of X-ray and neutron powder diffraction data (a = 11.8511(2) Å, b = 3.85076(4) Å, c = 33.0722(6) Å, S.G. Immm), unveiling the partially ordered distribution of Ti(4+) and Fe(3+) cations and indicating the presence of static random displacements of the Bi and O atoms. All Bi(3n+1)Ti7Fe(3n-3)O(9n+11) structures are composed of perovskite blocks separated by translational interfaces parallel to the (001)p perovskite planes. The thickness of the perovskite blocks increases with n, while the atomic arrangement at the interfaces remains the same. The interfaces comprise chains of double edge-sharing (Fe,Ti)O6 octahedra connected to the octahedra of the perovskite blocks by sharing edges and corners. This configuration shifts the adjacent perovskite blocks relative to each other over a vector ½[110]p and creates S-shaped tunnels along the [010] direction. The tunnels accommodate double columns of the Bi(3+) cations, which stabilize the interfaces owing to the stereochemical activity of their lone electron pairs. The Bi(3n+1)Ti7Fe(3n-3)O(9n+11) structures can be formally considered either as intergrowths of perovskite modules and polysynthetically twinned modules of the Bi2Ti4O11 structure or as intergrowths of the 2D perovskite and 1D anatase fragments. Transmission electron microscopy (TEM) on Bi10Ti7Fe6O38 reveals that static atomic displacements of Bi and O inside the perovskite blocks are not completely random; they are cooperative, yet only short-range ordered. According to TEM, the interfaces can be laterally shifted with respect to each other over ±1/3a, introducing an additional degree of disorder. Bi10Ti7Fe6O38 is paramagnetic in the 1.5-1000 K temperature range due to dilution of the magnetic Fe(3+) cations with nonmagnetic Ti(4+). The n = 3, 4 compounds demonstrate a high dielectric constant of 70-165 at room temperature.

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.

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).

Two new arsenides, Eu7Cu44As23 and Sr7Cu44As23, with a new filled variety of the BaHg11 structure.

Two new ternary arsenides, namely, Eu7Cu44As23 and Sr7Cu44As23, were synthesized from elements at 800 °C. Their crystal structure represents a new filled version of the BaHg11 motif with cubic voids alternately occupied by Eu(Sr) and As atoms, resulting in a 2 × 2 × 2 superstructure of the aristotype: space group Fm3̅m, a = 16.6707(2) Å and 16.7467(2) Å, respectively. The Eu derivative exhibits ferromagnetic ordering below 17.5 K. In agreement with band structure calculations both compounds are metals, exhibiting relatively low thermopower, but high electrical and low thermal conductivity.

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.

Discovery of a superhard iron tetraboride superconductor.

Single crystals of novel orthorhombic (space group Pnnm) iron tetraboride FeB4 were synthesized at pressures above 8 GPa and high temperatures. Magnetic susceptibility and heat capacity measurements demonstrate bulk superconductivity below 2.9 K. The putative isotope effect on the superconducting critical temperature and the analysis of specific heat data indicate that the superconductivity in FeB4 is likely phonon mediated, which is rare for Fe-based superconductors. The discovered iron tetraboride is highly incompressible and has the nanoindentation hardness of 62(5) GPa; thus, it opens a new class of highly desirable materials combining advanced mechanical properties and superconductivity.

Effect of lone-electron-pair cations on the orientation of crystallographic shear planes in anion-deficient perovskites.

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, ß = 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.