Fast ion conductivity in strained defect-fluorite structure created by ion tracks in Gd2Ti2O7

Nanoscale core-shell structure and recrystallization of swift heavy ion tracks in SrTiO3

It is widely accepted that the interaction of swift heavy ions with many complex oxides is predominantly governed by the electronic energy loss that gives rise to nanoscale amorphous ion tracks along the penetration direction. The question of how electronic excitation and electron-phonon coupling affect the atomic system through defect production, recrystallization, and strain effects has not yet been fully clarified. To advance the knowledge of the atomic structure of ion tracks, we irradiated single crystalline SrTiO3 with 629 MeV Xe ions and performed comprehensive electron microscopy investigations complemented by molecular dynamics simulations. This study shows discontinuous ion-track formation along the ion penetration path, comprising an amorphous core and a surrounding few monolayer thick shell of strained/defective crystalline SrTiO3. Using machine-learning-aided analysis of atomic-scale images, we demonstrate the presence of 4-8% strain in the disordered region interfacing with the amorphous core in the initially formed ion tracks. Under constant exposure of the electron beam during imaging, the amorphous part of the ion tracks readily recrystallizes radially inwards from the crystalline-amorphous interface under the constant electron-beam irradiation during the imaging. Cation strain in the amorphous region is observed to be significantly recovered, while the oxygen sublattice remains strained even under the electron irradiation due to the present oxygen vacancies. The molecular dynamics simulations support this observation and suggest that local transient heating and annealing facilitate recrystallization process of the amorphous phase and drive Sr and Ti sublattices to rearrange. In contrast, the annealing of O atoms is difficult, thus leaving a remnant of oxygen vacancies and strain even after recrystallization. This work provides insights for creating and transforming novel interfaces and nanostructures for future functional applications.

Composition-dependent photoluminescence in nanocrystalline La2Hf2-xZrxO7:Eu phosphor: role of chemical twin Zr/Hf environments around a luminescent center

Based on chemical intuition, linear trends are anticipated in Eu3+ photoluminescence performance inside a pyrochlore matrix of the chemical twins, Hf and Zr, owing to probable geometrical and chemical similarity around the luminescent center. The present work reports the drastically fluctuating result of doping Eu3+ in nanocrystalline pyrochlore, La2Hf2-xZrxO7 (LHZO), matrix on composition variation; the variation is counter to the anticipation-based chemical brotherhood of Hf and Zr. Zirconium-enriched samples of LHZO improve asymmetry around Eu3+ ion leading to enhanced photoluminescence quantum yield (PLQY). The samples with compositions 0.7Hf and 1.3Zr depict the lowest non-radiative channels with the highest theoretically calculated PLQY of ∼71% and excellent thermal stability (∼91%). Synergistic experimental and theoretical analysis reveals that Eu does not unbiasedly occupy La-sites in the pyrochlore LHZO matrix towards chemical twins of Hf and Zr; rather, it energetically prefers to occupy Zr-rich vicinal sites. When the composition with Zr is in the low-medium range, Eu has a higher probability of occupying Zr-rich vicinal sites depicting higher lifetime and PLQY. When Zr-content goes beyond 70-80%, the other site occupancies start contributing leading to a reduction in both lifetime and quantum yield. This work paves a great strategy and provides a futuristic potential to utilize europium luminescence in separating chemically close Hf-Zr for various technological applications.

Synthesis and feasibility studies of doping U at Ti site of Y2Ti2O7 as a radioactive waste immobilization matrix

In pursuit of clean and green nuclear energy one of the major challenges is to effectively immobilize the nuclear waste. In this context A2B2O7 type pyrochlore owing to its structural flexibility, ability to accommodate ions at both A/B-sites and high radiation tolerance has demonstrated excellent capability to store highly radioactive actinide ions. To fill the major gap area of actinide doping at the B site we have taken up the challenge of doping uranium ions at the Ti site of Y2Ti2O7 type pyrochlore. An yttria titanate (Y2Ti2-xUxO7; x = 0.05, 0.075, 0.1, 0.2, and 0.3) based matrix with uranium doped at the Ti site was synthesized using a simple gel combustion route under an air atmosphere. Rietveld refined X-ray diffraction (XRD) demonstrated that Y2Ti2O7 can accommodate U up to 5 mol% in the Ti site without any phase separation, which was further confirmed using Raman spectroscopy. Y2Ti2O7 based matrices are found to be radiation stable up to 1000 kGy and at the same time they are moderately thermally stable and on a par with the values reported for pyrochlores. Uranium in Y2Ti2O7 stabilizes in +6 oxidation state in the form of uranyl ion distributed near and far off from titanium vacancies with distinct excited state lifetime. This work could provide a smart and strategic way for selecting a suitable advanced ceramic matrix for immobilization of high level waste with additional and important information on solubility limit, actinide speciation, radiation/thermal stability, actinide concentration, etc.

Laser Irradiation-Induced Nanoscale Surface Transformations in Strontium Titanate

We studied the structural transformations and atomic rearrangements in strontium titanate (SrTiO3) via nanosecond pulsed laser irradiation-induced melting and ultrafast quenching. Using scanning transmission electron microscopy, we determine that the laser-irradiated surface in single-crystalline SrTiO3 transforms into an amorphous phase with an interposing disordered crystalline region between amorphous and ordered phases. The formation of disordered phase is attributed to the rapid recrystallization of SrTiO3 from the melt phase constrained by an epitaxial relation with the pristine region, which eases up on the surface, leading to amorphous phase formation. With electron energy-loss spectroscopic analysis, we confirm the transformation of Ti+4 to Ti+3 due to oxygen vacancy formation as a result of laser irradiation. In the disordered region, the maximum transformation of Ti+4 is observed to be 16.2 ± 0.2%, whereas it is observed to be 20.2 ± 0.2% in the amorphous region. Finally, we deduce that the degree of the disorder increases from atomically disordered to amorphous transition in SrTiO3 under laser-irradiation. The signatures of short-range ordering remain similar, leading to a comparable fingerprint of electronic structure. With these results, this study addresses the gap in understanding the atomic and electronic structure modified by pulsed laser irradiation and functionalizing pristine SrTiO3 for electronic, magnetic, and optical applications.

Proton Conductivity of La2(Hf2−xLax)O7−x/2 “Stuffed” Pyrochlores

The design of new oxygen- and proton-conducting materials is of paramount importance for their possible utilization in solid oxide fuel cells. In the present work, La2(Hf2–xLax)O7–x/2 (x = 0, 0.1) ceramics were prepared using ball milling of oxide mixtures (La2O3 and HfO2) followed by high-temperature annealing at 1600 °C for 10 h in air. La2Hf2O7 ceramics exhibit an ordered pyrochlore-type structure, whereas La2(Hf1.9La0.1)O6.95 has a defect pyrochlore structure type with oxygen vacancies at the 48f positions. The oxygen ion and proton conductivity of La2(Hf1.9La0.1)O6.95 “stuffed” pyrochlore ceramics was investigated by electrochemical impedance spectroscopy (two-probe AC) and four-probe DC measurements in a dry and a wet atmosphere (air and nitrogen). The use of two distinct conductivity measurement techniques ensured, for the first time, the collection of reliable data on the proton conductivity of the La2(Hf1.9La0.1)O6.95 “stuffed” hafnate pyrochlore. La2Hf2O7 was found to be a dielectric in the range 400–900 °C, whereas the La2(Hf1.9La0.1)O6.95 “stuffed” pyrochlore had both oxygen ion and proton conductivities in this temperature range. The proton conductivity level was found to be equal to ~8 × 10−5 S/cm at 700 °C. Clearly, the proton conductivity of the La2(Hf1.9La0.1)O6.95 “stuffed” hafnate pyrochlore is mainly due to the hydration of oxygen vacancies at 48f positions.

Atomic-Scale Insights on Large-Misfit Heterointerfaces in LSMO/MgO/c-Al2O3

Understanding the interfaces in heterostructures at an atomic scale is crucial in enabling the possibility to manipulate underlying functional properties in correlated materials. This work presents a detailed study on the atomic structures of heterogeneous interfaces in La0.7Sr0.3MnO3 (LSMO) film grown epitaxially on c-Al2O3 (0001) with a buffer layer of MgO. Using aberration-corrected scanning transmission electron microscopy, we detected nucleation of periodic misfit dislocations at the interfaces of the large misfit systems of LSMO/MgO and MgO/c-Al2O3 following the domain matching epitaxy paradigm. It was experimentally observed that the dislocations terminate with 4/5 lattice planes at the LSMO/MgO interface and with 12/13 lattice planes at the MgO/c-Al2O3 interface. This is consistent with theoretical predictions. Using the atomic-resolution image data analysis approach to generate atomic bond length maps, we investigated the atomic displacement in the LSMO/MgO and MgO/c-Al2O3 systems. Minimal presence of residual strain was shown at the respective interface due to strain relaxation following misfit dislocation formation. Further, based on electron energy-loss spectroscopy analysis, we confirmed an interfacial interdiffusion within two monolayers at both LSMO/MgO and MgO/c-Al2O3 interfaces. In essence, misfit dislocation configurations of the LSMO/MgO/c-Al2O3 system have been thoroughly investigated to understand atomic-scale insights on atomic structure and interfacial chemistry in these large misfit systems.

Probing Multiscale Disorder in Pyrochlore and Related Complex Oxides in the Transmission Electron Microscope: A Review

Transmission electron microscopy (TEM), and its counterpart, scanning TEM (STEM), are powerful materials characterization tools capable of probing crystal structure, composition, charge distribution, electronic structure, and bonding down to the atomic scale. Recent (S)TEM instrumentation developments such as electron beam aberration-correction as well as faster and more efficient signal detection systems have given rise to new and more powerful experimental methods, some of which (e.g., 4D-STEM, spectrum-imaging, in situ/operando (S)TEM)) facilitate the capture of high-dimensional datasets that contain spatially-resolved structural, spectroscopic, time- and/or stimulus-dependent information across the sub-angstrom to several micrometer length scale. Thus, through the variety of analysis methods available in the modern (S)TEM and its continual development towards high-dimensional data capture, it is well-suited to the challenge of characterizing isometric mixed-metal oxides such as pyrochlores, fluorites, and other complex oxides that reside on a continuum of chemical and spatial ordering. In this review, we present a suite of imaging and diffraction (S)TEM techniques that are uniquely suited to probe the many types, length-scales, and degrees of disorder in complex oxides, with a focus on disorder common to pyrochlores, fluorites and the expansive library of intermediate structures they may adopt. The application of these techniques to various complex oxides will be reviewed to demonstrate their capabilities and limitations in resolving the continuum of structural and chemical ordering in these systems.

Decoding ionic conductivity and reordering in cation-disordered pyrochlores

The ordered structure A₂B₂O₆O' in pyrochlores engenders twin rows of inequivalent anion sublattices each centred on alternating cations. While it is known that cation antisite disorder augments the ionic conductivity by several orders of magnitude, the local cation environment around the anions and the dynamic anion reordering during the cation disordering are not well-elucidated. Using atomistic simulations on Gd₂Zr₂O₇, we first show that the anions engage in concerted hops to the neighbouring tetrahedral sites mostly along with the 〈1 0 0〉 direction while completely avoiding the octahedral sites. While the initially vacant 8a sites start accommodating oxygen ions with increasing cation disorder, they show noticeable reluctance even at significant levels of disorder. We have also tracked both the distribution of available oxygen sites following random cation disorder, which is dependent only on cation disordering, and the probability of occupation of these sites. Interestingly, the available oxygen sites show a non-monotonic dependence on the number of B ions in the nearest neighbouring shell while the occupation probability of all the available oxygen sites increases monotonically. A tetrahedral oxygen site thus has a better probability of being occupied when it has a greater number of second neighbour B ions. This article is part of the Theo Murphy meeting issue 'Understanding fast-ion conduction in solid electrolytes'.

∑3 Twin Boundaries in Gd2Ti2O7 Pyrochlore: Pathways for Oxygen Migration

Understanding the chemistry at twin boundaries (TB) is a well-recognized challenge, which could enable the capabilities to manipulate the functional properties in complex oxides. The study of this atomic imperfection becomes even more important, as the presence of twin boundaries has been widely observed in materials, regardless of the dimensionalities, due to the complexities in growth methods. In the present study, we provide atomic-scale insights into a ∑3(111̅) ⟨11̅0⟩ twin boundary present in pyrochlore-structured Gd₂Ti₂O₇ using atomic-resolution electron microscopy and atomistic modeling. The formation of the observed TB occurs along (111̅) with a 71° angle between two symmetrically arranged crystals. We observe distortions (∼3 to 5% strain) in the atomic structure at the TB with an increase in Gd-Gd (0.66 ± 0.03 nm) and Ti-Ti (0.65 ± 0.02 nm) bond lengths in the (11̅0) plane, as compared to 0.63 nm in the ordered structure. Using atomistic modeling, we further calculate the oxygen migration barrier for vacancy hopping at 48f-48f sites in the pyrochlore structure, which is the primary diffusion pathway for fast oxygen transport. The mean migration barrier is lowered by ∼25% to 0.9 eV at the TB as compared to 1.23 eV in the bulk, suggesting the ease in oxygen transport through the ∑3 twin boundaries. Overall, these results offer a critical understanding of the atomic arrangement at the twin boundaries in pyrochlores, leading to control of the interplay between defects and properties.

Sculpting Nanoscale Functional Channels in Complex Oxides Using Energetic Ions and Electrons

The formation of metastable phases has attracted significant attention because of their unique properties and potential functionalities. In the present study, we demonstrate the phase conversion of energetic-ion-induced amorphous nanochannels/tracks into a metastable defect fluorite in A₂B₂O₇ structured complex oxides by electron irradiation. Through in situ electron irradiation experiments in a scanning transmission electron microscope, we observe electron-induced epitaxial crystallization of the amorphous nanochannels in Yb₂Ti₂O₇ into the defect fluorite. This energetic-electron-induced phase transformation is attributed to the coupled effect of ionization-induced electronic excitations and local heating, along with subthreshold elastic energy transfers. We also show the role of ionic radii of A-site cations (A = Yb, Gd, and Sm) and B-site cations (Ti and Zr) in facilitating the electron-beam-induced crystallization of the amorphous phase to the defect-fluorite structure. The formation of the defect-fluorite structure is eased by the decrease in the difference between ionic radii of A- and B-site cations in the lattice. Molecular dynamics simulations of thermal annealing of the amorphous phase nanochannels in A₂B₂O₇ draw parallels to the electron-irradiation-induced crystallization and confirm the role of ionic radii in lowering the barrier for crystallization. These results suggest that employing guided electron irradiation with atomic precision is a useful technique for selected area phase formation in nanoscale printed devices.

Disorder-induced transition from grain boundary to bulk dominated ionic diffusion in pyrochlores

We use molecular dynamics simulations to investigate the role of grain boundaries (GBs) on ionic diffusion in pyrochlores, as a function of the GB type, chemistry of the compound, and level of cation disorder. We observe that the presence of GBs promotes oxygen transport in ordered and low-disordered systems, as the GBs are found to have a higher concentration of mobile carriers with higher mobilities than in the bulk. Thus, in ordered samples, the ionic diffusion is 2D, localized along the grain boundary. When cation disorder is introduced, bulk carriers begin to contribute to the overall diffusion, while the GB contribution is only slightly enhanced. In highly disordered samples, the diffusive behavior at the GBs is bulk-like, and the two contributions (bulk vs. GB) can no longer be distinguished. There is thus a transition from 2D/GB dominated oxygen diffusivity to 3D/bulk dominated diffusivity versus disorder in pyrochlores. These results provide new insights into the possibility of using internal interfaces to enhance ionic conductivity in nanostructured complex oxides.

Strain Engineering to Modify the Electrochemistry of Energy Storage Electrodes

Strain engineering has been a critical aspect of device design in semiconductor manufacturing for the past decade, but remains relatively unexplored for other applications, such as energy storage. Using mechanical strain as an input parameter to modulate electrochemical potentials of metal oxides opens new opportunities intersecting fields of electrochemistry and mechanics. Here we demonstrate that less than 0.1% strain on a Ni-Ti-O based metal-oxide formed on superelastic shape memory NiTi alloys leads to anodic and cathodic peak potential shifts by up to ~30 mV in an electrochemical cell. Moreover, using the superelastic properties of NiTi to enable strain recovery also recovers the electrochemical potential of the metal oxide, providing mechanistic evidence of strain-modified electrochemistry. These results indicate that mechanical energy can be coupled with electrochemical systems to efficiently design and optimize a new class of strain-modulated energy storage materials.

Insights on dramatic radial fluctuations in track formation by energetic ions

We report on unexpected dramatic radial variations in ion tracks formed by irradiation with energetic ions (2.3 GeV (208)Pb) at a constant electronic energy-loss (~42 keV/nm) in pyrochlore-structured Gd2TiZrO7. Though previous studies have shown track formation and average track diameter measurements in the Gd2TixZr(1-x)O7 system, the present work clearly reveals the importance of the recrystallization process in ion track formation in this system, which leads to more morphological complexities in tracks than currently accepted behavior. The ion track profile is usually considered to be diametrically uniform for a constant value of electronic energy-loss. This study reveals the diameter variations to be as large as ~40% within an extremely short incremental track length of ~20 nm. Our molecular dynamics simulations show that these fluctuations in diameter of amorphous core and overall track diameter are attributed to the partial substitution of Ti atoms by Zr atoms, which have a large difference in ionic radii, on the B-site in pyrochlore lattice. This random distribution of Ti and Zr atoms leads to a local competition between amorphous phase formation (favored by Ti atoms) and defect-fluorite phase formation (favored by Zr atoms) during the recrystallization process and finally introduces large radial variations in track morphology.