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Ionic conductivity is pivotal for solid-state battery performance. While the garnet oxide electrolyte Li 7 La 3 Zr 2 O 12 (LLZO) boasts high ionic conductivity due to its distinct crystal structure and lithium-ion mobility, lithium loss during fabrication hampers its potential. In this study, we introduce a method that merges synthesis optimization with a post-lithiation process, enhancing LLZO's ionic conductivity. This approach compensates lithium loss with a gas-phase diffusion process, which stabilizes the cubic LLZO phase and amplifies its ionic conductivity by more than three orders of magnitude compared to electrolytes without post-lithiation. Through our comprehensive experimental procedure, we have conclusively determined that the film deposited at 700 °C and subsequently annealed at 700 °C with LiOH exhibits the highest conductivity, with a notable value of 1.11 × 10 -2 S cm -1 at 200 °C. This is a significant boost compared to the as-deposited film (3.54 × 10 -6 S cm -1 at 200 °C). Our findings present an additional approach to boosting lithium ion diffusion. The approach employed in this work has the potential to be applicable to films produced through other deposition methods, as it addresses the prevalent issue of lithium loss, a significant barrier to the utilization of lithium-rich thin films., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The heterostructure formed by depositing nanoscale thin films of spinel γ-Al 2 O 3 on perovskite SrTiO 3 exhibits a range of exciting properties including room temperature epitaxial growth, high electron mobility, strain-tunable magnetic order, and a symmetry-related reordering of the conduction bands. In comparison to the benchmark LaAlO 3 /SrTiO 3 heterostructure, the γ-Al 2 O 3 /SrTiO 3 heterostructure has been more sparsely investigated, which leaves plenty of room for scientific and technological discoveries. In this perspective article, I describe the key findings of the γ-Al 2 O 3 /SrTiO 3 heterostructure and propose five directions for future research: (1) an exploration of novel phenomena emerging when relaxing the conventional epitaxial constraint of matching crystal structures across the interface, (2) a dynamic switching of a strong polarization through nanoscale electromigration of aluminum vacancies, (3) autonomous and forced enhancement of the electron mobility via oxygen vacancy diffusion, (4) writing and erasing of magnetic and conducting nanolines using ferroelastic domain walls, and (5) a multiferroic state formed by combining ferroelectricity, ferromagnetism, and ferroelasticity. The proposed research directions may shed light on both fundamental aspects of the heterostructure and pave the way for applications within green energy devices and nanoelectronics.

The electronic structure as well as the mechanism underlying the high-mobility two-dimensional electron gases (2DEGs) at complex oxide interfaces remain elusive. Herein, using soft X-ray angle-resolved photoemission spectroscopy (ARPES), we present the band dispersion of metallic states at buffered LaAlO 3 /SrTiO 3 (LAO/STO) heterointerfaces where a single-unit-cell LaMnO 3 (LMO) spacer not only enhances the electron mobility but also renders the electronic structure robust toward X-ray radiation. By tracing the evolution of band dispersion, orbital occupation, and electron-phonon interaction of the interfacial 2DEG, we find unambiguous evidence that the insertion of the LMO buffer strongly suppresses both the formation of oxygen vacancies as well as the electron-phonon interaction on the STO side. The latter effect makes the buffered sample different from any other STO-based interfaces and may explain the maximum mobility enhancement achieved at buffered oxide interfaces.

The exploration for thermoelectric thin films of complex oxides such as SrTiO 3 -based oxides is driven by the need for miniaturized harvesting devices for powering the Internet of Things (IoT). However, there is still not a clear consensus in the literature for the underlying influence of film thickness on thermoelectric properties. Here, we report the fabrication of epitaxial thin films of 6% Nb-doped SrTiO 3 on (001) (LaAlO 3 ) 0.3 (Sr 2 AlTaO 6 ) 0.7 (LSAT) single crystal using pulsed laser deposition (PLD) where the film thickness was varied from 2 nm to 68 nm. The thickness dependence shows a subtle increase of tetragonality of the thin film lattice and a gradual drop of the electrical conductivity, the density of charge carriers, and the thermoelectric Seebeck coefficient as the film thickness decreases. DFT-based calculations show that ∼2.8% increase in tetragonality results in an increased splitting between t 2g and e g orbitals to ∼42.3 meV. However, experimentally observed tetragonality for films between 68 to 13 nm is only 0.06%. Hence, the effect of thickness on tetragonality is neglected. We have discussed the decrease of conductivity and the Seebeck coefficient based on the decrease of carriers and change in the scattering mechanism, respectively.

We report the electrical, magnetic and magnetotransport properties of Na and Mo dual doped Ca 3-2 x Na 2 x Co 4- x Mo x O 9 (0 ≤ x ≤ 0.15) polycrystalline samples. The results indicate that the strength of ferrimagnetic interaction decreases with increase in doping, as is evident from the observed decrease in Curie temperatures ( T C ). The substitution of non-magnetic Mo 6+ ions (4d 0 ) in CoO 2 layers and the presence of oxygen vacancies are responsible for decrease in ligand field strength, which results in an enhanced magnetization in the low doped x = 0.025 sample due to a change from the low spin to partial high spin electron configuration. The electrical resistivity of samples exhibits a semiconducting-like behavior in the low temperature range, a strongly correlated Fermi liquid-like behavior in the intermediate temperature range, and an incoherent metal-like behavior in the temperature range 210-300 K. All the samples show a large negative magnetoresistance (MR) at low temperature with a maximum MR value of -59% for the x = 0.025 sample at 2 K and 16 T applied field. The MR values follow the observed trend in magnetization at 5 K and sharply increase below the Curie temperatures of the samples, suggesting that the ferrimagnetic interactions are mainly responsible for the decrease in electrical resistivity under an applied magnetic field., Competing Interests: There is no conflict of interest to declare., (This journal is © The Royal Society of Chemistry.)

The metallic interface between two oxide insulators, such as LaAlO 3 /SrTiO 3 (LAO/STO), provides new opportunities for electronics and spintronics. However, due to the presence of multiple orbital populations, tailoring the interfacial properties such as the ground state and metal-insulator transitions remains challenging. Here, an unforeseen tunability of the phase diagram of LAO/STO is reported by alloying LAO with a ferromagnetic LaMnO 3 insulator without forming lattice disorder and at the same time without changing the polarity of the system. By increasing the Mn-doping level, x, of LaAl 1- x Mn x O 3 /STO (0 ≤ x ≤ 1), the interface undergoes a Lifshitz transition at x = 0.225 across a critical carrier density of n c = 2.8 × 10 13 cm -2 , where a peak T SC ≈255 mK of superconducting transition temperature is observed. Moreover, the LaAl 1- x Mn x O 3 turns ferromagnetic at x ≥ 0.25. Remarkably, at x = 0.3, where the metallic interface is populated by only d xy electrons and just before it becomes insulating, a same device with both signatures of superconductivity and clear anomalous Hall effect (7.6 × 10 12 cm -2 < n s ≤ 1.1 × 10 13 cm -2 ) is achieved reproducibly. This provides a unique and effective way to tailor oxide interfaces for designing on-demand electronic and spintronic devices., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The detailed crystal structures and high temperature thermoelectric properties of polycrystalline Ca 3-2 x Na 2 x Co 4- x W x O 9 (0 ≤ x ≤ 0.075) samples have been investigated. Powder X-ray diffraction data show that all samples are phase pure, with no detectable traces of impurity. The diffraction peaks shift to lower angle values with increase in doping ( x ), which is consistent with larger ionic radii of Na + and W 6+ ions. X-ray photoelectron spectroscopy data reveal that a mixture of Co 2+ , Co 3+ and Co 4+ valence states are present in all samples. It has been observed that electrical resistivity ( ρ ), Seebeck coefficient ( S ) and thermal conductivity ( κ ) are all improved with dual doping of Na and W in Ca 3 Co 4 O 9 system. A maximum power factor (PF) of 2.71 × 10 -4 W m -1 K -2 has been obtained for x = 0.025 sample at 1000 K. The corresponding thermoelectric figure of merit ( zT ) for x = 0.025 sample is calculated to be 0.21 at 1000 K, which is ∼2.3 times higher than zT value of the undoped sample. These results suggest that Na and W dual doping is a promising approach for improving thermoelectric properties of Ca 3 Co 4 O 9 system., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

A chemical redox reaction can lead to a two-dimensional electron gas at the interface between a TiO 2 -terminated SrTiO 3 (STO) substrate and an amorphous LaAlO 3 capping layer. When replacing the STO substrate with rutile and anatase TiO 2 substrates, considerable differences in the interfacial conduction are observed. On the basis of X-ray photoelectron spectroscopy (XPS) and transport measurements, we conclude that the interfacial conduction comes from redox reactions, and that the differences among the materials systems result mainly from variations in the activation energies for the diffusion of oxygen vacancies at substrate surfaces.

The two-dimensional metal forming at the interface between an oxide insulator and SrTiO&#95;{3} provides new opportunities for oxide electronics. However, the quantum Hall effect, one of the most fascinating effects of electrons confined in two dimensions, remains underexplored at these complex oxide heterointerfaces. Here, we report the experimental observation of quantized Hall resistance in a SrTiO&#95;{3} heterointerface based on the modulation-doped amorphous-LaAlO&#95;{3}/SrTiO&#95;{3} heterostructure, which exhibits both high electron mobility exceeding 10,000  cm^{2}/V s and low carrier density on the order of ∼10^{12}  cm^{-2}. Along with unambiguous Shubnikov-de Haas oscillations, the spacing of the quantized Hall resistance suggests that the interface is comprised of a single quantum well with ten parallel conducting two-dimensional sub-bands. This provides new insight into the electronic structure of conducting oxide interfaces and represents an important step towards designing and understanding advanced oxide devices.