Le Wang

Substoichiometric tungsten oxides (WOx) host multiple W valence states and local W–O coordination motifs, but their respective roles in determining electronic and optical behavior remain less understood. Here, we integrate scanning transmission electron microscopy, x-ray photoemission and absorption spectroscopies, spectroscopic ellipsometry, and density functional theory calculations to disentangle these effects in epitaxially stabilized WOx thin films with systematically varied oxygen deficiency. Lowering the oxygen partial pressure induces a structural evolution from a pseudocubic WO3 to W18O49-like Magnéli motifs, together with the gradual emergence of W5+ and W4+ states. Motif-resolved calculations show that reduced W sites are electronically inequivalent: W4+-rich edge-sharing environments are associated with localized in-gap states that enhance subgap optical absorption, whereas intermediate-valence W sites contribute to broader states near the Fermi level that are more favorable for electronic transport. In contrast, W6+-rich sites contribute primarily to high-energy O 2p-to-W 5d transitions characteristic of pseudocubic WO3 regions. Together, these results identify local W–O connectivity as a key determinant of electron localization in the WOx lattice, linking atomic-scale motifs to distinct electronic and optical properties.

Evaporating refractory metals such as tungsten (W) to grow their corresponding oxides remains challenging for molecular beam epitaxy (MBE) due to their high melting points and low vapor pressures at conventional effusion-cell temperatures. Conventional approaches using oxide precursors like WO3 often suffer from incongruent evaporation and flux instability caused by oxygen loss. Here, we utilize an in situ surface-oxidation method to generate a volatile oxide flux directly from a refractory W metal source for epitaxial tungsten-oxide thin film growth. In this approach, the W metal source is heated under a controlled oxygen partial pressure, dynamically forming and evaporating a surface oxide layer. This process produces a stable, high-purity, and tunable oxide flux at temperatures well below those required for metal evaporation. By controlling the oxygen partial pressure and electron-beam power, we can selectively generate tungsten-oxide species for epitaxial growth. In the low-flux regime (1–3 × 10−3 Å s−1), the deposition rate scales linearly with oxygen partial pressure and remains constant for over an hour, enabling the reproducible growth of ultrathin epitaxial WO3 films. Extending this approach to thermal-laser epitaxy achieves deposition rates up to ∼1.5 Å s−1 while maintaining excellent crystallinity. This surface-oxidation-assisted synthesis approach offers several advantages including the use of high-purity elemental sources, precise flux control, long-term stability, and selective generation of oxide species during growth, providing an alternative pathway for the MBE growth of high-quality oxides involving refractory metals with improved control and reliability.

We report the stabilization of the metastable body-centered tetragonal (BCT) phase of BaCoO₃ (BCT-BaCoO₃) under high-pressure (15 GPa) and high-temperature (1200 °C) conditions using a mixture precursor. This double perovskite adopts the EuTiO₃-type structure (space group I4/mcm), as confirmed by powder X-ray diffraction and high-resolution STEM. X-ray photoelectron spectroscopy indicates a predominant Co⁴⁺ oxidation state without detectable oxygen vacancies. Magnetization and heat capacity measurements reveal ferromagnetic ordering at T _C ∼ 107 K, attributable to the BCT-BaCoO₃ phase. Above this temperature, the mixed-phase sample exhibits Curie–Weiss paramagnetism, a low-spin to high-spin crossover upon cooling, and a possible intermediate-spin state at elevated temperatures. Resistivity data indicates insulating behavior with weak magnetoresistance. DFT and DFT + DMFT calculations suggest that the insulating state originates from an orbitally selective transition sensitive to the nominal valence of the Co-d shell. The metastable BCT-BaCoO₃ phase cannot be retained in pure form at ambient pressure but can be stabilized by embedding it in a disordered mixture, offering a potential route to discover and preserve other high-pressure phases under ambient conditions.

Understanding how water vapor interacts with transition metal oxides (TMOs) is critical for tailoring material properties to improve performance and enable new technologies. Despite extensive research efforts, atomic-scale mechanisms underpinning dynamic reactions and reaction-induced phase transitions remain elusive. Here, we use in situ environmental transmission electron microscopy to investigate how water vapor oxidizes vacancy-ordered SrCoO _2.5 at moderately elevated temperatures, demonstrating that water molecules can initiate oxidation more effectively than oxygen under comparable conditions. We discover a distinct “staging” behavior during the oxidation process: A fully ordered intermediate phase, SrCoO _2.75 , forms before transitioning into a near-perovskite SrCoO _3−δ . In addition, antiphase boundaries, originating at step terraces of SrTiO ₃ , alleviate strain by creating reversible nanoscale “gaps” during lattice contraction under oxidation, providing a pathway for preserving structural integrity throughout redox cycling. This work provides atomic-level guidance for engineering TMOs by leveraging water vapor to control their redox behavior and tailor functional properties.

High-entropy oxides (HEOs) offer exceptional compositional flexibility and structural stability, making them promising materials for energy and catalytic applications. Here, we investigate Sr doping effects on B-site cation oxidation states, local composition, and structure in epitaxial La_1-xSr_x(Cr_0.2Mn_0.2Fe_0.2Co_0.2Ni_0.2)O₃ thin films. X-ray spectroscopies reveal that Sr doping preferentially promotes Cr oxidation from Cr³⁺ to Cr⁶⁺, partially oxidizes Co and Ni, while leaving Mn⁴⁺ and Fe³⁺ unchanged. Atomic-resolution scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy shows pronounced Cr segregation, with depletion at the interface and enrichment at the surface, along with partial amorphization in heavily Sr-doped samples. This segregation is likely driven by oxidation-induced migration of smaller, high-valence Cr cations during growth. These findings highlight the critical interplay between charge compensation, local strain, and compositional fluctuations in HEOs, indicating that precise control over growth conditions is critical for tuning their surface composition and electronic structure toward more robust electrocatalyst design.

High-entropy oxides (HEOs) offer exceptional compositional flexibility and structural stability, making them promising materials for energy and catalytic applications. Here, we investigate Sr doping effects on B-site cation oxidation states, local composition, and structure in epitaxial La1-xSrx(Cr0.2Mn0.2Fe0.2Co0.2Ni0.2)O3 thin films. X-ray spectroscopies reveal that Sr doping preferentially promotes Cr oxidation from Cr3+ to Cr6+, partially oxidizes Co and Ni, while leaving Mn4+ and Fe3+ unchanged. Atomic-resolution scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy shows pronounced Cr segregation, with Cr exhibiting depletion at the film-substrate interface and enrichment at the film surface, along with the formation of a partially amorphous phase in heavily Sr-doped samples. This segregation is likely driven by oxidation-induced migration of smaller, high-valence Cr cations during the growth. These findings underscore the critical interplay between charge transfer, local strain, and compositional fluctuations, providing strategies to control surface composition and electronic structure in HEOs for more robust electrocatalyst design.

Multiferroic materials host both ferroelectricity and magnetism, offering potential for magnetic memory and spin transistor applications. Here, we report a multiferroic chalcogenide semiconductor Cu _1−x Mn _1+y SiTe ₃ (0.04 ≤ x ≤ 0.26; 0.03 ≤ y ≤ 0.15), which crystallizes in a polar monoclinic structure ( Pm space group). It exhibits a canted antiferromagnetic state below 35 kelvin, with magnetic hysteresis and remanent magnetization under 15 kelvin. We demonstrate multiferroicity and strong magnetoelectric coupling through magnetodielectric and magnetocurrent measurements. At 10 kelvin, the magnetically induced electric polarization reaches ~0.8 microcoulombs per square centimeter, comparable to the highest value in oxide multiferroics. We also observe possible room-temperature ferroelectricity. Given that multiferroicity is very rare among transition metal chalcogenides, our finding sets up a unique materials platform for designing multiferroic chalcogenides.

Charge transfer or redistribution at oxide heterointerfaces is a critical phenomenon, often leading to remarkable properties such as two-dimensional electron gas and interfacial ferromagnetism. Despite studies on LaNiO ₃ /LaFeO ₃ superlattices and heterostructures, the direction and magnitude of the charge transfer remain debated, with some suggesting no charge transfer due to the high stability of Fe ³⁺ (3d ⁵ ). Here, we synthesized a series of epitaxial LaNiO ₃ /LaFeO ₃ superlattices and demonstrated partial (up to ~0.5 e ⁻ /interface unit cell) charge transfer from Fe to Ni near the interface, supported by density functional theory simulations and spectroscopic evidence of changes in Ni and Fe oxidation states. The electron transfer from LaFeO ₃ to LaNiO ₃ and the subsequent rearrangement of the Fe 3d band create an unexpected metallic ground state within the LaFeO ₃ layer, strongly influencing the in-plane transport properties across the superlattice. Moreover, we establish a direct correlation between interfacial charge transfer and in-plane electrical transport properties, providing insights for designing functional oxide heterostructures with emerging properties.