Taifeng Liu

Point defects in solid-state materials are conventionally modeled as lattice-derived defects, generated by adding or removing atoms from symmetric bulk lattice structures, followed by local geometry optimization. This approach often yields symmetric, weakly relaxed local minima that remain close to the parent lattice. Here, we show that this assumption can fail qualitatively for vacancies in many metal oxides. Rather than retaining a lattice-derived geometry with localized holes on oxygen sites, cation vacancies can drive extensive reconstruction, producing defect configurations in which the O–O bonds and O–O–O chains become the preferred charge-compensation motifs. We combined the minima hopping (MH) structure prediction algorithm with machine-learned interatomic potentials (MLIPs), hybrid QM/MM embedded-cluster calculations, and periodic density functional theory (DFT) calculations to investigate cation vacancies across a broad series of metal oxides. Our results show a clear oxidation-state-dependent trend. In the charge-neutral state, cation vacancies in oxides with formal oxidation states of +4 or higher favor reconstructed defect configurations containing O–O bonds or O–O–O chains with bond lengths of approximately 1.4 Å, lowering the defect formation energy by up to several electronvolts relative to the conventional structure. By contrast, in oxides with lower cation oxidation states, such reconstructions are generally unfavorable at charge neutrality but can become energetically more favorable at positive charge states. These findings define oxygen-bond formation as a general reconstruction pathway for cation vacancies in metal oxides and show that global structure searches are essential for predictive defect modeling beyond conventional lattice-derived local minima.

Mid-infrared pulsed lasers are pivotal for biomedical engineering and atmospheric remote sensing, yet power scaling is severely bottlenecked by poor thermal stability and low damage thresholds of conventional saturable absorbers. Here, we address this limitation through a thermodynamic optimization strategy applied to atomically thin iron-based crystals grown by space-confined chemical vapor deposition. Screening anionic chalcogens (Se, S, and O) paired with Fe identifies iron oxide as the most stable matrix, while Mn doping further stabilizes the matrix and modulates carrier dynamics. Mn incorporation enhances the laser-induced damage threshold approximately 3-fold and accelerates carrier relaxation to 23.63 ps. Consequently, a record-high average output power exceeding 1.0 W was achieved in an Er3+-doped fiber laser at 2.8 μm, accompanied by nanosecond pulse compression and operation stability over 30 days. Our findings establish lattice thermodynamics as a material route to high-energy mid-infrared pulsed lasers and highlight its potential for on-chip long-wave terahertz photonic devices.

Holes in PbTiO ₃ form 2D large polarons on oxygen sites (weak coupling), while electrons form small polarons at Titanium sites (strong coupling). Spin–orbit coupling barely affects hole polarons but significantly delocalizes electron polarons.

This work investigates polaron formation in BiOX (X = Cl, Br, I) using the EPW method. Results show hole polarons tend to localize, while electron polarons form large polarons.

High‐entropy alloys (HEAs) offer unique catalytic advantages due to their multielement composition and severe lattice distortion. However, most HEA‐based electrocatalysts rely on a single‐anionic component, limiting interfacial electronic tunability and active‐site optimization. Herein, we report an entropy‐driven dual‐anion interface strategy that integrates both selenium (Se) and phosphorus (P) into a multimetallic NiCoFeMnMo (HEA) catalyst supported on Ni foam. A hydrothermal process yields the HEA precursor, followed by solid‐state phosphorization to establish a conductive phosphide backbone. Subsequent hydrothermal selenization generates Se‐rich surface domains, creating synergistic P‐in‐bulk and Se‐on‐surface dual‐anion architecture. This configuration preserves the high configurational entropy of the NiCoFeMnMo cationic framework while introducing heterogeneous Se–P coordination environments and interfacial electronic disorder that modulate electron distribution. The resulting HEA–Se–P electrode exhibits markedly enhanced hydrogen evolution reaction (HER) activity in 1.0 M KOH, delivering an overpotential of 17 mV at 10 mA·cm ^–2 , and outstanding long‐term durability. DFT calculations confirm that dual‐anion engineering produces an optimized interfacial ensemble with near‐thermoneutral hydrogen adsorption, enabling balanced H ^* adsorption energetics and enhanced catalytic turnover. This work demonstrates that dual‐anion entropy engineering is a powerful and generalizable strategy to design highly efficient HER catalysts for sustainable hydrogen production.

Mid-infrared (MIR) pulsed lasers are pivotal for molecular spectroscopy, atmospheric remote sensing, and surgical medicine, yet their power scaling is severely bottlenecked by the poor thermal stability and low damage threshold of conventional low-dimensional saturable absorbers (SAs). Here, we overcome this bottleneck through a rational thermodynamic optimization strategy based on atomically thin iron-based crystals via space-confined chemical vapor deposition. By screening anionic chalcogens (Se, S, O) paired with Fe, we identify iron oxide as the most thermodynamically stable matrix, while subsequent Mn doping further stabilizes the lattice thermodynamically and modulates carrier dynamics. Experimental analyses reveal that Mn-ion doping significantly enhances the laser-induced damage threshold (a ~3-fold increase over undoped counterparts) and accelerates carrier relaxation (23.63 ps). Consequently, a record-high average output power of >1.0 W was achieved in an Er ³⁺ -doped fiber laser at 2.8 µm, accompanied by a nanosecond pulse compression and exceptional long-term stability over 30 days. This work establishes lattice thermodynamics as a critical pathway for developing high-energy on-chip MIR ultrashort pulsed lasers, while demonstrating the design flexibility of two-dimensional crystals toward long-wave terahertz photonic devices

The exceptional performance of ceria (CeO₂) in catalysis and energy conversion is fundamentally governed by its defect chemistry, particularly oxygen vacancies. The formation of each oxygen vacancy (V_O^••) is assumed to be compensated by two localized electrons on cations (Ce³⁺). Here, we show by combining theory with experiment that while this 1 V_O^••: 2Ce³⁺ ratio accounts for the global charge compensation, it does not apply at the local scale, particularly in nanoparticles. Hybrid quantum mechanical/molecular mechanical (QM/MM) defect calculations, together with synchrotron X-ray photoelectron spectroscopy (XPS) measurements, show that electrons have a strong preference to localize and segregate on surfaces, which can overcome the trapping force from the V_O^•• sites in the bulk. At a given Fermi level, the surface V_O^•• tends to trap more electrons than those in bulk, resulting in a higher Ce³⁺ to V_O^•• ratio on surfaces than that in the bulk, driven by the preferential localization of electrons and enhanced V_O^••-Ce³⁺coupling. Large-scale unbiased Monte Carlo simulations on ceria nanoparticles confirmed this trend and further show that the surface segregation of electrons is more pronounced at low reduction levels and in smaller nanoparticles. In highly reduced ceria nanoparticles, however, the enhanced repulsive interactions lead to a less significant extent of defect heterogeneity or even reverse the location preference of defects in some nanoparticles. Our findings underscore the need to consider both the overall nonstoichiometry and local defect behavior in easily reducible oxides, with direct relevance to their performance in catalytic and energy applications.