Halide double perovskites (HDPs) have emerged as promising alternatives to their lead-based counterparts. However, their structural dynamics is less explored than that of conventional halide perovskites. In this work, we investigate octahedral tilting at 0 K and the relative stability of tetragonal and cubic phases of a set of 57 HDPs. By combining structural and energetic descriptors with simple geometric metrics, we identify the main trends controlling the stabilization of one-tilt tetragonal phases across this family. We find that both the magnitude of the tilt angles and the energetic preference for tilted phases correlate primarily with the Goldschmidt tolerance factor t. The presence of ns2 lone-pair cations also correlates with enhanced tilting; however, this trend largely reflects that lone-pair chemistries in HDPs occur together with ionic sizes that shift t away from unity. Consistent with this picture, we observe several compounds without lone pairs that nonetheless exhibit strong octahedral tilting. Finally, using machine-learned interatomic potentials, we connect the 0 K tilting energetics to finite-temperature behavior: compounds with more strongly stabilized tilt phases exhibit higher transition temperatures, and phonon spectra at 350 K reveal soft and broad modes that are consistent with the trends in tolerance factors, tilt angles, and tilt energies at 0 K. Our results provide a systematic reference for structure-stability relationships in HDPs and clarify that lone-pair chemistry is correlated with, rather than the primary cause of, octahedral tilting.

We study the electronic properties of lead-free layered Cs _3 Bi _2 Br _9 (CBB) perovskite, which has recently emerged as a promising material for photocatalysis. Our investigation, prompted by optical measurements suggesting self-trapping of excess charges and corroborated by ab initio electronic-structure calculations and molecular dynamics simulations, reveals that photogenerated electrons are assimilated in the material as small polarons, a consequence of sizable structural reorganization of both the inorganic sublattice and of A-site cations. The electron polaron exhibits an energy level at 0.6 eV below the conduction band edge, which is consistent with the physical picture ensuing from spectroscopy, and is suitably aligned with respect to redox potentials associated with common photoreduction processes. Estimation of the electron mobility, in the framework of polaron hopping, indicates a remarkable anisotropy, with interlayer movement of electron polarons being at least two orders of magnitude slower than intralayer diffusion. This suggests that heterojunctions, separating holes and electrons, are the most viable architecture to exploit CBB for photocatalysis, and that morphology as well as loading and size of CBB nanostructures are key in determining whether photogenerated electrons might reach the relevant interface or be lost due to recombination.

Graphitic carbon nitride (g-C3N4) is a promising metal-free photocatalyst whose activity is often enhanced by nitrogen vacancies, though their microscopic role remains unclear. Using advanced ab initio calculations with large periodic supercells, we show that long-range buckling is essential to correctly evaluate defect energetics and thus determine the stability of distinct vacancy configurations. The most stable defects are found to introduce localized in-gap states corresponding to shallow acceptor and deep donor levels. These features explain (i) the experimental red-shifted absorption and (ii) suppressed photoluminescence observed in N-deficient g-C3N4 samples. Most importantly (iii) energy-level alignment at the water-semiconductor interface explains the enhanced photocatalytic reduction and reduced oxidation activity reported experimentally. Overall, our results provide a unified microscopic picture that quantitatively connects defect-induced electronic structure changes and experimental observables, offering a concrete predictive strategy for designing defect engineered carbon nitride and related metal-free photocatalysts.

Abstract Complex semiconductors are increasingly used in energy conversion, optoelectronics, and photocatalysis. Their performance is often controlled by local atomic environments that are not represented by a single ideal crystal structure. In this Perspective, I discuss how atomistic models can include effects such as thermal motion, disorder, charge localization, defects, and interfaces when these are relevant to the property of interest. I focus on examples where including these effects changes the physical interpretation of the calculation, and discuss how machine-learned interatomic potentials can help sample the relevant configurations for subsequent electronic structure calculations.

Electrochemical CO2 mitigation offers a transformative route to sustainability, offering a powerful solution to the twin challenges of rising global energy demand and climate change. The formation of multicarbon products, particularly hydrocarbons and oxygenates, is of considerable industrial significance owing to their higher energy density and value as key chemical feedstocks. Despite significant progress, pivotal challenges persist in optimizing selectivity, streamlining reaction pathways, and enabling scalable implementation. This review outlines recent progress in the rational design of electrocatalysts for the selective CO2 reduction reaction (CO2RR), emphasizing operational conditions and strategies that manipulate the local reaction microenvironment to steer selectivity and efficiency. It presents an in-depth discussion of the stability of Cu- and non-Cu-based electrocatalysts, including single-atom and molecular catalysts, and their degradation mechanisms, with both ex/in situ analysis and computational insights. Special attention is given to structure-performance relationships and dynamic surface reconstructions under reaction conditions. Finally, it sheds light on promising pathways for achieving highly selective formation of higher-order hydrocarbons (C3-C6+). The review concludes by identifying pressing challenges and highlighting emerging frontiers in CO2RR that are poised to accelerate progress. These insights collectively chart a course for translating CO2R research into viable industrial applications, thereby advancing global efforts toward carbon neutrality.

Antimony selenide (Sb2Se3) has an optimal bandgap and absorption coefficient for thin film solar cell applications and comprises earth abundant elements. The rate of increase in reported power conversion efficiencies has slowed due to a persistently large open circuit voltage deficit attributed to detrimental concentrations of point defects. Here we use depth-profiling positron annihilation lifetime spectroscopy to study Sb2Se3 crystals and thin films. The method is specific to neutral and negative charge states of vacancy-related defects. Both monovacancy and divacancy defects are identified in intrinsic and n-type samples but no monovacancy defects are detected in the p-type sample. Comparison of the experimental positron lifetimes with density functional theory calculated values provide evidence for the observation of Sb monovacancies in the –3 state and of Se monovacancies in the –2 state. The results are consistent with recent density function theory predictions that the Sb and the Se monovacancy defects both have accessible negative charge states. Antimony selenide is a promising photovoltaic material, but the presence of point defects degrades performance. Here, the authors use positron annihilation spectroscopy combined with theory to detect and identify vacancy-type point defects.

Photoelectrocatalytic water splitting using bismuth vanadate (BiVO4) is a promising approach for sustainable hydrogen production, but its efficiency is limited by charge carrier dynamics. Though charge trapping in the form of polarons is well-studied, the behavior of self-trapped excitons (STEs), particularly whether they remain stable or dissociate under operating conditions, remains far less understood. Using hybrid density functional theory with the nudged elastic band method, we quantify activation barriers for STE hopping, dissociation and transformation in BiVO4, revealing distinct behaviors and kinetic time scales for two STE types: a separated, more mobile state and a compact, more stable one with higher barriers. Additionally, we study an alternative charge trapping mechanism via O-O dimers, providing an alternative multipolaron binding pathway with distinct kinetics. These findings provide fundamental insights into the kinetic stability and mobility of trapped charges in BiVO4, aiding the interpretation of charge trapping dynamics under operating conditions.

Bismuth vanadate is a promising photoanode material for photoelectrocatalytic water splitting. Localized charges play a crucial role in the water-splitting mechanism by introducing charge-transition levels within the band gap and thereby modifying the band alignment. Additionally, the introduction of oxygen vacancies, which can interact with electron polarons, has been reported to improve the photoelectrocatalytic efficiency. However, charge-transition levels are typically evaluated at 0 K, whereas operating conditions involve finite temperatures and, in thin films, epitaxial strain. Capturing these effects requires free energies and extensive sampling that are generally prohibitive for ab initio methods. In this work, we train a machine-learned interatomic potential for BiVO_{4}, including electron polarons and oxygen vacancies in various charge states, to resolve how free energies, transition levels, and charge trapping evolve under operando temperature and strain conditions. For oxygen vacancies, the use of a machine-learned potential enables efficient sampling of charge-localization sites in the vicinity of the vacancy, revealing favored configurations. The energies of the localized polaron and oxygen-vacancy states remain essentially unchanged with temperature, while their transition levels relative to the band edges vary due to temperature-dependent band-edge shifts. In contrast, strain more strongly modifies both the localized states and the band edges. These results provide new insights into the stability of localized charges and their role in photoelectrocatalytic water splitting at finite temperature.

Abstract Molecular single-source precursors are a promising way of obtaining multi-element extended solids directly. We show that thermal decomposition of well-defined mono-, bi- and trimetallic polyoxovanadates (POVs) proceeds through a series of intermediate amorphous and crystalline species which we characterise using solid-state NMR spectroscopy, pair-distribution function (PDF) analysis and in-situ X-ray diffraction, before forming crystalline V2O5 and BiVO4 products. This synthetic strategy enables the formation of phases inaccessible using other routes, including a previously unknown polymorph of BiVO4 which we name β-BiVO4 due to its similarity to β-SnWO4. Local structure information also reveals the temperature dependent incorporation of Zn do pants into BiVO4. The study also explores the electrochemical properties of amorphous mixed-valence vanadium oxides as Li-ion battery electrodes. We suggest that careful analysis of the thermal decomposition of molecular species may be a way of obtaining hitherto unknown kinetically stabilised polymorphs and amorphous variants of extended solids.

We report the structural and optoelectronic properties of lead-free CsGeI3 and CsGeBr3 perovskites, unveiling the critical role of local symmetry distortions in defining their emission properties. CsGeBr3 exhibits broad photoluminescence from self-trapped excitons, due to local octahedral distortion and a large distribution of the average bond lengths. On the contrary, by using temperature-dependent pair distribution function analysis and hybrid-functional molecular dynamics simulations, we demonstrate that CsGeI3 adopts a monoclinic local structure responsible for its narrow near-infrared (NIR) emission (∼745 nm, FWHM ≈ 110 meV at room temperature), the narrowest reported for Ge-based perovskites and in line with tin iodide perovskites. Notably, the high level of structural order also supports the achievement of amplified spontaneous emission (ASE) at room temperature with an exceptionally low threshold (75 μJ/cm2), positioning it as a promising candidate for lead-free NIR light-emitting and laser applications.

Although electrostatics can be incorporated into machine-learned interatomic potentials, existing approaches are computationally very demanding, limiting large-scale, long-time simulations of electrostatics-driven phenomena such as dielectric response, infrared activity, and field-matter coupling. Here, we extend the neuroevolution potential (NEP), a highly efficient machine-learned interatomic potential, to a charge-aware framework (qNEP) by introducing explicit, environment-dependent partial charges. Each ionic partial charge is represented by a neural network as a function of the local descriptor vector, analogous to the NEP site-energy model. This formulation enables the direct prediction of the Born effective charge tensor for each ion and, consequently, the polarization. As a result, dielectric properties, infrared spectra, and coupling to external electric fields can be evaluated within a unified framework. We derive consistent expressions for the forces and virials that explicitly account for the position dependence of the partial charges. The qNEP method has been implemented in the free-and-open-source GPUMD package, with support for both Ewald summation and particle-particle particle-mesh treatments of electrostatics. We demonstrate the accuracy and efficiency of the qNEP approach through representative applications to water, Li7La3Zr2O12, BaTiO3, and a magnesium-water interface. These results show that qNEP enables accurate atomistic simulations with explicit long-range electrostatics, scalable to million-atom systems on nanosecond time scales using consumer-grade GPUs.

Two-dimensional (2D) semiconductors have emerged as exciting candidates for the development of low-power and multifunctional computing applications, thanks to their qualities such as layer-dependent band gap tunability, high carrier mobility, and excellent electrostatic control. Here, we explore a pair of 2D semiconductors with nearly broken-gap (Type-III-like) band alignment and demonstrate a highly gate-tunable p-MoTe2/n-SnS2 heterojunction with multifunctional behavior. Employing a dual-gated asymmetric device geometry, we unveil its functionality as both a forward and backward rectifying device. Moreover, we observe a highly gate-tunable negative differential resistance (NDR), with a gate-coupling efficiency of η ≃ 0.5 and a peak-to-valley ratio of ∼ 3 down to 150 K. By employing density functional theory, we determine that the observed NDR is dominated by valence band-to-valence band tunneling, while additional interband tunneling contributions arise at higher bias. The combination of tunneling driven transport and gate controllability of NDR opens the pathway for realizing gate-tunable 2D material-based neuromorphic and energy-efficient electronics.

Mixed halide perovskites are highly versatile semiconductors with applications in photovoltaics, light-emitting diodes, and photodetectors. Understanding their thermodynamic phase behavior is central to guiding compositional design and improving device stability. Here, we train machine-learned interatomic potentials (MLIPs) on density functional theory reference data for CsxRb1-xPbBr3yI3-3y, CsxRb1-xPbBr3yCl3-3y, and CsxRb1-xPbCl3yI3-3y halide perovskites, enabling large-scale hybrid Monte Carlo-molecular dynamics simulations that sample both configurational and vibrational degrees of freedom. All three binary halide systems exhibit a miscibility gap, the extent of which correlates with halide ion size mismatch. The gaps in Br-Cl and Br-I close at low temperatures, while the Cl-I gap extends above room temperature. At temperatures above the miscibility gap (200 K to 500 K), all systems show a tendency toward layered halide ordering, with halide species preferentially occupying apical or equatorial octahedral sites. In CsPbBr3yI3-3y, this ordering occurs in a device-relevant temperature regime and is linked to the structural phase transitions, shifting transition temperatures by up to 100 K relative to randomly mixed structures. We attribute the strongly non-linear composition dependence of the orthorhombic-tetragonal phase boundary observed experimentally (a linear decrease followed by a plateau) to halide ordering. Introducing Rb on the A-site weakens halide ordering and eliminates the non-linear behavior, while narrowing the miscibility gap in both the Br-I and Br-Cl systems. These results establish halide ordering as a key determinant of structural phase stability in mixed-halide perovskites.

B-site mixing is a common strategy for tuning properties of halide perovskites. In the lead-free system MAGe1-xSn_xI3, it brings tilting and off-centering into competition. Using large-scale molecular dynamics driven by a machine-learned interatomic potential, we map the structural behavior across the full composition range. MAGeI3 exhibits strong polar B-site off-centering that remains nearly constant up to the cubic transition, together with methylammonium (MA) orientational order that weakens progressively on heating. By contrast, MASnI3 combines octahedral tilting with weaker, predominantly antipolar off-centering. Ge-like behavior persists upon alloying and gives way to Sn-like behavior only beyond roughly 65

Lead-free halide double perovskites are promising materials for light-emitting and photovoltaic applications. However, compositional tuning of their structural and optoelectronic properties has largely focused on B-site and halide alloying. In this work, we show that Cs + on the perovskite A-site of Cs 2 AgInCl 6 can be partly substituted with Rb + using simple apolar liquid-assisted mechanochemistry. By comparing Rb + with Na + substitution we show that ion size drives the site-selectivity of alkali ion substitution into double perovskites, with Na + preferentially occupying the perovskite B-site. By combining multinuclear solid-state nuclear magnetic resonance spectroscopy with X-ray diffraction, we show that Rb+ not only enters the double-perovskite site-selectively but also induces local symmetry breaking of the cubic structure, comparable to the effect of B-site Na + /Ag + alloying, with important implications for symmetry-forbidden photoluminescence emissivity. This work extends the tuneable compositional range of lead-free double perovskites to include A-site mixing while also establishing the site-selectivity of alkali ion speciation into the double perovskite structure.

Lead halide perovskites are promising optoelectronic materials for photovoltaics, light emission and detection. Their efficiencies in PV now approach the detailed-balance limit, leaving stability as the principal barrier. The intrinsic instabilities studied to date centre on ionic motion within a fixed, homogeneous lattice. Here we identify a further source of intrinsic structural instability, hidden in the lattice dynamics. Mapping caesium, methylammonium and formamidinium-based compositions with Cl, Br, I and mixed X-sites through all accessible phases, using single crystal X-ray and neutron diffuse scattering, machine-learning-assisted molecular dynamics, a phenomenological octahedral tilt model and hyperspectral photoluminescence, we find that nearly every composition hosts equilibrium local structural fluctuations: dynamic nanodomains of correlated octahedral tilts, a few nanometres in size, that locally break the crystallographic symmetry. Three complementary levers control them. The A-site cation sets their symmetry, shape and anisotropy, from sparse, isotropic and tetragonal in formamidinium-based compositions to dense, anisotropic and orthorhombic in nominally cubic caesium-based ones, the most locally disordered we studied. The halide controls the dynamic disorder and the phase-transition sequence. Thermal history is the third: different ramp rates drive nominally identical compositions into distinct crystallographic phases, each with its own hidden local order. In MAPbI3, the heating rate alone changes the photoluminescence quantum efficiency across the phase transition. Because these transitions lie within device operating ranges, from terrestrial thermal cycling to the extremes of space, thermal history may shape the local structure, and hence the optoelectronic response, throughout fabrication and operation, establishing it as a design variable alongside composition.

Understanding the phase behavior of mixed-cation halide perovskites is critical for optimizing their structural stability and optoelectronic performance. Here, we map the phase diagram of MA1-xFAxPbI3 using a machine-learned interatomic potential in molecular dynamics simulations. We identify a morphotropic phase boundary (MPB) at approximately 27% FA content, delineating the transition between out-of-phase and in-phase octahedral tilt patterns. Phonon mode projections reveal that this transition coincides with a mode crossover composition, where the free energy landscapes of the M and R phonon modes become nearly degenerate. This results in nanoscale layered structures with alternating tilt patterns, suggesting minimal interface energy between competing phases. Our results provide a systematic and consistent description of this important system, complementing earlier partial and sometimes conflicting experimental assessments. Furthermore, density functional theory calculations show that band edge fluctuations peak near the MPB, indicating an enhancement of electron-phonon coupling and dynamic disorder effects. These findings establish a direct link between phonon dynamics, phase behavior, and electronic structure, providing a further composition-driven pathway for tailoring the optoelectronic properties of perovskite materials. By demonstrating that phonon overdamping serves as a hallmark of the MPB, our study offers insights into the design principles for stable, high-performance perovskite solar cells.

Understanding the phase behavior of mixed-cation halide perovskites is critical for optimizing their structural stability and optoelectronic performance. Here, we map the phase diagram of MA$_{1-x}$FA$_x$PbI$_3$ using a machine-learned interatomic potential in molecular dynamics simulations. We identify a morphotropic phase boundary (MPB) at approximately 27% FA content, delineating the transition between out-of-phase and in-phase octahedral tilt patterns. Phonon mode projections reveal that this transition coincides with a mode crossover composition, where the free energy landscapes of the M and R phonon modes become nearly degenerate. This results in nanoscale layered structures with alternating tilt patterns, suggesting minimal interface energy between competing phases. Our results provide a systematic and consistent description of this important system, complementing earlier partial and sometimes conflicting experimental assessments. Furthermore, density functional theory calculations show that band edge fluctuations peak near the MPB, indicating an enhancement of electron-phonon coupling and dynamic disorder effects. These findings establish a direct link between phonon dynamics, phase behavior, and electronic structure, providing a further composition-driven pathway for tailoring the optoelectronic properties of perovskite materials. By demonstrating that phonon overdamping serves as a hallmark of the MPB, our study offers new insights into the design principles for stable, high-performance perovskite solar cells.