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This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Applied Materials and Interfaces, copyright © 2022 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acsami.2c02675. Metallic 1T-phase MoS2 exhibits superior hydrogen evolution reaction (HER) performance than natural 2H-phase MoS2 owing to its higher electrical conductivity and abundance of active sites. However, the reported 1T-MoS2 catalysts usually suffer from extreme instability, which results in quick phase transformation at ambient conditions. Herein, we present a facile approach to engineer the phase of MoS2 by introducing intercalated hydrazine. Interestingly, the as-synthesized 1T-dominant MoS2 sample demonstrates excellent ambient stability without noticeable degradation for 3 months. Additionally, the 1T-dominant MoS2 exhibits superior electrical conductivity (~700 times higher than that of 2H-MoS2) and improved electrochemical catalytic performance (current density ~12 times larger than that of 2H-MoS2 at an overpotential of 300 mV vs the reversible hydrogen electrode, RHE). Through experimental characterizations and density functional theory (DFT) calculation, we conclude that the stabilization of the metallic phase could be attributed to the electron donation from hydrazine molecules to the adjacent Mo atoms. The phase control strategy in this work provides a guideline to develop other highly efficient and stable two-dimensional (2D) electrocatalysts.

Solid-state electrolytes (SSEs) with high ion conductivity are pivotal for the development and large-scale adoption of green-energy conversion and storage technologies such as fuel cells, electrocatalysts and solid-state batteries. Yet, SSEs are extremely complex materials for which general rational design principles remain indeterminate. Here, we combine first-principles materials modelling, computational power and modern data analysis techniques to advance towards the solution of such a fundamental and technologically pressing problem. Our data-driven survey reveals that the correlations between ion diffusivity and other materials descriptors in general are monotonic, although not necessarily linear, and largest when the latter are of vibrational nature and explicitly incorporate anharmonic effects. Surprisingly, principal component and k-means clustering analyses show that elastic and vibrational descriptors, rather than the usual ones related to chemical composition and ion mobility, are best suited for reducing the high complexity of SSEs and classifying them into universal classes. Our findings highlight the need for considering databases that incorporate temperature effects to improve our understanding of SSEs and point towards a generalized approach to the design of energy materials., We acknowledge financial support from the MCIN/AEI/10.13039/501100011033 under Grant no. PID2020-119777GB-I00, the “Ramón y Cajal” fellowship RYC2018-024947-I, the Severo Ochoa Centres of Excellence Program (CEX2019-000917-S), the Generalitat de Catalunya under Grant no. 2017SGR1506, and the CSIC under the “JAE Intro SOMdM 2021” grant program. The authors acknowledge computational support from the Red Española de Supercomputación (RES) under the grants FI-2022-1-0006, FI-2022-2-0003 and FI-2022-3-0014., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Lithium-rich manganese-based oxides (LRMO) are regarded as promising cathode materials for powering electric applications due to their high capacity (250 mAh g–1) and energy density (~900 Wh kg–1). However, poor cycle stability and capacity fading have impeded the commercialization of this family of materials as battery components. Surface modification based on coating has proven successful in mitigating some of these problems, but a microscopic understanding of how such improvements are attained is still lacking, thus impeding systematic and rational design of LRMO-based cathodes. In this work, first-principles density functional theory (DFT) calculations are carried out to fill out such a knowledge gap and to propose a promising LRMO-coating material. It is found that SrTiO3 (STO), an archetypal and highly stable oxide perovskite, represents an excellent coating material for Li1.2Ni0.2Mn0.6O2 (LNMO), a prototypical member of the LRMO family. An accomplished atomistic model is constructed to theoretically estimate the structural, electronic, oxygen vacancy formation energy, and lithium-transport properties of the LNMO/STO interface system, thus providing insightful comparisons with the two integrating bulk materials. It is found that (i) electronic transport in the LNMO cathode is enhanced due to partial closure of the LNMO band gap (~0.4 eV) and (ii) the lithium ions can easily diffuse near the LNMO/STO interface and within STO due to the small size of the involved ion-hopping energy barriers. Furthermore, the formation energy of oxygen vacancies notably increases close to the LNMO/STO interface, thus indicating a reduction in oxygen loss at the cathode surface and a potential inhibition of undesirable structural phase transitions. This theoretical work therefore opens up new routes for the practical improvement of cost-affordable lithium-rich cathode materials based on highly stable oxide perovskite coatings.

A high-throughput screening of piezo-photocatalytic materials based on first-principles calculations and a simple electrostatic model is presented that identifies new bulk compounds able to catalyse the water splitting reaction under sunlight.

Introducing large oxygen deficiencies while retaining low resistivity is important for enhancing the overall thermoelectric properties in 3d transition-metal oxides. In this study, a new synthesis route to reconstruct the insulating brownmillerite SrCoO2.5 is adapted. Through a step-by-step nano-blocks modification, a series of highly-conductive layered structures is evolved, which are [Sr2O2H2]0.5CoO2, [Sr2O2]0.4CoO2, and [Sr2CoO3]0.57CoO2, while still retaining considerable Seebeck coefficient (˜100 µV K-1). Coexistence of low resistivity and high oxygen deficiency is realized in the latter two polymorphs by forming a majority of sintered oxygen vacancies in the rock-salt layer and a minority of normal oxygen vacancies in the CoO2 layer. A room-temperature in-plane power factor of 3.6 mW K-2 m-1, power output density of 4.5 W m-2 at a temperature difference of 28 K, and an out-of-plane thermal conductivity of 0.33 W K-1 m-1 are obtained in the [Sr2O2]0.4CoO2 thin film that exhibits the highest oxygen deficiency (d = 2.95), which is on par with Bi2Te3, the benchmark. It is pointed out that proper distribution of oxygen vacancy is essential in tailoring the physical and chemical properties of transition-metal oxides. The sintered/normal oxygen vacancy layer model provides guidance to the exploration of materials with both low electric resistivity and thermal conductivity.

We demonstrate that the heat capacity Boson peak (BP)-like anomaly appearing in fully ordered anharmonic molecular crystals emerges as a result of the strong interactions between propagating (acoustic) and low-energy quasi-localized (optical) phonons. In particular, we experimentally determine the low-temperature (

The new class of oxide heterojunctions with mixed dimensionalities holds great promise for energy and environmental applications. However, the existing fabrication strategies typically involve low-yield and multistep processes leading to the formation of powders that necessitate binding agents when used for electrochemical applications; thereby, the durability and performance of the resultant electrode may be adversely impacted. To address these challenges, the present work first reports a high-temperature counter-current gas flow technique for rapid fabrication (5–10 min) of centimetre-size, self-adhesive, free-standing 3D patches made of ZnO-based woven nanowires. Furthermore, the high applicability of the method was shown by layer-by-layer assembly of the ZnO and layers of 0D heteroatoms including Bi2O3, CdO, SnO2, and carbon forming stratified oxide heterojunction (SOH) nanostructures with midgap states within their electronic bandgap region. This work is innovative in that the ZnO and the fabricated SOHs are synthesised through a sustainable and large-scale method based on microrecycling of waste materials. The engineering of the electronic band positions can modify the functionality of the SOH patches by optimising the potentials required for catalytic reactions. As a representative, the SOH nanostructures were tested for anodic electrocatalytic water oxidation to H2O2. The results showed that the ZnO–CdO patch has the lowest overpotential of 150 mV and outstanding stability at 2.33 V vs. RHE. Furthermore, the results of first-principles density functional theory (DFT) calculations (i) confirmed realignments of the band position due to the formation of midgap states, and (ii) revealed that significant improvements in the electrocatalytic H2O2 performance can be achieved with overpotentials as low as 0.19 and 0.31 V for ZnO–CdO and ZnO–Bi2O3, respectively. This work offers an ultrafast fabrication strategy to synthesise binder-free SOH nanostructures with an engineered electronic structure that can be an alternative to state-of-the-art noble metal electrocatalysts such as Pt.

Multiferroics in which giant ferroelectric polarization and magnetism coexist are of tremendous potential for engineering disruptive applications in information storage and energy conversion. Yet the functional properties of multiferroics are thought to be affected detrimentally by the presence of point defects, which may be abundant due to the volatile nature of some constituent atoms and high temperatures involved in materials preparation. Here, we demonstrate with theoretical methods that oxygen vacancies may enhance the functionality of multiferroics by radically changing their magnetic interactions in thin films. Specifically, oxygen vacancies may restore missing magnetic super-exchange interactions in large axial ratio phases, leading to full antiferromagnetic spin ordering, and induce the stabilization of ferrimagnetic states with a significant net magnetization of 0.5 uB per formula unit. Our theoretical study should help to clarify the origins of long-standing controversies in bismuth ferrite and improve the design of technological applications based on multiferroics., Comment: 11 pages, 5 figures

Secondary electrocaloric effect (ECE) arising from piezoelectricity combined with crystal thermal expansion was often overlooked in ECE studies of ferroelectric ceramics. This work mainly evaluated the primary ECE (ΔTECE) and secondary ECE (∆T2) of eco-friendly BaSnxTi1-xO3 (BSnT100x, 0

Organic–inorganic hybrid lead halide perovskite solar cells have made unprecedented progress in improving photovoltaic efficiency during the past decade, while still facing critical stability challenges. Herein, the natural organic dye Indigo is explored for the first time to be an efficient molecular passivator that assists in the preparation of high-quality hybrid perovskite film with reduced defects and enhanced stability. The Indigo molecule with both carbonyl and amino groups can provide bifunctional chemical passivation for defects. In-depth theoretical and experimental studies show that the Indigo molecules firmly binds to the perovskite surfaces, enhancing the crystallization of perovskite films with improved morphology. Consequently, the Indigo-passivated perovskite film exhibits increased grain size with better uniformity, reduced grain boundaries, lowered defect density, and retarded ion migration, boosting the device efficiency up to 23.22%, and ˜21% for large-area device (1 cm2). Furthermore, the Indigo passivation can enhance device stability in terms of both humidity and thermal stress. These results provide not only new insights into the multipassivation role of natural organic dyes but also a simple and low-cost strategy to prepare high-quality hybrid perovskite films for optoelectronic applications based on Indigo derivatives.

Structural defects are ubiquitous for polycrystalline perovskite films, compromising device performance and stability. Herein, a universal method is developed to overcome this issue by incorporating halide perovskite quantum dots (QDs) into perovskite polycrystalline films. CsPbBr3 QDs are deposited on four types of halide perovskite films (CsPbBr3, CsPbIBr2, CsPbBrI2, and MAPbI3) and the interactions are triggered by annealing. The ions in the CsPbBr3 QDs are released into the thin films to passivate defects, and concurrently the hydrophobic ligands of QDs self‐assemble on the film surfaces and grain boundaries to reduce the defect density and enhance the film stability. For all QD‐treated films, PL emission intensity and carrier lifetime are significantly improved, and surface morphology and composition uniformity are also optimized. Furthermore, after the QD treatment, light‐induced phase segregation and degradation in mixed‐halide perovskite films are suppressed, and the efficiency of mixed‐halide CsPbIBr2 solar cells is remarkably improved to over 11% from 8.7%. Overall, this work provides a general approach to achieving high‐quality halide perovskite films with suppressed phase segregation, reduced defects, and enhanced stability for optoelectronic applications., A universal approach is reported to fabricate perovskite films by incorporating inorganic CsPbBr3 quantum dots (QDs) into halide perovskite bulk films. Upon post‐annealing, the released elements from QDs compensate vacancies, and hydrophobic ligands on QDs passivate under‐charged Pb atoms and self‐assemble on surface. Therefore, the resulting films with reduced trap density, suppressed phase segregation, improved surface uniformity and enhanced stability are enabled.

Harvesting energy from ubiquitous moisture is attracting growing interest for directly powering electronic devices. However, it is still challenging to fabricate high-performing moisture-electric generators (MEGs) with high and stable electric output. Herein, we report a simple strategy to modify the oxygen-based groups of graphene oxide using hydrochloric acid treatment, which boosts the electric output based on the device structure of graphene oxide/polyvinyl alcohol (GO/PVA) MEGs. The resulting MEG enables a stable voltage of 0.85 V and a current of 9.28 µA (92.8 µA·cm-2), which are among the highest values reported so far. More excitingly, electric output gets further improved by simply assembling four MEG units in series or parallel. Moreover, the MEG shows great commercial potential for flexible and wearable applications. Driven by these advancements, the assembled MEGs can successfully power sensors and calculators. This work opens a new era of advance for a new energy conversion technology able to directly powering electronic devices. C.C. acknowledges support from the Spanish Ministry of Science, Innovation and Universities under the "Ramon y Cajal" fellowship RYC2018-024947-I

This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Materials Letters, copyright © 2021 American Chemical Societ, after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acsmaterialslett.1c00592. A cost-effective and environmentally friendly Pb source is a prerequisite for achieving large-scale, low-cost perovskite photovoltaic devices. Currently, the commonly used method to prepare the lead source is based on a fire smelting process, requiring a high temperature of more than 1000 °C, which results in environmental pollution. Spent car lead acid batteries are an environmentally hazardous waste; however, they can alternatively serve as an abundant and inexpensive Pb source. Due to “self-purification”, quantum dots feature a high tolerance of impurities in the precursor since the impurities tend to be expelled from the small crystalline cores during colloidal nucleation. Herein, PbI2 recycled from spent lead acid batteries via a facile low-temperature solution process is used to synthesize CsPbI3 quantum dots, which simultaneously brings multiple benefits including (1) avoiding pollution originating from the fire smelting process; (2) recycling the Pb waste from batteries; and (3) synthesizing high-quality quantum dots. The resulting CsPbI3 quantum dots have photophysical properties such as PLQY and carrier lifetimes on par with those synthesized from the commercial PbI2 due to expelling of the impurity Na atoms. The resulting solar cells deliver comparable power conversion efficiencies: 14.0% for the cells fabricated using recycled PbI2 and 14.7% for the cells constructed using commercial PbI2. This work paves a new and feasible path to applying recycled Pb sources in perovskite photovoltaics.

Protonated 3d transition-metal oxides often display low electronic conduction, which hampers their application in electric, magnetic, thermoelectric, and catalytic fields. Electronic conduction can be enhanced by co-inserting oxygen acceptors simultaneously. However, the currently used redox approaches hinder protons and oxygen ions co-insertion due to the selective switching issues. Here, a thermal hydration strategy for systematically exploring the synthesis of conductive protonated oxides from 3d transition-metal oxides is introduced. This strategy is illustrated by synthesizing a novel layered-oxide SrCoO3H from the brownmillerite SrCoO2.5. Compared to the insulating SrCoO2.5, SrCoO3H exhibits an unprecedented high electronic conductivity above room temperature, water uptake at 250 °C, and a thermoelectric power factor of up to 1.2 mW K-2 m-1 at 300 K. These findings open up opportunities for creating high-conductive protonated layered oxides by protons and oxygen ions co-doping. CC acknowledges support from the Spanish Ministry of Science, Innovation, and Universities under the “Ramón y Cajal” fellowship RYC2018-024947-I.

A pressure-induced structural phase transition and its intimate link with the superconducting transition was studied for the first time in TiSe2 up to 40 GPa at room temperature using X-ray diffraction, transport measurement, and first-principles calculations. We demonstrate the occurrence of a first-order structural phase transition at 4 GPa from the standard trigonal structure (S.G.P3¯m1) to another trigonal structure (S-G-P3¯c1). Additionally, at 16 GPa, the P3¯c1 phase spontaneously transforms into a monoclinic C2/m phase, and above 24 GPa, the C2/m phase returns to the initial P3¯m1 phase. Electrical transport results show that metallization occurs above 6 GPa. The charge density wave observed at ambient pressure is suppressed upon compression up to 2 GPa with the emergence of superconductivity at 2.5 GPa, with a critical temperature (Tc) of 2 K. A structural transition accompanies the emergence of superconductivity that persists up to 4 GPa. The results demonstrate that the pressure-induced phase transitions explored by the experiments along with the theoretical predictions may open the door to a new path for searching and controlling the phase diagrams of transition metal dichalcogenides. C.C. acknowledges support from the Spanish Ministry of Science, Innovation, and Universities under the “Ramon y Cajal” fellowship RYC2018-024947-I.

In this work, a new surface engineering technique through UV/ozone treatment to improve the oxygen evolution reaction performance of nickel-iron hydroxide based catalysts has been developed. It is found that UV/ozone treatment can significantly improve the hydrophilicity of the catalysts as well as modify the electronic structure at the Ni–Fe center and oxidize Ni2+ to Ni3+, which consequently enables better electrode–electrolyte contacts, stronger charge-transfer and more numbers of active sites with higher intrinsic activity during the OER. This work demonstrates UV/ozone treatment as a highly effective strategy towards the rational design of high-performance electrocatalysts by exploiting surface modifications.

The present work reports a detailed mechanistic interpretation of the role of the solubility of dopants and resultant midgap defect energies in band gap engineering. While there is a general perception that a single dopant is associated with single solubility and defect mechanisms, in reality, the potential for multiple solubility and defect mechanisms requires a more nuanced interpretation. Similarly, Kroger–Vink defect equilibria assume that stoichiometries during substitutional and interstitial solid solubility as well as Schottky and Frenkel pair formation are compensated by the diffusion of matrix ions to the grain boundaries or surface. However, this approach does not allow the possibility that stoichiometry is uncompensated, where diffusion of the matrix ion to lattice interstices occurs, followed by charge compensation by redox of this ion. Consequently, a modified defect equilibria formalism has been developed in order to allow description of this situation. Experimental data for the structural, chemical, semiconducting, and photocatalytic properties as a function of doping level are correlated with conceptual structural models, a comprehensive energy band diagram, and the corresponding defect equilibria. These correlations reveal the complex mechanisms of the interrelated solubility and defect formation mechanisms, which change significantly and irregularly as a function of small changes in doping level. The analyses confirm that the assumption of single mechanisms of solid solubility and defect formation may be simplifications of more complex processes. The generation of (1) a matrix of complementary characterisation and analytical data, (2) the calculation of a complete energy band diagram, (3) consideration of charge compensation mechanisms and redox beyond the limitations of Kroger–Vink approaches, and (4) the development of models of corresponding structural analogies combine to create a new approach to interpret and explain experimental data. These strategies allow deconstruction of these complex issues and thus targeting of optimal and possibly unique doping levels to achieve lattice configurations that may be energetically and structurally unfavorable. These approaches then can be applied to other doped semiconducting systems.

Conventional refrigeration methods based on compression-expansion cycles of greenhouse gases are environmentally threatening and cannot be miniaturized. Electrocaloric effects driven by electric fields are especially well suited for implementation of built-in cooling in portable electronic devices. However, most known electrocaloric materials present poor cooling performances near room temperature, contain toxic substances, and require high electric fields. Here, we show that lead-free ferroelectric thin-film bilayers composed of (Bi

We report a strategy for the surface engineering of electrode materials for water splitting by constructing a nickel–iron (Ni0.95Fe0.05) layered double hydroxide (LDH) nanosheets on silver nanowire...

We measured the specific heat Cp of normal (C4H4S) and deuterated (C4D4S) thiophene in the temperature interval of 1 = T, K = 25. C4H4S exhibits a metastable phase II2 and a stable phase V, both with frozen orientational disorder (OD), whereas C4D4S exhibits a metastable phase II2, which is analogous to the OD phase II2 of C4H4S and a fully ordered stable phase V. Our measurements demonstrate the existence of a large bump in the heat capacity of both stable and metastable C4D4S and C4H4S phases at temperatures of ~10 K, which significantly departs from the expected Debye temperature behavior of Cp ˜ T3. This case study demonstrates that the identified low-temperature Cp anomaly, typically referred to as a “Boson-peak” in the context of glassy crystals, is not exclusive of disordered materials.

The morphologies of ceria nanocrystals play an essential role in determining their redox and catalytic performances in many applications, yet the effects of synthesis variables on the formation of ceria nanoparticles of different morphologies and their related growth mechanisms have not been systematised. The design of these morphologies is underpinned by a range of fundamental parameters, including crystallography, optical mineralogy, the stabilities of exposed crystallographic planes, CeO2-x stoichiometry, phase equilibria, thermodynamics, defect equilibria, and the crystal growth mechanisms. These features are formalised and the key analytical methods used for analysing defects, particularly the critical oxygen vacancies, are surveyed, with the aim of providing a source of design parameters for the synthesis of nanocrystals, specifically CeO2-x. However, the most important aspect in the design of CeO2-x nanocrystals is an understanding of the roles of the main variables used for synthesis. While there is a substantial body of data on CeO2-x morphologies fabricated using low cerium concentrations ([Ce]) under different experimental conditions, the present work fully maps the effects of the relevant variables on the resultant CeO2-x morphologies in terms of the commonly used raw materials [Ce] (and [NO3-] in Ce(NO3)3·6H2O) as feedstock, [NaOH] as precipitating agent, temperature, and time (as well as the complementary vapour pressure). Through the combination of consideration of the published literature and the generation of key experimental data to fill in the gaps, a complete mechanistic description of the development of the main CeO2-x morphologies is illustrated. Further, the mechanisms of the conversion of nanochains into the two variants of nanorods, square and hexagonal, have been elucidated through crystallographic reasoning. Other key conclusions for the crystal growth process are the critical roles of (1) the formation of Ce(OH)4 crystallite nanochains as the precursors of nanorods and (2) the disassembly of the nanorods into Ce(OH)4 crystallites and NO3--assisted reassembly into nanocubes (and nanospheres) as an unrecognised intermediate stage of crystal growth.

Metal oxide pseudocapacitors are limited by low electrical and ionic conductivities. The present work integrates defect engineering and architectural design to exhibit, for the first time, intercalation pseudocapacitance in CeO2–x. An engineered chronoamperometric electrochemical deposition is used to synthesize 2D CeO2–x nanoflakes as thin as ~12 nm. Through simultaneous regulation of intrinsic and extrinsic defect concentrations, charge transfer and charge–discharge kinetics with redox and intercalation capacitances together are optimized, where reduction increases the gravimetric capacitance by 77% to 583 F g–1, exceeding the theoretical capacitance (562 F g–1). Mo ion implantation and reduction processes increase the specific capacitance by 133%, while the capacitance retention increases from 89 to 95%. The role of ion-implanted Mo6+ is critical through its interstitial solid solubility, which is not to alter the energy band diagram but to facilitate the generation of electrons and to establish the midgap states for color centers, which facilitate electron transfer across the band gap, thus enhancing n-type semiconductivity. Critically, density functional theory simulations reveal, for the first time, that the reduction causes the formation of ordered oxygen vacancies that provide an atomic channel for ion intercalation. These channels enable intercalation pseudocapacitance but also increase electrical and ionic conductivities. In addition, the associated increased active site density enhances the redox such that the 10% of the Ce3+ available for redox (surface only) increases to 35% by oxygen vacancy channels. These findings are critical for any oxide system used for energy storage systems, as they offer both architectural design and structural engineering of materials to maximize the capacitance performance by achieving accumulative surface redox and intercalation-based redox reactions during the charge/discharge process.

Facile patterning technologies of silver nanowires (AgNWs) with low-cost, high-resolution, designable, scalable, substrate-independent, and transferable characteristics are highly desired. However, it remains a grand challenge for any material processing method to fulfil all desirable features. Herein, a new patterning method is introduced by combining inkjet printing with adhesion manipulation of substrate interfaces. Both positive and negative patterns (i.e., AgNW grid and rectangular patterns) have been simultaneously achieved, and the pattern polarity can be reversed through adhesion modification with judiciously selected supporting layers. The electrical performance of the AgNW grids depends on the AgNW interlocking structure, manifesting a strong structure-property correlation. High-resolution and complex AgNW patterns with line width and spacing as small as 10 μm have been demonstrated through selective deposition of poly(methyl methacrylate) layers. In addition, customized AgNW patterns, such as logos and words, can be fabricated onto A4-size samples and subsequently transferred to targeted substrates, including Si wafers, a curved glass vial, and a beaker. This reported inkjet-assisted process therefore offers a new effective route to manipulate AgNWs for advanced device applications.

Organic-inorganic perovskites have attracted increasing attention in recent years owing to their excellent optoelectronic properties and photovoltaic performance. In this work, the prototypical hybrid perovskite CH

Hybrid halide perovskites have emerged as highly promising photovoltaic materials because of their exceptional optoelectronic properties, which are often optimized via compositional engineering like mixing halides. It is well established that hybrid perovskites undergo a series of structural phase transitions as temperature varies. In this work, the authors find that phase transitions are substantially suppressed in mixed-halide hybrid perovskite single crystals of MAPbI3-xBrx (MA = CH3NH3+ and x = 1 or 2) using a complementary suite of diffraction and spectroscopic techniques. Furthermore, as a general behavior, multiple crystallographic phases coexist in mixed-halide perovskites over a wide temperature range, and a slightly distorted monoclinic phase, hitherto unreported for hybrid perovskites, is dominant at temperatures above 100 K. The anomalous structural evolution is correlated with the glassy behavior of organic cations and optical phonons in mixed-halide perovskites. This work demonstrates the complex interplay between composition engineering and lattice dynamics in hybrid perovskites, shedding new light on their unique properties.

The lack of understanding in engineering cation defects in metal oxides has impeded the development of high performance, and transparent electronic devices. Through studying the formation energy of various cationic defects in Mn-doped SnO2 via simulation, we found Mn3+ cations occupy the interstitial sites of SnO2 nanocrystals, and we proved that such defects can be engineered to significantly improve resistive switching performance of tin oxide-based devices. With this finding, a new solution-processed approach has been developed to synthesize Mn-doped SnO2 nanocrystals with a self-assembly technique for high quality transparent Mn-doped SnO2 thin film fabrication. Defect migration behavior of the Mn-doped SnO2 thin film was studied by building a metal-oxide-metal sandwich device. The effects of cationic defects, such as Mn interstitials, on the charge transport behavior were further studied to reveal the underlying mechanism. This study provides new insights into the design and engineering of defects in transparent oxides for high-density data storage applications. Keywords: Nanocrystal growth, Self-assembly, Tin oxide, Liquid liquid interface

Magnetic materials with large magnetic anisotropy are essential for workaday applications such as permanent magnets and magnetic data storage. There is widespread interest in finding efficient ways of controlling magnetic anisotropy, among which strain control has proven to be a very powerful technique. Here, we demonstrate the strain-mediated magnetic anisotropy in SrCoO3−δ thin film, a perovskite oxide that is metallic and adopts a cubic structure at δ ≤ 0.25. We find that the easy-magnetization axis in SrCoO3−δ can be rotated by 90° upon application of moderate epitaxial strains ranging from −1.2 to +1.8%. The magnetic anisotropy in compressive SrCoO3−δ thin films is huge, as shown by magnetic hysteresis loops rendering an anisotropy energy density of ∼106 erg/cm3. The local variance in magnetic force microscopy upon temperature and magnetic field reveals that the evolution of magnetic domains in the SCO thin film is strongly dependent on magnetic anisotropy.

Interfacial thermal transport plays a prominent role in the thermal management of nanoscale objects and is of fundamental importance for basic research and nanodevices. At metal/insulator interfaces, a configuration commonly found in electronic devices, heat transport strongly depends upon the effective energy transfer from thermalized electrons in the metal to the phonons in the insulator. However, the mechanism of interfacial electron-phonon coupling and thermal transport at metal/insulator interfaces is not well understood. Here, the observation of a substantial enhancement of the interfacial thermal resistance and the important role of surface charges at the metal/ferroelectric interface in an Al/BiFeO3 membrane are reported. By applying uniaxial strain, the interfacial thermal resistance can be varied substantially (up to an order of magnitude), which is attributed to the renormalized interfacial electron-phonon coupling caused by the charge redistribution at the interface due to the polarization rotation. These results imply that surface charges at a metal/insulator interface can substantially enhance the interfacial electron-phonon-mediated thermal coupling, providing a new route to optimize the thermal transport performance in next-generation nanodevices, power electronics, and thermal logic devices., Y.Z., C.D., and Z.G. contributed equally to this work. This work was supported by the National Natural Science Foundation of China (nos. 11774153, 51772143, 11474158, 11890700, 11904162, 1861161004, 11625418, 11974163, 51732006, 52027803, 61704074, and 91963211), the National Basic Research (Key R&D) Program of China (2017YFA0303702 and 2018YFA0306200), the introduced innovative R&D team of Guangdong (2017ZT07C062), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (51720001), and the Fundamental Research Funds for the Central Universities (nos. 0213-14380198 and 0213-14380167). Y.N. was supported by High-Level Entrepreneurial and Innovative Talents Introduction, Jiangsu Province. Theoretical work was funded by the Luxembourg National Research Fund through project FNR/C18/MS/12705883/REFOX. (H.A. and J.Í.). C.C. acknowledges support from the Spanish Ministry of Science, Innovation, and Universities under the “Ramón y Cajal” fellowship RYC2018-024947-I. R.R. acknowledges financial support by the Ministerio de Ciencia e Innovación (MICINN) under grant FEDER-MAT2017-90024-P and the Severo Ochoa Centres of Excellence Program under Grant CEX2019-000917-S and by the Generalitat de Catalunya under grants no. 2017 SGR 1506. The authors thank the Centro de Supercomputación de Galicia (CESGA) for the use of their computational resources. The authors thank Yaya Zhou for the support of SEM and EDS measurement., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Finding solid-state electrolytes with high ionic conductivity near room temperature is an important prerequisite for developing all-solid-state electrochemical batteries. Here, we investigate the effects of point defects (vacancies) and biaxial stress on the superionic properties of fast-ion conductors (represented by the archetypal compounds CaF2, Li-rich antiperovskite Li3OCl, and AgI) by using classical molecular dynamics and first-principles simulation methods. We find that the critical superionic temperature of all analyzed families of fast-ion conductors can be reduced by several hundreds of degrees through the application of relatively small biaxial stresses (|σ| ≤ 1 GPa) on slightly defective samples (cv ∼ 1%). In AgI, we show that superionicity can be triggered at room temperature by applying a moderate compressive biaxial stress of ∼1 GPa. In this case, we reveal the existence of a σ-induced order–disorder phase transition involving sizable displacements of all the ions with respect to the equil...

In this study, we report the effect of doping in morphotropic BiFeO3 (BFO) thin films on mechanical properties, revealing variations in the elasticity across the competing phases and their boundaries. Spectroscopic force–distance (F–D) curves and force mapping images by AFM are used to characterize the structure and elastic properties of three BFO thin-film candidates (pure-BFO, Ca-doped BFO, La-doped BFO). We show that softening behavior is observed in isovalent La-doped BFO, whereas hardening is seen in aliovalent Ca-doped BFO. Furthermore, quantitative F–D measurements are extended to show threshold strengths for phase transitions, revealing their dependence on doping in the system. First-principles simulation methods are also employed to understand the observed mechanical properties in pure and doped BFO thin films and to provide microscopic insight on them. These results provide key insight into doping as an effective control parameter to tune nanomechanical properties and suggest an alternative fram...

CeO2 is a promising candidate for materials utilized in solid oxide fuel cells (SOFCs) due to its high ionic conductivity. The high operating temperature of SOFCs results in residual thermal stress in the composing materials. In this work, we studied simultaneously the mechanical and electronic behavior of CeO2 under different uniaxial tensile loading directions using density functional theory. CeO2 shows strong anisotropic mechanical and electronic behavior under uniaxial tensile strain that it has the highest ideal strength and fracture strain along [100] direction. Meanwhile, [100] tensile strain also leads to the largest band gap reduction compared with the other two strain directions. The analysis of the mechanism shows that the highest strength along [100] direction is from the highest Young's modulus and surface energy. Meanwhile, the analysis on the band gap variation using a theoretical model previously developed by us suggest that the largest average bond length and dielectric susceptibility variation leads to the largest band gap reduction when [100] tensile strain is applied to CeO2. Therefore, the current study provides a meaningful insight into the mechanical and electronic properties of CeO2 under stress, which is vital for its application as SOFCs’ materials.

Mechanocaloric materials experience a change in temperature when a mechanical stress is applied on them adiabatically. Thus, far, only ferroelectrics and superelastic metallic alloys have been considered as potential mechanocaloric compounds to be exploited in solid-state cooling applications. Here we show that giant mechanocaloric effects occur in hitherto overlooked fast ion conductors (FIC), a class of multicomponent materials in which above a critical temperature, Ts, a constituent ionic species undergoes a sudden increase in mobility. Using first-principles and molecular dynamics simulations, we found that the superionic transition in fluorite-structured FIC, which is characterized by a large entropy increase of the order of 10(2) JK(-1) kg(-1), can be externally tuned with hydrostatic, biaxial, or uniaxial stresses. In particular, Ts can be reduced several hundreds of degrees through the application of moderate tensile stresses due to the concomitant drop in the formation energy of Frenkel pair defects. We predict that the adiabatic temperature change in CaF2 and PbF2, two archetypal fluorite-structured FIC, close to their critical points are of the order of 10(2) and 10(1) K, respectively. This work advocates that FIC constitute a new family of mechanocaloric materials showing great promise for prospective solid-state refrigeration applications.

Photocatalytic materials are pivotal for the implementation of disruptive clean energy applications such as conversion of H$_{2}$O and CO$_{2}$ into fuels and chemicals driven by solar energy. However, efficient and cost-effective materials able to catalyze the chemical reactions of interest when exposed to visible light are scarce due to the stringent electronic conditions that they must satisfy. Chemical and nanostructuring approaches are capable of improving the catalytic performance of known photoactive compounds however the complexity of the synthesized nanomaterials and sophistication of the employed methods make systematic design of photocatalysts difficult. Here, we show by means of first-principles simulation methods that application of biaxial stress, $\eta$, on semiconductor oxide thin films can modify their optoelectronic and catalytic properties in a significant and predictable manner. In particular, we show that upon moderate tensile strains CeO$_{2}$ and TiO$_{2}$ thin films become suitable materials for photocatalytic conversion of H$_{2}$O into H$_{2}$ and CO$_{2}$ into CH$_{4}$ under sunlight. The band gap shifts induced by $\eta$ are reproduced qualitatively by a simple analytical model that depends only on structural and dielectric susceptibility changes. Thus, epitaxial strain represents a promising route for methodical screening and rational design of photocatalytic materials.

Altres ajuts: ICN2 is supported by the CERCA Programme/Generalitat de Catalunya. Holey 2D metal oxides have shown great promise as functional materials for energy storage and catalysts. Despite impressive performance, their processing is challenged by the requirement of templates plus capping agents or high temperatures; these materials also exhibit excessive thicknesses and low yields. The present work reports a metal-based coordination polymer (MCP) strategy to synthesize polycrystalline, holey, metal oxide (MO) nanosheets with thicknesses as low as two-unit cells. The process involves rapid exfoliation of bulk-layered, MCPs (Ce-, Ti-, Zr-based) into atomically thin MCPs at room temperature, followed by transformation into holey 2D MOs upon the removal of organic linkers in aqueous solution. Further, this work represents an extra step for decorating the holey nanosheets using precursors of transition metals to engineer their band alignments, establishing a route to optimize their photocatalysis. The work introduces a simple, high-yield, room-temperature, and template-free approach to synthesize ultrathin holey nanosheets with high-level functionalities.

In the context of novel solid electrolytes for solid-state batteries, first-principles calculations are becoming increasingly more popular due to their ability to reproduce and predict accurately the energy, structural, and dynamical properties of fast-ion conductors. To accelerate the discovery of new superionic conductors is convenient to establish meaningful relations between ionic transport and simple materials descriptors. Recently, several experimental studies on lithium fast-ion conductors suggested a correlation between lattice softness and enhanced ionic conductivity due to a concomitant decrease in the activation energy for ion migration ${E}_{a}$. In this article, we employ extensive ab initio molecular dynamics simulations based on density functional theory to substantiate the links between ionic transport and lattice dynamics in a number of structurally and chemically distinct lithium superionic conductors. Our first-principles results show no evidence for a direct and general correlation between ${E}_{a}$, or the hopping attempt frequency ${\ensuremath{\nu}}_{0}$, and lattice softness. However, we find that, in agreement with recent observations, the pre-exponential factor of lithium diffusivity ${D}_{0}$, which is proportional to ${\ensuremath{\nu}}_{0}$, follows the Meyer-Neldel rule $\ensuremath{\propto}exp\left({E}_{a}/\ensuremath{\langle}\ensuremath{\omega}\ensuremath{\rangle}\right)$ where $\ensuremath{\langle}\ensuremath{\omega}\ensuremath{\rangle}$ represents an average phonon frequency. Hence, lattice softness can be identified with enhanced lithium diffusivity, but only within families of superionic materials presenting very similar migration activation energies due to an increase in ${D}_{0}$ (or, equivalently, in ${\ensuremath{\nu}}_{0}$). On the technical side, we show that neglecting temperature effects in the estimation of ${E}_{a}$ may lead to huge inaccuracies of $\ensuremath{\sim}10%$. The limitations of zero-temperature harmonic approaches in describing the vibrational properties of lithium-ion conductors are also illustrated.

We study the structural and mobility properties of edge dislocations in rare-gas crystals with the hexagonal close-packed (hcp) structure by using classical simulation techniques. Our results are discussed in the light of recent experimental and theoretical studies on hcp 4 He, an archetypal quantum crystal. According to our simulations classical hcp rare-gas crystals present a strong tendency towards dislocation dissociation into Shockley partials in the basal plane, similarly to what is observed in solid helium. This is due to the presence of a low-energy metastable stacking fault, of the order of 0.1 mJ/m 2 , that can get further reduced by quantum nuclear effects. We compute the minimum shear stress that induces glide of dislocations within the hcp basal plane at zero temperature, namely, the Peierls stress, and find a characteristic value of the order of 1 MPa. This threshold value is similar to the Peierls stress reported for metallic hcp solids (Zr and Cd) but orders of magnitude larger than the one estimated for solid helium. We find, however, that in contrast to classical hcp metals but in analogy to solid helium, glide of edge dislocations can be thermally activated at very low temperatures, T∼10 K, in the absence of any applied shear stress.

We present a first-principles study of the energy and elastic properties of solid helium at pressures below the range in which it is energetically stable. We find that the limit of mechanical stability in hcp \(^{4}\)He is \(P_{s} = -33.8(1)\) bar, which lies significantly below the spinodal pressure found in the liquid phase (i.e., \(-9.6\) bar). Furthermore, we show that the pressure variation of the transverse and longitudinal sound velocities close to \(P_{s}\) does not follow a power law of the form \(\propto \left( P - P_{s} \right) ^{\gamma }\), in contrast to what is observed in the fluid.

Piezoelectric and other physical properties are significantly enhanced at (or near) a morphotropic phase boundary (MPB) in ferroelectrics. MPB materials have attracted significant attention owing to both fundamental physics as well as the possibility of well-regulated energy and information storage devices which are dominated by lead (Pb)-based materials. Here, we report the crystal structure, Raman spectra, dielectric constant and polarization near the MPB of lead free (1 - x) Na0.5Bi0.5TiO3 - x BaTiO3 (0.00 ⩽ x ⩽ 0.10) solid-solution, prepared by sol-gel auto combustion technique and sintered by microwave sintering technique. With the addition of BaTiO3 into Na0.5Bi0.5TiO3, it induces a structural phase transition from R3c (a single phase) to R3c + P4mm (a dual phase) close to x = 0.06 and 0.07 and transform to a high symmetry tetragonal phase P4mm at higher compositions (x = 0.08 to 0.10) as evident from our x-ray Rietveld refinement and Raman spectroscopic results. We perform first-principles calculations based on density functional theory that confirm a structural transition from a rhombohedral to a tetragonal phase under increasing x. In the prepared solid solution, an anomalous enhancement of remnant polarization ([Formula: see text]) was observed for x = 0.06 and 0.07, which has been explained based on the existence of the MPB. On the other hand, the value of coercive field [Formula: see text] was found to be decreased linearly from x = 0.00 to 0.06; it is constant for higher compositions. Further details of the ferroelectric properties on the electric field poled samples have been studied and compared with the as-grown (unpoled) samples.

The behavior of pairs and clusters of vacancies in solid 4He crystals is studied with diffusion Monte Carlo method. We use a trial function suitable for describing solid 4He with long range order, arbitrary number of unoccupied sites and explicit Bose symmetry. It is found that vacancy clusters are unstable and collapse in all considered systems. We find no signature of stability of bound vacancy clusters of any size. Vacancies are found to exhibit weak attraction and a rough estimation of the binding energy is reported.

The ground state of the ionic solids LiD and LiH is theoretically studied using the Variational Monte Carlo (VMC) method. Our main focus has been the calculation of relevant properties of the H− and D− ions using a fully quantum approach. In particular, we report results on their kinetic energies, mean-squared displacements, Einstein frequencies, radial distribution functions, and density profiles around the sites. The microscopic results obtained for both isotopes show corrections beyond trivial isotopic effects due to their quantum behavior. Finally, the VMC results are compared with predictions from self consistent average phonon (SCAP) theory at T = 0 K.

A quantum Monte Carlo study at zero temperature of twodimensional H2 on top of a solid Rb surface is presented. The lattice constituted by the alkali atoms frustrates the formation of solid H2, which is the natural phase of pure two-dimensional H2. Therefore, H2 with the Rb atoms acting like fixed impurities remains in a liquid phase even at zero temperature. We present resugts for the H2 equation of state, structural properties, superfluid density and condensate fraction for a fixed Rb lattice.

We study the zero-temperature phase diagrams of H2 adsorbed on the three structures predicted for graphane (chair, boat and washboard graphane), using a dif- fusion Monte Carlo technique. Graphane is the hydrogenated version of graphene, in which each carbon atom changes its hybridization to sp 3 and forms a covalent bond with a hydrogen atom. Our results show that the ground state of H2 adsorbed on all three types of graphane is a √ 3 × √ 3 solid, similar to the structures found both for H2 and D2 on graphene. When the H2 density increases, the system undergoes a first order phase transition to a triangular incommensurate solid. This change is direct in the case of washboard graphane, but indirect via different commensurate structures in the other cases. The total hydrogen weight percentage on the three graphane types in their ground states is in the range 10 % to 12 %, depending on if one or both graphane surfaces are covered with H2.

We present a combined theoretical and experimental study of the high-pressure behavior of thallium. X-ray diffraction experiments have been carried out at room temperature up to 125 GPa using diamond-anvil cells, nearly doubling the pressure range of previous experiments. We have confirmed the hcp-fcc transition at 3.5 GPa and determined that the fcc structure remains stable up to the highest pressure attained in the experiments. In addition, HP-HT experiments have been performed up to 8 GPa and 700 K by using a combination of x-ray diffraction and a resistively heated diamond-anvil cell. Information on the phase boundaries is obtained, as well as crystallographic information on the HT bcc phase. The equation of state for different phases is reported. Ab initio calculations have also been carried out considering several potential high-pressure structures. They are consistent with the experimental results and predict that, among the structures considered in the calculations, the fcc structure of thallium is stable up to 4.3 TPa. Calculations also predict the post-fcc phase to have a close-packed orthorhombic structure above 4.3 TPa., Comment: 29 pages, 14 figures

In a recent study we have reported a new type of trial wave function symmetric under the exchange of particles and which is able to describe a supersolid phase. In this work, we use the diffusion Monte Carlo method and this model wave function to study the properties of solid 4He in two- and quasi two-dimensional geometries. In the purely two-dimensional case, we obtain results for the total ground-state energy and freezing and melting densities which are in good agreement with previous exact Monte Carlo calculations performed with a slightly different interatomic potential model. We calculate the value of the zero-temperature superfluid fraction \rho_{s} / \rho of 2D solid 4He and find that it is negligible in all the considered cases, similarly to what is obtained in the perfect (free of defects) three-dimensional crystal using the same computational approach. Interestingly, by allowing the atoms to move locally in the perpendicular direction to the plane where they are confined to zero-point oscillations (quasi two-dimensional crystal) we observe the emergence of a finite superfluid density that coexists with the periodicity of the system., Comment: 16 pages, 8 figures

We study the zero-temperature equation of state (EOS) of solid 4He in the hexagonal closed packet (hcp) phase over the 0-57 GPa pressure range by means of the Diffusion Monte Carlo (DMC) method and the semi-empirical Aziz pair potential HFD-B(HE). In the low pressure regime (P ~ 0-1 GPa) we assess excellent agreement with experiments and we give an accurate description of the atomic kinetic energy, Lindemann ratio and Debye temperature over a wide range of molar volumes (22-6 cm^{3}/mol). However, on moving to higher pressures our calculated P-V curve presents an increasingly steeper slope which ultimately provides differences within ~40 % with respect to measurements. In order to account for many-body interactions arising in the crystal with compression which are not reproduced by our model, we perform additional electronic density-functional theory (DFT) calculations for correcting the computed DMC energies in a perturbative way. We explore both generalized gradient and local density approximations (GGA and LDA, respectively) for the electronic exchange-correlation potential. By proceeding in this manner, we show that discrepancies with respect to high pressure data are reduced to 5-10 % with few computational extra cost. Further comparison between our calculated EOSs and ab initio curves deduced for the perfect crystal and corrected for the zero-point motion of the atoms enforces the reliability of our approach., 29 pages, 9 figures. To be published in Journal of Physics: Condensed Matter

We determined the zero-temperature phase diagram of D$_2$ physisorbed on graphane using the diffusion Monte Carlo method. The substrate used was C-graphane, an allotropic form of the compound that has been experimentally obtained through hydrogenation of graphene. We found that the ground state is the $\delta$ phase, a commensurate structure observed experimentally when D$_2$ is adsorbed on graphite, and not the registered $\sqrt 3 \times \sqrt 3$ structure characteristic of H$_2$ on the same substrate., Comment: 6 figures