Origin of band gaps in 3d perovskite oxides

Quantum fundaments of catalysis: true electronic potential energy

Catalysis is a quantum phenomenon enthalpically driven by electronic correlations with many-particle effects in all of its branches, including electro-photo-catalysis and electron transfer. This means that only probability amplitudes provide a complete relationship between the state of catalysis and observations. Thus, in any atomic system material), competing space-time electronic interactions coexist to define its (related) properties such as stability, (super)conductivity, magnetism (spin-orbital ordering), chemisorption and catalysis. Catalysts, reactants, and chemisorbed and transition states have the possibility of optimizing quantum correlations to improve reaction kinetics. Active sites with closed-shell orbital configurations share a maximum number of spin-paired electrons, mainly optimizing coulombic attractions and covalency and defining weakly correlated closed-shell (WCCS) structures. However, in compositions with open-shell orbital configurations, at least, quantum spin exchange interactions (QSEIopenshells) arise, stabilising unpaired electrons in less covalent bonds and differentiating non-weakly (or strongly) correlated open-shell (NWCOS) systems. In NWCOS catalysts, electronic ground states can have bonds with diverse and rival spin-orbital orderings as well as ferro-, ferri- and multiple antiferro-magnetic textures, which deeply define their activities. Particularly in inter-atomic ferromagnetic (FM) bonds, the increase in relevance of non-classical quantum potentials can significantly optimize chemisorption energies, transition states (TSs), activation energies (overpotential) and spin-dependent electron transfer (conductivity), overall implying the need for explaining the thermodynamic and kinetic origin of catalysis from its true quantum electronic energy. To do so, we use the connection between the Born-Oppenheimer approximation and Virial theorem in the treatment of electronic kinetic and potential energies. Thus, the exact fundamental interactions that decompose TSs appear. The possibility of increasing the stabilization of TSs, due to quantum correlations on NWCO catalysts, opens the possibility of simultaneously reducing chemisorption enthalpies and activation barriers of reaction mechanisms, which implies the anticipation and explanation of positive deviations from the Brønsted-Evans-Polanyi principle.

Unraveling the Correlation between the Interface Structures and Tunable Magnetic Properties of La1-xSrxCoO3-δ/La1-xSrxMnO3-δ Bilayers Using Deep Learning Models

Perovskite oxides are gaining significant attention for use in next-generation magnetic and ferroelectric devices due to their exceptional charge transport properties and the opportunity to tune the charge, spin, lattice, and orbital degrees of freedom. Interfaces between perovskite oxides, exemplified by La1-xSrxCoO3-δ/La1-xSrxMnO3-δ (LSCO/LSMO) bilayers, exhibit unconventional magnetic exchange switching behavior, offering a pathway for innovative designs in perovskite oxide-based devices. However, the precise atomic-level stoichiometric compositions and chemophysical properties of these interfaces remain elusive, hindering the establishment of surrogate design principles. We leverage first-principles simulations, evolutionary algorithms, and neural network searches with on-the-fly uncertainty quantification to design deep learning model ensembles to investigate over 50,000 LSCO/LSMO bilayer structures as a function of oxygen deficiency (δ) and strontium concentration (x). Structural analysis of the low-energy interface structures reveals that preferential segregation of oxygen vacancies toward the interfacial La0.7Sr0.3CoO3-δ layers causes distortion of the CoOx polyhedra and the emergence of magnetically active Co2+ ions. At the same time, an increase in the Sr concentration and a decrease in oxygen vacancies in the La0.7Sr0.3MnO3-δ layers tend to retain MnO6 octahedra and promote the formation of Mn4+ ions. Electronic structure analysis reveals that the nonuniform distributions of Sr ions and oxygen vacancies on both sides of the interface can alter the local magnetization at the interface, showing a transition from ferromagnetic (FM) to local antiferromagnetic (AFM) or ferrimagnetic regions. Therefore, the exotic properties of La1-xSrxCoO3-δ/La1-xSrxMnO3-δ are strongly coupled to the presence of hard/soft magnetic layers, as well as the FM to AFM transition at the interface, and can be tuned by changing the Sr concentration and oxygen partial pressure during growth. These insights provide valuable guidance for the precise design of perovskite oxide multilayers, enabling tailoring of their functional properties to meet specific requirements for various device applications.

Emerging Two-Dimensional Conductivity at the Interface between Mott and Band Insulators

Intriguingly, conducting perovskite interfaces between ordinary band insulators are widely explored, whereas similar interfaces with Mott insulators are still not quite understood. Here, we address the (001), (110), and (111) interfaces between the LaTiO_{3} Mott, and large band gap KTaO_{3} insulators. Based on first-principles calculations, we reveal a mechanism of interfacial conductivity, which is distinct from a formerly studied one applicable to interfaces between polar wideband insulators. Here, the key factor causing conductivity is the matching of oxygen octahedra tilting in KTaO_{3} and LaTiO_{3} which, due to a small gap in the LaTiO_{3} results in its sensitivity to the crystal structure, yields metallization of its overlayer and following charge transfer from Ti to Ta. Our findings, also applicable to other Mott insulators interfaces, shed light on the emergence of conductivity observed in LaTiO_{3}/KTaO_{3} (110) where the "polar" arguments are not applicable and on the emergence of superconductivity in these structures.

Surface triggered stabilization of metastable charge-ordered phase in SrTiO3

Charge ordering (CO), characterized by a periodic modulation of electron density and lattice distortion, has been a fundamental topic in condensed matter physics, serving as a potential platform for inducing novel functional properties. The charge-ordered phase is known to occur in a doped system with high d-electron occupancy, rather than low occupancy. Here, we report the realization of the charge-ordered phase in electron-doped (100) SrTiO3 epitaxial thin films that have the lowest d-electron occupancy i.e., d1-d0. Theoretical calculation predicts the presence of a metastable CO state in the bulk state of electron-doped SrTiO3. Atomic scale analysis reveals that (100) surface distortion favors electron-lattice coupling for the charge-ordered state, and triggering the stabilization of the CO phase from a correlated metal state. This stabilization extends up to six unit cells from the top surface to the interior. Our approach offers an insight into the means of stabilizing a new phase of matter, extending CO phase to the lowest electron occupancy and encompassing a wide range of 3d transition metal oxides.

Giant adiabatic temperature change and its direct measurement of a barocaloric effect in a charge-transfer solid

Solid refrigerants exhibiting a caloric effect upon applying external stimuli are receiving attention as one of the next-generation refrigeration technologies. Herein, we report a new inorganic refrigerant, rubidium cyano-bridged manganese-iron-cobalt ternary metal assembly (cyano-RbMnFeCo). Cyano-RbMnFeCo shows a reversible barocaloric effect with large reversible adiabatic temperature changes of 74 K (from 57 °C to -17 °C) at 340 MPa, and 85 K (from 88 °C to 3 °C) at 560 MPa. Such large reversible adiabatic temperature changes have yet to be reported among caloric effects in solid-solid phase transition refrigerants. The reversible refrigerant capacity is 26000 J kg-1 and the temperature window is 142 K. Additionally, cyano-RbMnFeCo shows barocaloric effects even at low pressures, e.g., reversible adiabatic temperature change is 21 K at 90 MPa. Furthermore, direct measurement of the temperature change using a thermocouple shows +44 K by applying pressure. The temperature increase and decrease upon pressure application and release are repeated over 100 cycles without any degradation of the performance. This material series also possesses a high thermal conductivity value of 20.4 W m-1 K-1. The present barocaloric material may realize a high-efficiency solid refrigerant.

Electronic and structural properties of mixed-cation hybrid perovskites studied using an efficient spin-orbit included DFT-1/2 approach

Fundamental understanding and optimization of the emerging mixed organic-inorganic hybrid perovskites for solar cells require multiscale modeling starting from ab initio quantum mechanics methods. Particularly, it is important to correctly predict the structural and electronic properties such as phase stability, lattice parameters, band gaps, and band structures. Although density functional theory is the method of choice to address these properties and generate the input for subsequent multiscale, high-throughput, and data-driven approaches, standard exchange correlation functionals fail to reproduce the bandgap, particularly if spin-orbit coupling (SOC) is correctly taken into account. While many SOC-included hybrid functionals suffer from low transferability between different molecular ions and are computationally costly, we propose an efficient multistep simulation protocol based on the DFT-1/2 method. We apply this approach to APbI3 with A: FA, MA, Cs, and systems with mixed cations and show how the choice of the A-cation modifies the Pb-I scaffold and the hydrogen bonding and discuss their interplay with structural stability. Furthermore, band gaps, band structures, Rashba band splitting, Born effective charges as well as partial density of states (PDOS) are compared for different cases w/wo the SOC effect and the DFT-1/2 approach.

Complex Oxides for Brain-Inspired Computing: A Review

The fields of brain-inspired computing, robotics, and, more broadly, artificial intelligence (AI) seek to implement knowledge gleaned from the natural world into human-designed electronics and machines. In this review, the opportunities presented by complex oxides, a class of electronic ceramic materials whose properties can be elegantly tuned by doping, electron interactions, and a variety of external stimuli near room temperature, are discussed. The review begins with a discussion of natural intelligence at the elementary level in the nervous system, followed by collective intelligence and learning at the animal colony level mediated by social interactions. An important aspect highlighted is the vast spatial and temporal scales involved in learning and memory. The focus then turns to collective phenomena, such as metal-to-insulator transitions (MITs), ferroelectricity, and related examples, to highlight recent demonstrations of artificial neurons, synapses, and circuits and their learning. First-principles theoretical treatments of the electronic structure, and in situ synchrotron spectroscopy of operating devices are then discussed. The implementation of the experimental characteristics into neural networks and algorithm design is then revewed. Finally, outstanding materials challenges that require a microscopic understanding of the physical mechanisms, which will be essential for advancing the frontiers of neuromorphic computing, are highlighted.

Correlation versus hybridization gap in CaMn[Formula: see text]Bi[Formula: see text]

We study the interplay between electronic correlations and hybridization in the low-energy electronic structure of CaMn[Formula: see text]Bi[Formula: see text], a candidate hybridization-gap semiconductor. By employing a DFT+U approach we find both the antiferromagnetic Néel order and band gap in good agreement with the corresponding experimental values. Under hydrostatic pressure, we find a crossover from hybridization gap to charge-transfer insulting physics due to the delicate balance of hybridization and correlations. Increasing the pressure above [Formula: see text] GPa we find a simultaneous pressure-induced volume collapse, plane-to-chain, insulator to metal transition. Finally, we have also analyzed the topology in the antiferromagnetic CaMn[Formula: see text]Bi[Formula: see text] for all pressures studied.

Effect of pressure on the electronic structure of antiferromagnetic and paramagnetic YNiO3: the role of disproportionation

The dependence of electronic properties of quantum materials on external controls (e.g., pressure and temperature) is one of the fundamentals of neuromorphic computing, sensors, etc. Until recently, it has been believed that the theoretical description of such compounds cannot be accomplished using "traditional" density functional theory, and more advanced methods like dynamic mean-field theory should be utilized instead. Focusing here on the example of long-range ordered antiferromagnetic and paramagnetic YNiO₃ phases, we show the interplay between spin and structural motifs under pressure and their impact on electronic properties. We successfully describe the insulating nature of both YNiO₃ phases and the role of symmetry-breaking motifs in the band gap opening. Moreover, by analyzing the pressure-dependent distribution of local motifs, we show that external pressure can significantly reduce the band gap energy of both phases, originating from the reduction of structural and magnetic disproportionation - change in the distribution of local motifs. These results thus demonstrate that some of the experimental observations in quantum materials (e.g., YNiO₃ compounds) can be fully understood without accounting for dynamic correlation.

Competing electronic states emerging on polar surfaces

Excess charge on polar surfaces of ionic compounds is commonly described by the two-dimensional electron gas (2DEG) model, a homogeneous distribution of charge, spatially-confined in a few atomic layers. Here, by combining scanning probe microscopy with density functional theory calculations, we show that excess charge on the polar TaO2 termination of KTaO3(001) forms more complex electronic states with different degrees of spatial and electronic localization: charge density waves (CDW) coexist with strongly-localized electron polarons and bipolarons. These surface electronic reconstructions, originating from the combined action of electron-lattice interaction and electronic correlation, are energetically more favorable than the 2DEG solution. They exhibit distinct spectroscopy signals and impact on the surface properties, as manifested by a local suppression of ferroelectric distortions.

Core-hole delocalization for modeling X-ray spectroscopies: A cautionary tale

The influence of core-hole delocalization for X-ray photoelectron, X-ray absorption, and X-ray emission spectrum calculations is investigated in detail, using approaches including response theory, transition-potential methods, and ground state schemes. The question of a localized/delocalized vacancy is relevant for systems with symmetrically equivalent atoms, as well as near-degeneracies which can distribute the core-orbitals over several atoms. We show that issues relating to core-hole delocalization are present for calculations considering explicit core-hole states, e.g. when using a core-excited or core-ionized reference state, or for fractional occupation numbers. Including electron correlation eventually alleviates the issues, but even using CCSD(T) there is a noticable discrepancy between core-ionization energies obtained with a localized and delocalized core-hole (0.5 eV for the carbon K-edge). Within density functional theory, the discrepancy correlates to the exchange interaction involving the core orbitals of the same spin symmetry as the delocalized core-hole. The use of a localized core-hole allows for a reasonably good inclusion of relaxation at lower level of theory, whereas the proper symmetry solution involving a delocalized core-hole requires higher levels of theory to account for the correlation effects involved in orbital relaxation. For linear response methods, we further show that if X-ray absorption spectra are modelled by considering symmetry-unique sets of atoms, care has to be taken such that there are no delocalizations of the core orbitals, which would otherwise introduce shifts in absolute energies and relative features.

Designing Magnetism in High Entropy Oxides

In magnetic systems, spin and exchange disorder can provide access to quantum criticality, frustration, and spin dynamics, but broad tunability of these responses and a deeper understanding of strong limit disorder are lacking. Here, it is demonstrated that high entropy oxides present a previously unexplored route to designing materials in which the presence of strong local compositional disorder may be exploited to generate tunable magnetic behaviors-from macroscopically ordered states to frustration-driven dynamic spin interactions. Single-crystal La(Cr0.2 Mn0.2 Fe0.2 Co0.2 Ni0.2 )O3 films are used as a model system hosting a magnetic sublattice with a high degree of microstate disorder in the form of site-to-site spin and exchange type inhomogeneity. A classical Heisenberg model simplified to represent the highest probability microstates well describes how compositionally disordered systems can paradoxically host magnetic uniformity and demonstrates a path toward continuous control over ordering types and critical temperatures. Model-predicted materials are synthesized and found to possess an incipient quantum critical point when magnetic ordering types are designed to be in direct competition, this leads to highly controllable exchange bias behaviors previously accessible only in intentionally designed bilayer heterojunctions.

Unlocking surface octahedral tilt in two-dimensional Ruddlesden-Popper perovskites

Molecularly soft organic-inorganic hybrid perovskites are susceptible to dynamic instabilities of the lattice called octahedral tilt, which directly impacts their carrier transport and exciton-phonon coupling. Although the structural phase transitions associated with octahedral tilt has been extensively studied in 3D hybrid halide perovskites, its impact in hybrid 2D perovskites is not well understood. Here, we used scanning tunneling microscopy (STM) to directly visualize surface octahedral tilt in freshly exfoliated 2D Ruddlesden-Popper perovskites (RPPs) across the homologous series, whereby the steric hindrance imposed by long organic cations is unlocked by exfoliation. The experimentally determined octahedral tilts from n = 1 to n = 4 RPPs from STM images are found to agree very well with out-of-plane surface octahedral tilts predicted by density functional theory calculations. The surface-enhanced octahedral tilt is correlated to excitonic redshift observed in photoluminescence (PL), and it enhances inversion asymmetry normal to the direction of quantum well and promotes Rashba spin splitting for n > 1.

Oxygen vacancy distributions and electron localization in a CeO2(100) nanocube

Oxygen vacancy distributions in a 5 nm CeO2 nanocube were determined using the Reverse Monte Carlo method. The oxygen vacancies tend to be located on the surface of the CeO2 nanocube, with far fewer in subsurface and internal regions.

Interplay of domain structure and phase transitions: theory, experiment and functionality

Domain walls and phase boundaries are fundamental ingredients of ferroelectrics and strongly influence their functional properties. Although both interfaces have been studied for decades, often only a phenomenological macroscopic understanding has been established. The recent developments in experiments and theory allow to address the relevant time and length scales and revisit nucleation, phase propagation and the coupling of domains and phase transitions. This review attempts to specify regularities of domain formation and evolution at ferroelectric transitions and give an overview on unusual polar topological structures that appear as transient states and at the nanoscale. We survey the benefits, validity, and limitations of experimental tools as well as simulation methods to study phase and domain interfaces. We focus on the recent success of these tools in joint scale-bridging studies to solve long lasting puzzles in the field and give an outlook on recent trends in superlattices.

Designing and controlling the properties of transition metal oxide quantum materials

This Perspective addresses the design, creation, characterization and control of synthetic quantum materials with strong electronic correlations. We show how emerging synergies between theoretical/computational approaches and materials design/experimental probes are driving recent advances in the discovery, understanding and control of new electronic behaviour in materials systems with interesting and potentially technologically important properties. The focus here is on transition metal oxides, where electronic correlations lead to a myriad of functional properties including superconductivity, magnetism, Mott transitions, multiferroicity and emergent behaviour at picoscale-designed interfaces. Current opportunities and challenges are also addressed, including possible new discoveries of non-equilibrium phenomena and optical control of correlated quantum phases of transition metal oxides.

Octahedral rotations trigger electronic and magnetic transitions in strontium manganate under volume expansion

The perovskite structure of manganate yields a series of intriguing physical properties. Based on the results of first-principles calculations, strontium manganate appears to undergo a magnetic phase transition and a metal-insulator transition-from antiferromagnetic insulator to ferromagnetic metal and then to ferromagnetic insulator-under isotropic volume expansion combined with oxygen octahedral distortions. Interestingly, the results show that increasing the Mn-O bond length and adding rotation of the oxygen octahedra can soften the breathing distortion and account for the insulator phase. We further build a simple model to explain such transitions. Due to electron transfer and the favoring of a hole state of ligandporbitals, the electron state transfer from2(t2g3)to2(eg1+t2g2)and then tot2g3eg1+(t2g3L̲1). Such rearrangement of charges is responsible for the transitions of its magnetic order and electronic structure. Furthermore, we calculate spin susceptibility under the bare conditions and random phase approximation. The magnetic order of the intermediate metal state of itinerant electrons behaves as a ferromagnetic.

Magnetic and dielectric property control in the multivalent nanoscale perovskite Eu0.5Ba0.5TiO3

We report nanoscale Eu_0.5Ba_0.5TiO₃, a multiferroic in the bulk and candidate in the search to quantify the electric dipole moment of the electron. Eu_0.5Ba_0.5TiO₃, in the form of nanoparticles and other nanostructures is interesting for nanocomposite integration, biomedical imaging and fundamental research, based upon the prospect of polarizability, f-orbital magnetism and tunable optical/radio luminescence. We developed a [non-hydrolytic]sol-[H₂O-activated]gel route, derived from in-house metallic Ba_(s)/Eu_(s) alkoxide precursors and Ti{(OCH(CH₃)₂}₄. Two distinct nanoscale compounds of Ba:Ti:Eu with the parent perovskite crystal structure were produced, with variable dielectric, magnetic and optical properties, based on altering the oxidizing/reducing conditions. Eu_0.5Ba_0.5TiO₃ prepared under air/O₂ atmospheres produced a spherical core-shell nanostructure (30-35 nm), with perovskite Eu_0.5Ba_0.5TiO₃ nanocrystal core-insulating oxide shell layer (∼3 nm), presumed a pre-pyrochlore layer abundant with Eu³⁺. Fluorescence spectroscopy shows a high intensity ⁵D₀→⁷F₂ transition at 622 nm and strong red fluorescence. The core/shell structure demonstrated excellent capacitive properties: assembly into dielectric thin films gave low conductivity (2133 GΩ mm^-1) and an extremely stable, low loss permittivity of ε_eff∼25 over a wide frequency range (tan δ < 0.01, 100 kHz-2 MHz). Eu_0.5Ba_0.5TiO₃ prepared under H₂/argon produced more irregular shaped nanocrystals (20-25) nm, with a thin film permittivity around 4 times greater (ε_eff 101, tan δ < 0.05, 10 kHz-2 MHz, σ∼59.54 kΩ mm^-1). Field-cooled magnetization values of 0.025 emu g^-1 for EBTO-Air and 0.84 emu g^-1 for EBTO-Argon were observed. X-ray photoelectron spectroscopy analysis reveals a complex interplay of Eu^II/III/Ti^III/IV configurations which contribute to the observed ferroic and fluorescence behavior.

Lessons learned from first-principles calculations of transition metal oxides

Transition metal oxides (TMOs) are an important class of materials with diverse applications, ranging from memristors to photoelectrochemical cells. First-principles calculations are critical for understanding these complex materials at an atomic level and establishing relationships between atomic and electronic structures, particularly for probing quantities difficult or inaccessible to experiment. Here, we discuss computational strategies used to understand TMOs by focusing on two examples, a photoanode material, BiVO₄, and an oxide for low-power electronics, La_1-xSr_xCoO₃. We highlight key aspects required for the modeling of TMOs, namely, the descriptions of how oxygen vacancies, extrinsic doping, the magnetic state, and polaron formation impact their electronic and atomic structures and, consequently, many of the observed properties.

Assessing the Effects of Temperature and Oxygen Vacancy on Band Gap Renormalization in LaCrO3-δ: First-Principles and Experimental Corroboration

Understanding the temperature dependence of functional properties in high-temperature gas sensors is vital for applications in combustion environments. Temperature effect on the electronic structure due to electron-phonon coupling is a key property of interest as this influences other responses of sensors. In this work, we assess the impact of temperature on band gap renormalization of pristine and oxygen-vacant LaCrO_3-δ perovskite employing Allen-Heine-Cardona theory with first-principles simulations and corroborate with experimental observation. Antiferromagnetic cubic LaCrO₃ shows a direct ground-state band gap of 2.62 eV that is reduced by over 1 eV due to the presence of oxygen vacancies, which can form endothermically. We find excellent agreement in temperature-dependent band gap shift in LaCrO₃ between theory and an in-house experiment, proving that the theory can adequately predict renormalization on the band gap in a magnetic system. Band gaps in cubic LaCrO_3-δ are found to monotonically narrow by 1.13 eV in pristine and by around 0.62 eV in oxygen-vacant structures as temperature increases from 0 to 1500 K. The predicted band gap variations are rationalized using an analytical model. The experimental zero-temperature band gaps are extracted from the model fits that can provide useful insights on the simulated band gaps.

Theoretical investigation of tetrahedral distortion of four-coordinate iron(II) centres in FePd(CN)4

The tetrahedral distortion of iron(ii) centres in the cyanide-bridged framework FePd(CN)4 was recently demonstrated experimentally. Here, we theoretically confirmed the electronically driven tetrahedral distortion of iron(ii) by comparing the density of states and total energies of FePd(CN)4 (d6) and ZnPd(CN)4 (d10). The calculation results suggested that a Jahn-Teller-like effect is caused on the tetrahedral geometry by the electronic effect of unequally occupied non-bonding 3d orbitals in the corresponding structure.

On the lack of monoclinic distortion in the insulating regime of EuNiO3 and GdNiO3 perovskites by high-angular resolution synchrotron X-ray diffraction: a comparison with YNiO3

Rare-earth nickelates RNiO3 (R = Y, LaLu) are electron-correlated perovskite materials where the interplay between charge and spin order results in a rich phase diagram, evolving from antiferromagnetic insulators to paramagnetic metals. They are well-known to undergo metal-insulator (MI) transitions as a function of temperature and the size of the rare-earth ion. For intermediate-size Eu3+ and Gd3+ ions, the MI transitions are described to happen at TMI = 463 K and 511 K, respectively. We have investigated their structural evolution across TMI with the excellent angular resolution of synchrotron X-ray diffraction, using high-crystalline quality samples prepared at elevated hydrostatic pressures. Unlike YNiO3, synthesized and measured under the same conditions, exhibiting a characteristic monoclinic phase (space group P21/n) in the insulating regime (below TMI), the present EuNiO3 and GdNiO3 samples do not exhibit such a symmetry, but their crystal structures can be defined in an orthorhombic superstructure of perovskite (space group Pbnm) in all the temperature interval, between 100 and 623 K for Eu and 298 K and 650 K for Gd. Nevertheless, an abrupt evolution of the unit-cell parameters is observed upon metallization above TMI. A prior report of a charge disproportionation effect by Mössbauer spectroscopy on Fe-doped perovskite samples seems to suggest that the distribution of two distinct Ni sites must not exhibit the required long-range ordering to be effectively detected by diffraction methods. An abrupt contraction of the b parameter of EuNiO3 in the 175-200 K range, coincident with the onset of antiferromagnetic ordering, suggests a magnetoelastic coupling, not described so far in rare-earth nickelates. The magnetic susceptibility is dominated by the paramagnetic signal of the rare-earth ions; however, the AC susceptibility curves yield a Néel temperature corresponding to the antiferromagnetic ordering of the Ni moments of TN = 197 K for EuNiO3, corroborated by specific heat measurements. For GdNiO3, a χT vs. T plot presents a clear change in the slope at TN = 187 K, also consistent with specific heat data. Magnetization measurements at 2 K under large fields up to 14 T show a complete saturation of the magnetic moments with a rather high ordered moment of 7.5μB per f.u.

First-principles exploration of oxygen vacancy impact on electronic and optical properties of ABO3-δ (A = La, Sr; B = Cr, Mn) perovskites

ABO3-δ perovskites are utilized in many applications including optical gas sensing for energy systems. Understanding the opto-electronic properties allows rational selection of the perovskite-based sensors from a diverse family of ABO3-δ perovskites, associated with the choices of A and B cations and range of oxygen concentrations. Herein, we assess the impact of oxygen vacancies on the electronic structure and optical response of pristine and oxygen-vacant ABO3-δ (A = La, Sr; B = Cr, Mn) perovskites via first-principles calculations. The endothermic formation energy for oxygen vacancies shows that the generation of ABO3-δ defect structures is thermodynamically possible. LaCrO3 and LaMnO3 have direct and indirect ground-state band gaps, respectively, whereas SrCrO3 and SrMnO3 are metallic. In the presence of an oxygen mono-vacancy, however, the band gap decreases in LaCrO3-δ and vanishes in LaMnO3-δ. In contrast to the decrease in the band gaps, the oxygen vacancies in ABO3-δ are found to increase optical absorption in the visible to near-infrared wavelength regime, and thus lower the onset energy of absorption compared with the pristine materials. Our assessments emphasize the role of the oxygen vacancy, or other possible oxygen non-stoichiometry defects, in perovskite oxides with respect to the opto-electronic performance parameters that are of interest for optical gas sensors for energy generation process environments.

Harnessing the topotactic transition in oxide heterostructures for fast and high-efficiency electrochromic applications

Mobile oxygen vacancies offer a substantial potential to broaden the range of optical functionalities of complex transition metal oxides due to their high mobility and the interplay with correlated electrons. Here, we report a large electro-absorptive optical variation induced by a topotactic transition via oxygen vacancy fluidic motion in calcium ferrite with large-scale uniformity. The coloration efficiency reaches ~80 cm² C^-1, which means that a 300-nm-thick layer blocks 99% of transmitted visible light by the electrical switching. By tracking the color propagation, oxygen vacancy mobility can be estimated to be 10^-8 cm² s^-1 V^-1 near 300°C, which is a giant value attained due to the mosaic pseudomonoclinic film stabilized on LaAlO₃ substrate. First-principles calculations reveal that the defect density modulation associated with hole charge injection causes a prominent change in electron correlation, resulting in the light absorption modulation. Our findings will pave the pathway for practical topotactic electrochromic applications.

Review on Sensing Applications of Perovskite Nanomaterials

Recently, perovskite-based nanomaterials are utilized in diverse sustainable applications. Their unique structural characteristics allow researchers to explore functionalities towards diverse directions, such as solar cells, light emitting devices, transistors, sensors, etc. Many perovskite nanomaterial-based devices have been demonstrated with extraordinary sensing performance to various chemical and biological species in both solid and solution states. In particular, perovskite nanomaterials are capable of detecting small molecules such as O2, NO2, CO2, etc. This review elaborates the sensing applications of those perovskite materials with diverse cations, dopants and composites. Moreover, the underlying mechanisms and electron transport properties, which are important for understanding those sensor performances, will be discussed. Their synthetic tactics, structural information, modifications and real time sensing applications are provided to promote such perovskite nanomaterials-based molecular designs. Lastly, we summarize the perspectives and provide feasible guidelines for future developing of novel perovskite nanostructure-based chemo- and biosensors with real time demonstration.

Topotactic Hydrogen in Nickelate Superconductors and Akin Infinite-Layer Oxides ABO_{2}

Superconducting nickelates appear to be difficult to synthesize. Since the chemical reduction of ABO_{3} [rare earth (A), transition metal (B)] with CaH_{2} may result in both ABO_{2} and ABO_{2}H, we calculate the topotactic H binding energy by density functional theory (DFT). We find intercalating H to be energetically favorable for LaNiO_{2} but not for Sr-doped NdNiO_{2}. This has dramatic consequences for the electronic structure as determined by DFT+dynamical mean field theory: that of 3d^{9} LaNiO_{2} is similar to (doped) cuprates, 3d^{8} LaNiO_{2}H is a two-orbital Mott insulator. Topotactic H might hence explain why some nickelates are superconducting and others are not.

Strategy for achieving multiferroic E-type magnetic order in orthorhombic manganites RMnO3 (R = La-Lu)

Based on first-principles calculations, multiferroic properties of orthorhombic manganites (RMnO₃, R = La-Lu) with E-type ground state have been achieved by lanthanide contraction (chemical pressure) and/or external strain. Our research demonstrates that a smaller R radius within the octahedral voids in RMnO₃ results in the increase in the tilts of the octahedra but only a gentle change in the Jahn-Teller (JT) distortion. The reduction of the intraplane octahedral rotation angle and the narrowed e_g states and lifting t_2g band edge are mainly responsible for the intraplane magnetic transition from ferromagnetic (La-Gd) to zigzag-like spin arrangement (Ho-Lu). In turn, the center-broken E-type RMnO₃ bulk characterizes the dominated electronic polarization behavior, benefiting from their distortion response to small R substitution, which gives rise to the strong magnetoelectricity. Subsequently, we have figured out the strain strategy for obtaining an E-type transition in light rare-earth manganites (La-Gd) by imposing a series of hypothetical strains, where the small intraplane rotation angle (Θ) and large JT distortion favor the small aspect ratios of a/b and c/b, respectively. The strained LaMnO₃ and GdMnO₃ achieve E-type transitions successfully by imposing a modest compressive strain along the a- and c-axes and remaining free along the b-direction. Simultaneously, their polarization behaviors were comparatively studied. It was found that the size of the A-site rare-earth ions has a great influence on the external strain response, in addition to its effect on the magnetic phase transition.

Magneto-electric multiferroics: designing new materials from first-principles calculations

In parallel with the revival of interest for magneto-electric multiferroic materials in the beginning of the century, first-principles simulations have grown incredibly in efficiency during the last two decades. Density functional theory calculations, in particular, have so become a must-have tool for physicists and chemists in the multiferroic community. While these calculations were originally used to support and explain experimental behaviour, their interest has progressively moved to the design of novel magneto-electric multiferroic materials. In this article, we mainly focus on oxide perovskites, an important class of multifunctional material, and review some significant advances to which contributed first-principles calculations. We also briefly introduce the various theoretical developments that were at the core of all these advances.

Strain-dependent structure and Raman behaviours in the heavy-ion irradiated manganite at extreme low dose

The microstrains in heavy-ion irradiated manganite LaMnO₃ can be managed in linear response of irradiation dose, and the corresponding internal pressure up to 8 GPa can be induced by varying doses. The response of structure under stress is studied by means of Density Functional Theory and Lattice Dynamic Calculation. All obtained Raman scattering lines are discussed in details to shed light onto structural changes during ion implantation. There appears new resonance peak at around 550 cm^-1, which splits from broad features in the spectra, and attributes to the anti-symmetric vibrations of O₆ cages. The blue shift of this peak scales to ~2.4 cm^-1 per 1 GPa of stress. Another strong feature showing considerable blue shift is seen in the vicinity of 640 cm^-1 and corresponds to one of rhombohedral distortion related soft modes. A weak mode, not frequently reported, is seen at around 420 cm^-1 and corresponds to translation-like motions of fixed O₆ cages.

Design of Heteroanionic MoON Exhibiting a Peierls Metal-Insulator Transition

Using a first-principles approach, we design the heteroanionic oxynitride MoON to exhibit a first-order isosymmetric thermally activated Peierls-type metal-insulator transition (MIT). We identify a ground state insulating phase (α-MoON) with monoclinic Pc symmetry and a metastable high temperature metallic phase (β-MoON) of equivalent symmetry. We find that ordered fac-MoO_{3}N_{3} octahedra with edge and corner connectivity stabilize the twisted Mo-Mo dimers present in the α phase, which activate the MIT through electron localization within the 4d a_{1g} manifold. By analyzing the temperature dependence of the soft zone-boundary instability driving the MIT, we estimate an ordering temperature T_{MIT}∼900  K. Our work shows that electronic transitions can be designed by exploiting multiple anions, and heteroanionic materials could offer new insights into the microscopic electron-lattice interactions governing unresolved transitions in homoanionic oxides.

Local orbital degeneracy lifting as a precursor to an orbital-selective Peierls transition

Fundamental electronic principles underlying all transition metal compounds are the symmetry and filling of the d-electron orbitals and the influence of this filling on structural configurations and responses. Here we use a sensitive local structural technique, x-ray atomic pair distribution function analysis, to reveal the presence of fluctuating local-structural distortions at high temperature in one such compound, CuIr₂S₄. We show that this hitherto overlooked fluctuating symmetry-lowering is electronic in origin and will modify the energy-level spectrum and electronic and magnetic properties. The explanation is a local, fluctuating, orbital-degeneracy-lifted state. The natural extension of our result would be that this phenomenon is likely to be widespread amongst diverse classes of partially filled nominally degenerate d-electron systems, with potentially broad implications for our understanding of their properties.

A common feature of many transition metal materials is global symmetry breaking at low temperatures. Here the authors show that such materials are characterized by fluctuating symmetry-lowering distortions that exist pre-formed in higher temperature phases with greater average symmetry.