Crystal and Electronic Structure and Magnetic Properties of Divalent Europium Perovskite Oxides EuMO3 (M = Ti, Zr, and Hf): Experimental and First-Principles Approaches

KCoO2-type layered nitrides Ca1-xEuxTiN2: structural stability, electrical properties and Eu coordination chemistry

Eu2+ was used to substitute Ca in the orthorhombic KCoO2-type layered CaTiN2 to form a Ca1-xEuxTiN2 solid solution, which showed a limited substitution around x = 0.35 with the tetragonality enhanced but the orthorhombic symmetry retained and inaccessibility for the end member EuTiN2. This was in contrast with the full solid solution Ca1-xSrxTiN2, which realized a structural transition from orthorhombic to tetragonal at x = 0.5, even though Eu2+ and Sr2+ ions have similar sizes. The Eu substitution for Ca reduced the dielectric permittivity of CaTiN2 owing to the reduced structural distortion arising from the enhanced tetragonality with the substitution. First-principle theoretical calculations on the total energies and formation energies considering the 4f electrons of Eu ions and the related magnetism were performed to understand the structural stability of the hypothetical EuTiN2. Compared with CaTiN2 and SrTiN2, EuTiN2 has much higher formation energies, making it inaccessible at high temperature. The evolutions of the experimentally observed and calculated lattice parameters of the Ca1-xEuxTiN2 solid solution showed a preference for the orthorhombic phase over the tetragonal phase for the hypothetical EuTiN2, revealing a different coordination chemistry of Eu2+-N to Eu2+-O through the comparison of the structural variations of ATiN2 and ATiO3 (A = Ca, Sr, Eu).

Large Perpendicular Magnetic Anisotropy Induced by an Intersite Charge Transfer in Strained EuVO2H Films

Perovskite oxides ABO3 continue to be a major focus in materials science. Of particular interest is the interplay between A and B cations as exemplified by intersite charge transfer (ICT), which causes novel phenomena including negative thermal expansion and metal-insulator transition. However, the ICT properties were achieved and optimized by cationic substitution or ordering. Here we demonstrate an anionic approach to induce ICT using an oxyhydride perovskite, EuVO2H, which has alternating layers of EuH and VO2. A bulk EuVO2H behaves as a ferromagnetic insulator with a relatively high transition temperature (TC) of 10 K. However, the application of external pressure to the EuIIVIIIO2H bulk or compressive strain from the substrate in the thin films induces ICT from the EuIIH layer to the VIIIO2 layer due to the extended empty V dxy orbital. The ICT phenomenon causes the VO2 layer to become conductive, leading to an increase in TC that is dependent on the number of carriers in the dxy orbitals (up to a factor of 4 for 10 nm thin films). In addition, a large perpendicular magnetic anisotropy appears with the ICT for the films of <100 nm, which is unprecedented in materials with orbital-free Eu2+, opening new perspectives for applications. The present results provide opportunities for the acquisition of novel functions by alternating transition metal/rare earth layers with heteroanions.

Investigation of thermal control in phase-changing ABO3 perovskites via first-principles predictions: general mechanism of emittance

Phase-change thermal control has recently seen increased interest due to its significant potential for use in smart windows, building insulation, and optoelectronic devices in spacecraft. Tunable variation in infrared emittance can be achieved by thermally controlling the phase transitions of materials at different temperatures. A high emittance in the mid-infrared region is usually caused by resonant phonon vibrational modes. However, the fundamental mechanism of emittance variation during the phase-change process remains elusive. In this work, the electronic bandgaps, phononic structures, optical-spectrum properties, and formation energies of 76 kinds of phase-changing ABO₃ perovskites were predicted based on first-principles calculations in the mid-infrared region. The variation in emittance between two phases of a single material was found to have an exponential correlation with the bandgap difference (R² ∼ 0.92). Furthermore, a strong linear correlation (R² ∼ 0.92) was found between the emittance variation and the formation-energy difference, and the emittance variation was also strongly correlated with the volume-distortion rate (R² ∼ 0.90). Finally, it was concluded that a large lattice vibrational energy, a high formation energy, and a small cell volume are conducive to high emittance. This work provides a strong dataset for training machine-learning models, and it paves the way for further use of this novel methodology to seek efficient phase-change materials for thermal control.

Eu-Mn Charge Transfer and the Strong Charge-Spin-Electronic Coupling Behavior in EuMnO3

Based on first-principles calculations with the DFT + U method, the couplings of lattice, charge, spin, and electronic behaviors underlying the Eu-Mn charge transfer in a strongly correlated system of EuMnO₃ were investigated. The potential valence transition from Eu³⁺/Mn³⁺ to Eu²⁺/Mn⁴⁺ was observed in a compressed lattice with little distortions, which is achieved under hydrostatic pressure and external strain. The intraplane antiferromagnetism (AFM) of Mn is proved to be instrumental in the emergence of Eu²⁺. Furthermore, we calculated the magnetic exchange interactions within two equilibrium structures of Eu³⁺Mn³⁺O₃ and Eu²⁺Mn⁴⁺O₃. Mn-Mn ferromagnetic exchange in the ab-plane is enhanced strongly in the Eu²⁺Mn⁴⁺O₃ structure, contributing to the existence of mixed states. The versatile electronic structures were obtained within the Eu²⁺Mn⁴⁺O₃ phase by imposing different magnetic configurations on the Eu and Mn sublattice, attributed to the coupling of charge transfer and magnetic orderings. It is found that the intraplane ferromagnetic ordering of Mn leads to a metallic electronic structure with the coexistence of Eu²⁺ and Eu³⁺, while the intraplane AFM Mn spin ordering leads to insulating states only with Eu²⁺. Notably, a half-metallic characteristic emerges at the magnetic ground state of CF ordering (C-type AFM for the Eu sublattice and ferromagnetic for the Mn sublattice), which makes such a supposed phase more intriguing than the conventional experimental phase. Additionally, the mixture of delocalized 4f with 5d states of Eu in the background of Mn 3d and O 2p orbitals implies a pathway of Eu 4f 5d ↔ O 2p ↔ Mn 3d for charge transfer between Eu and Mn. Our calculation shows that the Eu-Mn charge transfer could be expected in compressed EuMnO₃ and the introduction of Eu²⁺ 4f states near the Fermi level plays an important role in manipulating the interlinks of charge and spin together with electronic behaviors.

Vertical Strain-Driven Antiferromagnetic to Ferromagnetic Phase Transition in EuTiO3 Nanocomposite Thin Films

Three-dimensional (3D) strain induced in self-assembled vertically aligned nanocomposite (VAN) epitaxial films provides an unrivaled method to induce very large strains in thin films. Here, by growing VAN films of EuTiO₃ (ETO)-Eu₂O₃ (EO) with different EO fractions, the vertical strain was systematically increased in ETO, up to 3.15%, and the Eu-Ti-Eu bond angle along ⟨111⟩ decreased by up to 1°, leading to a weakening of the antiferromagnetic interactions and switching from antiferromagnetic to ferromagnetic behavior. Our work has shown for the first time that Eu-Ti-Eu superexchange interactions play a key role in determining the magnetic ground state of ETO. More broadly, our work serves as an exemplar to show that multifunctionalities in strong spin-lattice coupling perovskite oxides can be uniquely tuned at the atomic scale using simple VAN structures.

A series of new layered lithium europium(II) oxoniobates(V) and -tantalates(V)

The new Ruddlesden-Popper-related phases A n +1 B n O3 n +1 (n = 3) with the compositions Li2Eu2Nb3O10, Li2Eu1.5Ta3O10, Li2EuKNb3O10, and Li2EuKTa3O10 were synthesized by solid-state reactions from Li2[CO3] (+ K2[CO3]) and the corresponding refractory metals along with their oxides in a high-frequency furnace at temperatures above T = 1600°C. Their structures have been determined by single-crystal X-ray diffraction studies. Characteristic features are triple layers of corner-sharing [MO6]7− octahedra (M = Nb and Ta), which are connected via [LiO4]7− tetrahedra. The Eu2+ cations are cuboctahedrally surrounded by 12 oxygen atoms and according to the Eu–O distances of around 275 pm, they have the oxidation state +2, as confirmed by XPS measurements. In the potassium-containing samples they share their positions with K+ cations. The black compounds are stable in air at room temperature. Measurements of the magnetic susceptibilities in the range of T = 5–300 K revealed Li2Eu2Nb3O10, Li2Eu1.5Ta3O10 and Li2EuKTa3O10 to be paramagnetic without any ordering.

Carrier-Density-Induced Ferromagnetism in EuTiO_{3} Bulk and Heterostructures

EuTiO_{3} is an antiferromagnetic (AFM) material showing strong spin-lattice interactions, large magnetoelectric response, and quantum paraelectric behavior at low temperatures. Using electronic-structure calculations, we show that adding electrons to the conduction band leads to ferromagnetism. The transition from antiferromagnetism to ferromagnetism is predicted to occur at ∼0.08  electrons/Eu (∼1.4×10^{21}  cm^{-3}). This effect is also predicted to occur in heterostructures such as LaAlO_{3}/EuTiO_{3}, where ferromagnetism is triggered by the formation of a high-density two-dimensional electron gas in the EuTiO_{3}. Our analysis indicates that the coupling between Ti 3d and Eu 5d plays a crucial role in lowering the Ti 3d conduction band in the ferromagnetic (FM) phase, leading to an almost linear dependence of the energy difference between the FM and AFM ordering on the carrier concentration. These findings open up possibilities in designing field-effect transistors using EuTiO_{3}-based heterointerfaces to probe fundamental interactions between highly localized spins and itinerant, polarized charge carriers.

Network analysis of synthesizable materials discovery

Assessing the synthesizability of inorganic materials is a grand challenge for accelerating their discovery using computations. Synthesis of a material is a complex process that depends not only on its thermodynamic stability with respect to others, but also on factors from kinetics, to advances in synthesis techniques, to the availability of precursors. This complexity makes the development of a general theory or first-principles approach to synthesizability currently impractical. Here we show how an alternative pathway to predicting synthesizability emerges from the dynamics of the materials stability network: a scale-free network constructed by combining the convex free-energy surface of inorganic materials computed by high-throughput density functional theory and their experimental discovery timelines extracted from citations. The time-evolution of the underlying network properties allows us to use machine-learning to predict the likelihood that hypothetical, computer-generated materials will be amenable to successful experimental synthesis.

Strain and temperature dependent absorption spectra studies for identifying the phase structure and band gap of EuTiO3 perovskite films

Post-annealing has been approved to effectively relax the out-of-plane strain in thin films. Epitaxial EuTiO3 (ETO) thin films, with and without strain, have been fabricated on (001) LaAlO3 substrates by pulsed laser deposition. The absorption and electronic transitions of the ETO thin films are investigated by means of temperature dependent transmittance spectra. The antiferrodistortive phase transition can be found at about 260-280 K. The first-principles calculations indicate there are two interband electronic transitions in ETO films. Remarkably, the direct optical band gap and higher interband transition for ETO films show variation in trends with different strains and temperatures. The strain leads to a band gap shrinkage of about 240 meV while the higher interband transition an expansion of about 140 meV. The hardening of the interband transition energies in ETO films with increasing temperature can be attributed to the Fröhlich electron-phonon interaction. The behavior can be linked to the strain and low temperature modified valence electronic structure, which is associated with rotations of the TiO6 octahedra.

Hidden magnetism in the paramagnetic phase of EuTiO₃

EuTiO3 is investigated experimentally by temperature dependent conductivity and dielectric constant measurements. Both data sets evidence a crossover behavior in their temperature dependence around T * ~ 200 K indicating a change of the electrical and magnetic behavior of EuTiO3 as already observed by muon spin rotation (μSR) measurements. Around T '  =  80 K an additional anomaly appears which is consistent with previous resonant ultrasound spectroscopy (RUS) data. By applying a magnetic field (1.2 T at room temperature) the bulk conductivity is anomalously enhanced by more than one order of magnitude. The bulk dielectric constant ε(r) decreases with increasing temperature and is enhanced by the magnetic field by up to 22%. All data reveal a substantial hysteresis which is diminished in a magnetic field. The unusual magnetic field dependence of all quantities is suggested to stem from magnetically active domain walls which are mobile between the structural phase transition temperature (T(S)  =  282 K) and T * and are pinned below it.

Tiny cause with huge impact: polar instability through strong magneto-electric-elastic coupling in bulk EuTiO3

EuTiO3 exhibits strong magneto-electric coupling at the onset of antiferromagnetic order below TN = 5.7 K. The dielectric permittivity drops at TN by 7% and recovers to normal values with increasing magnetic field. This effect is shown to stem from tiny lattice effects as seen in magnetostriction data which directly affect the soft optic mode and its polarizability coordinate. By combining experimental results with theory we show that marginal changes in the lattice parameter of the order of 0.01% have a more than 1000% effect on the transverse optic soft mode of ETO and thus easily induce a ferroelectric instability.

Hybrid functionals applied to perovskites

After being used for years in the chemistry community to describe molecular properties, hybrid functionals have been increasingly and successfully employed for a wide range of solid state problems which are not accurately accessible by standard density functional theory. In particular, the upsurge of interest in transition metal perovskite-based compounds, motivated by their technological relevance and functional ductility, has incentivized the use of hybrid functionals for realistic applications, as hybrid functionals appear to be capable of capturing the complex correlated physics of this class of oxide material, characterized by a subtle coupling between several competing interactions (lattice, orbital, spin). Here we present a map of recent applications of hybrid functionals to perovskites, aiming to cover an ample spectra of cases, including the 'classical' 3d compounds (manganites, titanates, nickelates, ferrites, etc.), less conventional examples from the the 4d (technetiates) and 5d (iridates) series, and the (non-transition metal) sp perovskite BaBiO3. We focus our attention on the technical aspects of the hybrid functional formalism, such as the role of the mixing and (for range-separated hybrids) screening parameters, and on an extended array of physical phenomena: pressure- and doping-induced insulator-to-metal and structural phase transitions, multiferroism, surface and interface effects, charge ordering and localization effects, and spin-orbit coupling.

Neutron powder diffraction study of the magnetic structure of EuZrO₃

Neutron powder diffraction experiments on the orthorhombic perovskite EuZrO₃ show it to have an antiferromagnetic G-type magnetic structure with the magnetic moments aligned parallel to the a-axis. The orthorhombic crystal structure is a consequence of cooperative rotations of the corner-sharing ZrO₆ octahedra. The crystal structure does not change significantly below the Néel temperature, ∼4.1 K, showing there to be only weak magnetoelastic coupling.

Lattice and polarizability mediated spin activity in EuTiO3

EuTiO3 is shown to exhibit novel strong spin-charge-lattice coupling deep in the paramagnetic phase. Its existence is evidenced by an, until now, unknown response of the paramagnetic susceptibility at temperatures exceeding the structural phase transition temperature T(S) = 282 K. The 'extra' features in the susceptibility follow the rotational soft zone boundary mode temperature dependence above and below TS. The theoretical modeling consistently reproduces this behavior and provides reasoning for the stabilization of the soft optic mode other than quantum fluctuations.

Nonlinear pressure dependence of TN in almost multiferroic EuTiO3

The antiferromagnetic (AFM) phase transition temperature TN of EuTiO3 has been studied as a function of pressure p. The data reveal a nonlinear dependence of TN on p with TN increasing with increasing pressure. The exchange interactions exhibit an analogous dependence on p as TN (if the absolute value of the nearest neighbor interaction is considered) and there is evidence that the AFM transition is robust with increasing pressure. The corresponding Weiss temperature ΘW remains anomalous since it always exhibits positive values. The data are analyzed within the Bloch power law model and provide excellent agreement with experiment.