arXiv subjects

Oleksandr I. Malyi

Publications and source records attributed to Oleksandr I. Malyi.

At least 19 recordsLinked to original sources

arXiv2025

Rational Design Principles for Na- and Li-ion Carbon Anodes from Interlayer Spacing Control

Graphite, the standard commercial anode for Li-ion batteries, is thermodynamically incompatible with Na-ion batteries, leading researchers to search for alternative C-based structures (e.g., hard carbon, expanded graphite). In a simplified picture, the main idea of such search relies on identifying disordered C structures with a large interlayer spacing and distribution of local structural motifs (e.g., pores) with target electrochemical properties. Such exploration is typically done via trial-and-error experimentation, which often does not allow precise understanding of the role of interlayer distance and even Na/Li-ion intercalation in the electrochemical performance. Motivated by this, using density-functional theory and cluster expansion, we establish a structure-property relationship for Li- and Na-intercalation across a range of graphite interlayer spacings and stacking arrangements. We show that Na intercalation becomes thermodynamically possible in large concentrations above 4.21 Å even without a change in interlayer spacing. Conversely, Li intercalation has a narrow optimal window, with maximum capacity (close to the commercial limit) at approximately 3.75 Å, while larger spacings (e.g., 4.58 Å) quickly reduce Li storage capacity. We also find that AA-stacked domains consistently offer stronger ion bonding and higher voltages than AB-stacked domains for both ions. Our results, thus, not only explain the role of metal ion intercalation in the electrochemical performance, but also clarify the fundamental design trade-offs for expanded C anodes, offering practical targets and interlayer distance ranges for the independent optimization of the next generation of negative electrodes for metal-ion batteries.

cond-mat.mtrl-sci↗

arXiv2023

Unveiling Defect Physics in Gapped Metals: A Theoretical Investigation into Defect Formation and Electronic Structure Interplay

In materials science, point defects play a crucial role in materials properties. This is particularly well known for the wide band gap insulators where the defect formation/compensation determines the equilibrium Fermi level and generally the doping response of a given material. Similarly, the main defect trends are also widely understood for regular metals (e.g., Cu and Zn discussed herein). With the development of electronic structure theory, a unique class of quantum materials - gapped metals (e.g., Ca6Al7O16, SrNbO3, In15SnO24, and CaN2) that exhibit characteristics of both metals and insulators - has been identified. While these materials have internal band gaps similar to insulators, their Fermi level is within one of the main band edges, giving a large intrinsic free carrier concentration. Such unique electronic structures give rise to unique defect physics, where the formation of acceptor or donor defect directly affects not only the electronic structure but also can substantially shift the Fermi level. Motivated by this, herein, we develop a fundamental first-principles theory of defect formation in gapped metal with a primary focus on accurate calculations of defect formation energy. We demonstrate that due to electron-hole recombination, the formation of acceptor defects in n-type gapped metals results in significant dependence of defect formation energy on supercell size, which we explain by the change of band filling and its effect on the defect formation energetics. To accurately describe defect formation energy, we revisit the phenomenology of band-filling corrections and demonstrate the effect of this correction, accurate potential alignment, and other factors that can affect defect energetics. Thus, this work not only sheds light on the intrinsic properties of gapped metals but, in general, establishes a theoretical foundation for analyzing defects in gapped metals.

cond-mat.mtrl-sci↗

arXiv2023

Temperature-induced suppression of structural disproportionation in paramagnetic quantum materials

With the development of electronic structure theory, a new class of materials - quantum ones has been recognized by the community. Traditionally, it has been believed that the properties of such compounds cannot be described within the framework of modern density functional theory, and indeed, more advanced post-mean-field theory methods are needed. Motivated by this, herein, we develop a fundamental understanding of such complex materials using the example of paramagnetic YNiO3, which is experimentally known to exhibit metal-to-insulator phase transition. We show that this material has a temperature-dependent distribution of local structural and spin motifs. Thus, while at low temperatures, YNiO3 has distinct structural disproportionation with the formation of large and small octahedra, as the temperature increases, this disproportionation is suppressed. We also explain the paramagnetic monoclinic to paramagnetic orthorhombic phase transition within the double-well to single-well energy profile, predicting the variation of the corresponding energy profile as a function of octahedral size distribution. In this way, we demonstrate a fundamental understanding of structural phase transitions in quantum materials, giving insight into how it can be used for different applications and what minimum level of theory is needed to describe such types of complex materials correctly.

cond-mat.mtrl-sci↗

arXiv2023

Physics of band-filling correction in defect calculations of solid-state materials

In solid-state physics/chemistry, a precise understanding of defect formation and its impact on the electronic properties of wide-bandgap insulators is a cornerstone of modern semiconductor technology. However, complexities arise in the electronic structure theory of defect formation when the latter triggers partial occupation of the conduction/valence band, necessitating accurate post-process correction to the energy calculations. Herein, we dissect these complexities, focusing specifically on the post-process band-filling corrections, a crucial element that often demands thorough treatment in defect formation studies. We recognize the importance of these corrections in maintaining the accuracy of electronic properties predictions in wide-bandgap insulators and their role in reinforcing the importance of a reliable common reference state for defect formation energy calculations. We explored solutions such as aligning deep states and electrostatic potentials, both of which have been used in previous works, showing the effect of band alignment on defect formation energy. Our findings demonstrate that the impact of defect formation on electronic structure (even deep states) can be significantly dependent on the supercell size. We also show that within band-filling calculations, one needs to account for the possible change of electronic structure induced by defect formation, which requires sufficient convergence of electronic structure with supercell size. Thus, this work emphasizes the critical steps to predict defect formation energy better and paves the way for future research to overcome these challenges and advance the field with more efficient and reliable predictive models.

cond-mat.mtrl-sci↗

arXiv2023

Noble gas functional defect with unusual relaxation pattern in solids

The conventional understanding has always been that noble gases are chemically inert and do not affect materials properties. This belief has led to their use as a standard reference in various experimental applications through noble gas implantation. However, in our research, using first-principles calculations, we delve into the effects of noble gas defects on the properties of several functional oxides, thereby questioning this long-held assumption. We provide evidence that noble gases can indeed serve as functional defects. They have the potential to decentralize the localized defect states and prompt a shift of electrons from a localized state to the main conduction band. Our investigation unveils that noble gas defects can indeed significantly alter material properties. Thus, we underscore the importance of factoring in such defects when assessing material properties.

cond-mat.mtrl-sci↗

arXiv2023

Insulating band gaps both below and above the Néel temperature in d-electron LaTiO3, LaVO3, SrMnO3, and LaMnO3 perovskites as a symmetry-breaking event

Compounds having an odd number of electrons with the same orbital character in occupied and unoccupied band edge states would be expected to have band degeneracy at the Fermi energy, making such reference system metals. Yet, many ABO3 oxide perovskites with a magnetic 3d B-atom are, in fact, insulators both below and above the Neel temperature. These inconsistencies between experimental observation and expectation have been traditionally resolved by invoking degeneracy-breaking physics based largely on high-order electron effects, such as strong interelectronic correlation (the Mott mechanism). Such explanations generally utilize the highest symmetry structure, considering microscopic degrees of freedom (m-DOF) as largely passive spectators. Yet, it has long been known that ABO3 perovskites can manifest an arrangement of m-DOFs in the form of octahedral tilting, bond dimerization, Jahn-Teller distortions, and ordering of local magnetic moments as part of the stabilizing intrinsic symmetry. Such m-DOFs are seen both by local experimental probes and theoretically in total energy minimization of its Born-Oppenheimer state. While such local structural and magnetic symmetry breaking motifs were often considered to be the reason for gapping of the reference system below the transition into Para phases, it was also often thought that above the transition those local motifs might vanish, requiring a different mechanism for gapping - such as strong correlation. Here we examine if such intrinsic structural and magnetic symmetry breaking might systematically explain the formation of insulating band gaps both below and above the magnetic transition and account at the same time for specific and non-accidental exceptions of the absence of gaping in some compounds, such as SrVO3.

cond-mat.str-el↗

arXiv2023

Optical Properties and Electronic Structures of Intrinsic Gapped Metals: Inverse Materials Design Principles for Transparent Conductors

Traditional solid-state physics has long correlated the optical properties of materials with their electronic structures. However, recent discoveries of intrinsic gapped metals have challenged this classical view. Gapped metals possess electronic properties distinct from both metals and insulators, with a large concentration of free carriers without any intentional doping and an internal band gap. This unique electronic structure makes gapped metals potentially superior to materials designed by intentional doping of the wide band gap insulators. Despite their promising applications, such as transparent conductors, designing gapped metals for specific purposes remains challenging due to the lack of understanding of the correlation between their electronic band structures and optical properties. This study focuses on representative examples of gapped metals and demonstrates the cases of (i) gapped metals (e.g., CaN2) with strong intraband absorption in the visible range, (ii) gapped metals (e.g., SrNbO3) with strong interband absorption in the visible range, (iii) gapped metals (e.g., Sr5Nb5O17) that are potential transparent conductors. We explore the complexity of identifying potential gapped metals for transparent conductors and propose inverse materials design principles for discovering new-generation transparent conductors.

cond-mat.mtrl-sci↗

arXiv2023

The rise and fall of Mott insulating gaps in YNiO$_3$ paramagnets as a reflection of symmetry breaking and remaking

The YNiO$_3$ nickelate is a paradigm d-electron oxide that manifests the intriguing temperature-mediated sequence of three phases transitions from (i) magnetically ordered insulator to (ii) paramagnetic (PM) insulator and then to (iii) PM metal. Such phenomena raised the question of the nature of the association of magnetism and structural symmetry breaking with the appearance in (i) and (ii) and disappearance in (iii) of insulating band gaps. It is demonstrated here that first-principles mean-field-like density functional theory (DFT), driven by molecular dynamics temperature evolution, can describe not only the origin of the magnetically long-range ordered insulating phase (i), but also the creation of an insulating paramagnet (ii) that lacks spin-long-range order, and of a metallic paramagnet (iii) as temperature rises. This approach provides the patterns of structural and magnetic symmetry breaking at different temperatures, in parallel with band gaps obtained when the evolving geometries are used as input to DFT electronic band structure calculations. This disentangles the complex interplay among spin, charge and orbital degrees of freedom. Analysis shows that the success in describing the rise and fall of the insulating band gaps along the phase transition sequence is enabled by allowing sufficient flexibility in describing diverse local structural and magnetic motifs as input to DFT. This entails using sufficiently large supercells that allow expressing structural disproportionation of octahedra, as well as a description of PM phases as a distribution of local magnetic moments. It appears that the historic dismissal of mean-field-like DFT as being unable to describe such Mott-like transitions was premature, as it was based on consideration of averaged crystallographic unit cells, a description that washes out local symmetry-breaking motifs.

cond-mat.mtrl-sci↗

arXiv2023

Spontaneous off-stoichiometry as the knob to control dielectric properties of gapped metals

Using the first-principles calculations and La3Te4 as an example of an n-type gapped metal, we demonstrate that gapped metals can develop spontaneous defect formation resulting in off-stoichiometric compounds. Importantly, these compounds have different free carrier concentrations and can be realized by optimizing synthesis conditions. The ability to manipulate the free carrier concentration allows to tailor intraband and interband transitions, thus controlling the optoelectronic properties of materials in general. Specifically, by realizing different off-stochiometric La3-xTe4 compounds, it is possible to reach specific crossings of the real part of the dielectric function with the zero line, reduce plasma frequency contribution to absorption spectra, or, more generally, induce metal-to-insulator transition. This is particularly important in the context of optoelectronic, plasmonic, and epsilon-near-zero materials, as it enables materials design with a target functionality. While this work is limited to the specific gapped metal, we demonstrate that the fundamental physics is transferable to other gapped metals and can be generally used to design a wide class of new optoelectronic/plasmonic materials.

cond-mat.mtrl-sci↗

arXiv2022

Fermi Level Instability as a Way to Tailor Properties of La3Te4

Traditionally, the formation of off-stoichiometric compounds is believed to be the growth effect rather than the intrinsic tendency of the system. However, here, using the example of La3Te4, we demonstrate that in n-type gapped metals having a large internal gap between principal band edges and the Fermi level inside of the principal conduction band, Fermi-level instability can develop, resulting in a reduction of formation energy for acceptor defects. Specifically, La vacancies in La3Te4 form spontaneously to produce the acceptor states and remove a fraction of free carriers from the principal conduction band via electron-hole recombination. Such a unique self-doping mechanism allows to stabilize a range of off-stoichiometric La3-xTe4 compounds, which have different electronic properties. Moreover, we thus show how controlling synthesis conditions can be used as a knob to reach the target functionality, including controllable metal-to-insulator transition.

cond-mat.mtrl-sci↗

arXiv2022

Perspective on first-principles studies of 2D materials

The successful exfoliation of graphene from graphite has brought significant attention to predicting new two-dimensional (2D) materials that can be realized experimentally. As a consequence, first-principles studies of novel 2D materials become a routine, with thousands of papers published every year. What makes these studies interesting is that they predict new materials which have not been realized yet but should be a panacea for topological insulators, next-generation battery materials, novel solar cell materials, etc. There is no doubt that some of the proposed materials can provide a specific solution and their properties/performance can be confirmed experimentally, at the same time there are many false predictions because of the computational errors or the Lego-land approach to study 2D materials. To reduce the gap between theoretical and experimental works, we perform a systematic review of computational and Lego-land factors that should be minimized in future theoretical works.

cond-mat.mtrl-sci↗

arXiv2022

Density Functional description of spin, lattice, and spin-lattice dynamics in antiferromagnetic and paramagnetic phases at finite temperatures

Describing the (a) electronic and magnetic properties (EMP) of antiferromagnetic or paramagnetic phases of compounds generally requires the knowledge of (b) the spin configurations and lattice structure (SCLS) of such phases at a given temperature. Indeed, studying the coupling between (a) and (b) has been an outstanding challenge in the theory of magnetism. The traditional approach to electronic phases of matter has generally focused on solving the problem of EMP regarding the SCLS as a spectator degree of freedom (DOF). Yet, it has been recognized that EMP of a compound generally respond self-consistently to changes in SCLS and vice versa. We construct here a practical, density functional theory (DFT)-based approach that provides the SCLS as a function of temperature, involving the description of spin, lattice, and spin-lattice dynamics of different magnetic phases. We distinguish three Levels of dynamics: (I) dynamics of the spin DOF treated via noncollinear Heisenberg Monte-Carlo with exchange energies from DFT, (II) dynamics of the lattice DOF treated by ab initio molecular dynamics (AIMD) employing a fixed spin configuration from Level I at the simulated temperature, and (III) coupling of spin and lattice dynamics via Landau-Lifshitz-Gilbert spin dynamics combined with AIMD. Such SCLS at each of the three levels are used as inputs to DFT supercell calculations, providing the EMP at each temperature. The results of this sequence include electronic band structures, band gaps, density of states, as well as the statistical distribution of local moments and the short-range order parameters, each as a function of temperature. Using NiO as a test case, we address the separability of the DOF in magnetic insulators for a minimal description of electronic and magnetic properties, demonstrating that inclusion of spin dynamics and, to some level, lattice dynamics is enough to explain the EMP.

physics.comp-ph↗

arXiv2021

Intrinsic local symmetry-breaking in nominally cubic paraelectric BaTiO3

Whereas low-temperature ferroelectrics have a well understood ordered spatial dipole arrangement, the fate of these dipoles in paraelectric phases remains poorly understood. Using density functional theory (DFT), we find that unlike the case in conventional non-polar ABO$_3$ compounds illustrated here for cubic BaZrO$_3$, the origin of the distribution of the B site off-centering in cubic paraelectric such as BaTiO$_3$ is an intrinsic, energy stabilizing symmetry breaking. Minimizing the internal energy E of a constrained cubic phase already reveals the formation of a distribution of intrinsic local displacements that (i) mimic the symmetries of the low temperature phases, while (ii) being the precursors of what finite temperature DFT Molecular Dynamics finds as thermal motifs. The implications of such symmetry breaking on the microscopic structures and anomalous properties in these kinds of PE materials are discussed.

cond-mat.mtrl-sci↗

arXiv2021

Premelting and formation of ice due to Casimir-Lifshitz interactions: Impact of improved parameterization for materials

Recently, the premelting and formation of ice due to the Casimir-Lifshitz interaction, proposed in early 1990s by Elbaum and Schick [Phys. Rev. Lett. 66, 1713-1716 (1991)], have been generalized to diverse practical scenarios, yielding novel physical intuitions and possibilities of application for those phenomena. The properties of materials, in particular, the electrical permittivity and permeability, exert significant influences on the Casimir-Cifshitz energies and forces, and hence on the corresponding premelting and formation of ice. To address these influences in detail and explore the resulting physics, here we revisit and extend the analyses of previous work, with both the dielectric data utilized there and the latest dielectric functions for ice and cold water. While our previous results are rederived, an error there has also been spotted. For the four-layer cases considered by some of us, the existence of stable configurations depending on the initial conditions has been confirmed, and different types of stability corresponding to minima of the Casimir-Lifshitz free energy are demonstrated. As the new dielectric functions for ice and cold water deviate considerably from those used by Elbaum and Schick, their vital impacts on three- and four-layer configurations are therefore being reconsidered.

cond-mat.mtrl-sci↗

arXiv2021

The effects of intrinsic local distortions vs. dynamic thermal motions on the stability and band gaps of cubic oxide and halide perovskites

Corner-shared ABX$_3$ perovskites have long featured prominently in solid-state chemistry and condensed matter physics. Still, the joint understanding of their two main subgroups-halides and oxides-has not been fully developed. Indeed, unlike the case that compounds having a single repeated motif ("monomorphous"), certain cubic perovskites can manifest a non-thermal distribution of local motifs ("polymorphous networks"). Such intrinsic deformations can include positional degrees of freedom. Unlike thermal motion, such intrinsic distortions do not time-average to zero. The present study compares electronic structure features of oxide and halide perovskites starting from the intrinsic polymorphous network described by DFT minimization of the internal energy, continuing to finite temperature thermal disorder using AIMD. We find that (i) different oxide vs. halide ABX$_3$ compounds adopt different energy-lowering distortion modes. The DFT calculated pair distribution function (PDF) of SrTiO$_3$ agrees with the recently measured PDF. (ii) In both oxides and halides, such intrinsic distortions lead to bandgap blueshifts with respect to monomorphous structure. (iii) For oxide perovskites, high-temperature AIMD simulations initiated from the polymorphous structures reveal that the thermally-induced distortions can lead to a bandgap redshift. (iv) In contrast, for cubic CsPbI$_3$, both the intrinsic distortions and the thermal distortions contribute in tandem to bandgap blueshift, the former, intrinsic effect being dominant. (v) In the oxide SrTiO$_3$ and CaTiO$_3$ (but not in halide), octahedral tilting leads to the emergence of a distinct $Γ$-$Γ$ direct bandgap component as a secondary valley minimum to the well-known indirect R-$Γ$ gap. Understanding such intrinsic vs. thermal effects on oxide vs. halide perovskites holds the potential for designing target electronic properties.

cond-mat.mtrl-sci↗

arXiv2021

Dependence of band gaps in d-electron perovskite oxides on magnetism

Understanding the controlling principles of band gaps trends in d electron perovskites is needed both for gauging metal-insulator transitions, as well as their application in catalysis and doping. The magnitude of this band gap is rather different for different magnetic spin configurations. We find via electronic structure theory that the factors that connect gapping magnitudes to magnetism depend on the nature of the band edge orbital character (BEOC) and surprisingly scale with the number of antiferromagnetic contacts z$_i$ between neighboring transition metal ions. The dependence is weak when the BEOC are (d,d)-like ("Mott insulators"), whereas this dependence is rather strong in (p,d)-like ("charge transfer" insulators). These unexpected rules are traced to the reduced orbital interactions through the increase in the number of antiferromagnetic contacts between transition metal ions. The impact of magnetic order is not limited to the band gap magnitude and includes also the magnitude of lattice distortions connected to the electronic structure. These results highlight the importance of establishing in electronic structure theory of gap-related phenomena (doping, transport, metal-insulator transitions, conductive interfaces) the appropriate magnetic order.

cond-mat.str-el↗

arXiv2021

Bulk NdNiO2 is thermodynamically unstable with respect to decomposition while hydrogenation reduces the instability and transforms it from metal to insulator

Through CaH2 chemical reduction of a parent R3+Ni3+O3 perovskite form, superconductivity was recently achieved in Sr-doped NdNiO2 on SrTiO3 substrate. Using density functional theory (DFT) calculations, we find that stoichiometric NdNiO2 is significantly unstable with respect to decomposition into 1/2[Nd2O3 + NiO + Ni] with exothermic decomposition energy of +176 meV/atom, a considerably higher instability than that for common ternary oxides. This poses the question if the stoichiometric NdNiO2 nickelate compound used extensively to model the electronic band structure of Ni-based oxide analog to cuprates and found to be metallic is the right model for this purpose. To examine this, we study via DFT the role of the common H impurity expected to be present in the process of chemical reduction needed to obtain NdNiO2. We find that H can be incorporated exothermically, i.e., spontaneously in NdNiO2, even from H2 gas. In the concentrated limit, such impurities can result in the formation of a hydride compound NdNiO2H, which has significantly reduced instability relative to hydrogen-free NdNiO2. Interestingly, the hydrogenated form has a similar lattice constant as the pure form (leading to comparable XRD patterns), but unlike the metallic character of NdNiO2, the hydrogenated form is predicted to be a wide gap insulator thus, requiring doping to create a metallic or superconducting state, just like cuprates, but unlike unhydrogenated nickelates. While it is possible that hydrogen would be eventually desorbed, the calculation suggests that pristine NdNiO2 is hydrogen-stabilized. One must exercise caution with theories predicting new physics in pristine stoichiometric NdNiO2 as it might be an unrealizable compound. Experimental examination of the composition of real NdNiO2 superconductors and the effect of hydrogen on the superconductivity is called for.

cond-mat.mtrl-sci↗

arXiv2021

Local positional and spin symmetry breaking as a source of magnetism and insulation in paramagnetic EuTiO3

We consider theoretically the paramagnetic phases of EuTiO3 that represent configurations created by two sets of microscopic degrees of freedom (m-DOF): positional symmetry breaking due to octahedral rotations and magnetic symmetry breaking due to spin disorder. The effect of these sets of m-DOFs on the electronic structure and properties of the para phases is assessed by considering sufficiently large (super) cells with the required nominal global average symmetry, allowing, however, the local positional and magnetic symmetries to be lowered. We find that tendencies for local symmetry breaking can be monitored by following total energy lowering in mean-field like density functional theory, without recourse for strong correlation effects. While most nominally cubic ABO3 perovskites are known for their symmetry breaking due to the B-atom sublattice, the case of f-electron magnetism in EuTiO3 is associated with A- sublattice symmetry breaking and its coupling to structural distortions. We find that (i) paramagnetic cubic EuTiO3 has an intrinsic tendency for both magnetic and positional symmetry breaking, while paramagnetic tetragonal EuTiO3 has only magnetic symmetry lowering and no noticeable positional symmetry lowering with respect to low-temperature antiferromagnetic tetragonal phase. (ii) Properly modeled paramagnetic tetragonal and cubic EuTiO3 have a nonzero local magnetic moment on each Eu ion, consistent with the experimental observations of local magnetism in the para phases of EuTiO3 significantly above the Néel temperature. Interestingly, (iii) the local positional distortion modes in the short-range ordered para phases are inherited from the long-range ordered low-temperature antiferromagnetic ground state phase.

cond-mat.mtrl-sci↗