arXiv subjects

Vladan Stevanović

Publications and source records attributed to Vladan Stevanović.

At least 19 recordsLinked to original sources

arXiv2026

Local distortions as a source of piezoelectric/stiffness decoupling in B-doped AlScN

We present a first-principles analysis of the wurtzite pseudo-ternary (Al,Sc,B)N to elucidate the structural origin of a decoupling between stiffness $C_{33}$ and piezoelectric response $e_{33}$ upon boron incorporation, using DFT-relaxed 100-atom special quasirandom structures across a broad composition range. Pair distribution function analysis reveals interstitial threefold-coordinated boron atoms that have displaced from the tetrahedral cation site. Direct structural analysis establishes their preferential orientation along the $c$-axis and identifies a scandium-activated creation mechanism. The vertical coordination asymmetry of each cation is quantified through a site-specific axial asymmetry ratio (AAR), showing that boron incorporation progressively symmetrizes the Sc environment. Correlation with Born effective charges demonstrates that this symmetrization is the mechanism behind the piezoelectric enhancement.

cond-mat.mtrl-sci↗

arXiv2025

First-principles study of the phase competition, mechanical and piezoelectric properties of pseudo-binary (SiC)(AlN) alloy

The ongoing search for new piezoelectric materials offering adequate balance between piezoelectric response and other application-relevant properties has lead to the investigation of various alloy systems. In this work we study the alloy of the widely used AlN with SiC for their relative abundance, current use in other electronics applications and expected phase competition between wurtzite and other polymorphs, the kind of which has lead to some of the most interesting results notably between AlN and ScN. Here the pseudo-binary (SiC)(AlN) alloy is studied from first-principles over the entire composition range. Relevant crystalline phases are identified using the First-Principles Random Structure Sampling approach which, in accordance with previous bulk experiments, finds wurtzite, zincblende and rhombohedral phases to be the only statistically relevant phases of the alloy. Further study of these phases is done through Special Quasi-random Structures (SQS) and, in the case of the wurtzite phase, predictions of the stiffness, piezoelectric and dielectric tensors. Analysis of these tensors is done through the scope of a Bulk AcousticWave (BAW) filter application, where trends and trade-offs between the c-axis acoustic velocity and piezoelectric response enable identification of relevant compositions.

cond-mat.mtrl-sci↗

arXiv2025

Stability, growth, and doping of In$_{2}$(Si, Ge)$_{2}$O$_{7}$ as promising n-type wide-gap semiconductors

In this paper we investigate, computationally and experimentally, the phase stability, electronic structure properties, and the propensity for n-type doping of In$_{2}$X$_{2}$O$_{7}$ (X=Si, Ge) ternary oxides. This family of materials contains promising novel wide-gap semiconductors based on their estimated high $n$-type Baliga figures of merit and acceptable thermal conductivity for power electronics applications. Here, we find that both In$_{2}$Si$_{2}$O$_{7}$ and In$_{2}$Ge$_{2}$O$_{7}$ to be n-type dopable, with Zr providing between 10$^{16}$ and above 10$^{21}$ cm$^{-3}$ net donor concentrations under O-poor conditions, depending on the chemistry, structure (ground-state thorvetite or high-pressure pyrochlore) and synthesis temperature. Initial thin-film growth and annealing leads to polycrystalline In$_{2}$Ge$_{2}$O$_{7}$ thin films in thorvetite structure with band gap over 4 eV, and confirms Zr doping predictions by achieving electron concentrations at 10$^{14}$-10$^{16}$ cm$^{-3}$ under O-rich condition. While future epitaxial growth development is still needed, this study establishes In$_{2}$X$_{2}$O$_{7}$ as promising n-type wide-gap semiconductors for power electronic applications.

cond-mat.mtrl-sci↗

arXiv2025

Modeling glasses from first-principles using random structure sampling

We present an approach to approximating static properties of glasses without experimental inputs rooted in the first-principles random structure sampling. In our approach, the glassy system is represented by a collection (composite) of periodic, small-cell (few 10s of atoms) local minima on the potential energy surface. These are obtained by generating a set of periodic structures with random lattice parameters and random atomic positions, which are then relaxed to their closest local minima on the potential energy surface using the first-principles methods. Using vitreous SiO2 as an example, we illustrate and discuss how well various atomic and electronic structure properties calculated as averages over the set of such local minima reproduce experimental data. The practical benefit of our approach, which can be rigorously thought of as representing an infinitely quickly quenched liquid, is in that it transfers the computational burden to linearly scaling and easy to converge averages of properties computed on small-cell structures, rather than simulation cells with 100s if not 1000s of atoms while retaining a good overall predictive accuracy. Because of this it enables the future use of high-cost/high-accuracy electronic structure methods thereby bringing modeling of glasses and amorphous phases closer to the state of modeling of crystalline solids.

cond-mat.mtrl-sci↗

arXiv2024

All "roads" lead to rocksalt structure

Rocksalt is the most common crystal structure among binary compounds. Moreover, no long-lived, metastable polymorphs are observed in compounds with the rocksalt ground state. We investigate the absence of polymorphism and the dominance of the rocksalt structure via first-principles random structure sampling and transformation kinetics modeling of three rocksalt compounds: MgO, TaC, and PbTe. Random structure sampling reveals that for all three systems the rocksalt local minimum on the potential energy surface is far larger than any other, making it statistically the most significant structure. The kinetics modeling shows that virtually all other relevant structures, including most of the 457 known A$_1$B$_1$ prototypes, transform rapidly to rocksalt, making it the only option at ambient conditions. These results explain the absence of polymorphism in binary rocksalts, answer why rocksalt is the structure of choice for high-entropy ceramics and disordered ternary nitrides, and help understand/predict metastable states in crystalline solids more generally.

cond-mat.mtrl-sci↗

arXiv2023

Computational insights into phase equilibria between wide-gap semiconductors and contact materials

Novel wide-band-gap semiconductors are needed for next-generation power electronic but there is a gap between a promising material and a functional device. Finding stable contacts is one of the major challenges, which is currently dealt with mainly via trial and error. Herein, we computationally investigate the thermochemistry and phase co-existence at the junction between three wide gap semiconductors, $β$-Ga$_{2}$O$_{3}$, GeO$_2$, and GaN, and possible contact materials. The pool of possible contacts includes 47 elemental metals and 4 common $n$-type transparent conducting oxides (ZnO, TiO$_2$, SnO$_2$, and In$_2$O$_3$). We use first-principles thermodynamics to model the Gibbs free energies of chemical reactions as a function of the gas pressure (p$_{\mathrm{O}_2}$/p$_{\mathrm{N}_2}$) and equilibrium temperature. We deduce whether a semiconductor/contact interface will be stable at relevant conditions, possibly influencing the long-term reliability and performance of devices. We generally find that most elemental metals tend to oxidize or nitridize and form various interface oxide/nitride layers. Exceptions include select late- and post-transition metals, and in case of GaN also the alkali metals, which are predicted to exhibit stable coexistence, although in many cases at relatively low gas partial pressures. Similar is true for the transparent conducting oxides, for which in most cases we predict a preference toward forming ternary oxides when in contact with $β$-Ga$_{2}$O$_{3}$ and GeO$_{2}$. The only exception is SnO$_2$, which can form stable contacts with both oxides. Finally, we show how the same approach can be used to predict gas partial pressure vs. temperature phase diagrams to help direct synthesis of ternary compounds. We believe these results provide a valuable guidance in selecting contact materials to wide-gap semiconductors and suitable growth conditions.

cond-mat.mtrl-sci↗

arXiv2023

Mechanistically-guided materials chemistry: synthesis of new ternary nitrides, CaZrN$_2$ and CaHfN$_2$

Recent computational studies have predicted many new ternary nitrides, revealing synthetic opportunities in this underexplored phase space. However, synthesizing new ternary nitrides is difficult, in part because intermediate and product phases often have high cohesive energies that inhibit diffusion. Here, we report the synthesis of two new phases, calcium zirconium nitride (CaZrN$_2$) and calcium hafnium nitride (CaHfN$_2$), by solid state metathesis reactions between Ca$_3$N$_2$ and $M$Cl$_4$ ($M$ = Zr, Hf). Although the reaction nominally proceeds to the target phases in a 1:1 ratio of the precursors via Ca$_3$N$_2$ + $M$Cl$_4$ $\rightarrow$ Ca$M$N$_2$ + 2 CaCl$_2$, reactions prepared this way result in Ca-poor materials (Ca$_xM_{2-x}$N$_2$, $x<1$). A small excess of Ca$_3$N$_2$ (ca. 20 mol\%) is needed to yield stoichiometric Ca$M$N$_2$, as confirmed by high-resolution synchrotron powder X-ray diffraction. In situ synchrotron X-ray diffraction studies reveal that nominally stoichiometric reactions produce Zr$^{3+}$ intermediates early in the reaction pathway, and the excess Ca$_3$N$_2$ is needed to reoxidize Zr$^{3+}$ intermediates back to the Zr$^{4+}$ oxidation state of CaZrN$_2$. Analysis of computationally-derived chemical potential diagrams rationalizes this synthetic approach and its contrast from the synthesis of MgZrN$_2$. These findings additionally highlight the utility of in situ diffraction studies and computational thermochemistry to provide mechanistic guidance for synthesis.

cond-mat.mtrl-sci↗

arXiv2022

The Mixing Thermodynamics and Local Structure of High-entropy Alloys from Randomly Sampled Ordered Configurations

A general method is presented for modeling high entropy alloys as ensembles of randomly sampled, ordered configurations on a given lattice. Statistical mechanics is applied post hoc to derive the ensemble properties as a function of composition and temperature, including the free energy of mixing and local structure. Random sampling is employed to address the high computational costs needed to model alloys with a large number of components. Doing so also provides rigorous convergence criteria, including the quantification of noise due to random sampling, and an estimation of the number of additional samples required to lower this noise to the needed/desired levels. This method is well-suited for a variety of cases: i) high entropy alloys, where standard lattice models are costly; ii) "medium" entropy alloys, where both the entropy and enthalpy play significant roles; and iii) alloys with residual short-range order. Binary to 5-component alloys of the group-IV chalcogenides are used as case examples, for which the predicted miscibility shows excellent agreement with experiment.

cond-mat.mtrl-sci↗

arXiv2022

Transition metal impurities in Silicon: Computational search for a semiconductor qubit

Semiconductors offer a promising platform for physical implementation of qubits, but their broad adoption is presently hindered by limited scalability and/or very low operating temperatures. Learning from the nitrogen-vacancy centers in diamond, our goal is to find equivalent optically active point defect centers in crystalline silicon, which could be advantageous for their scalability and integration with classical devices. Transition metal (TM) impurities in silicon are common paramagnetic deep defects, but a comprehensive theoretical study of the whole 3$d$ series that considers generalized Koopmans' condition is missing. We apply the HSE06(+U) method to examine their potential as optically active spin qubits and identify seven TM impurities that have optically allowed triplet-triplet transitions within the silicon band gap. These results provide the first step toward silicon-based qubits with higher operating temperatures for quantum sensing. Additionally, these point defects could lead to spin-photon interfaces in silicon-based qubits and devices for mid-infrared free-space communications.

cond-mat.mtrl-sci↗

arXiv2022

Ternary Wide Band Gap Oxides for High-Power Electronics Identified Computationally

As electricity grids become more renewable energy-compliant, there will be a need for novel semiconductors that can withstand high power, high voltage, and high temperatures. Wide band gap (WBG) semiconductors tend to exhibit large breakdown field, allowing high operating voltages. Currently explored WBG materials for power electronics are costly (GaN), difficult to synthesize as high-quality single crystals (SiC) and at scale (diamond, BN), have low thermal conductivity ($β$-Ga$_2$O$_3$), or cannot be suitably doped (AlN). We conduct a computational search for novel semiconductors across 1,340 known metal-oxides using first-principles calculations and existing transport models. We calculate the Baliga figure of merit (BFOM) and lattice thermal conductivity ($κ_L$) to identify top candidates for n-type power electronics. We find 40 mostly ternary oxides that have higher $κ_L$ than $β$-Ga$_2$O$_3$ and higher n-type BFOM than SiC and GaN. Among these, several material classes emerge, including 2-2-7 stoichiometry thortveitites and pyrochlores, II-IV spinels, and calcite-type borates. Within these classes, we propose In$_2$Ge$_2$O$_7$, Mg$_2$GeO$_4$, and InBO$_3$ as they are the most favorable for n-type doping based on our preliminary evaluation and could be grown as single crystals or thin film heterostructures. These materials could help advance power electronic devices for the future grid.

cond-mat.mtrl-sci↗

arXiv2022

The role of disorder in the synthesis of metastable zinc zirconium nitrides

In materials science, it is often assumed that ground state crystal structures predicted by density functional theory are the easiest polymorphs to synthesize. Ternary nitride materials, with many possible metastable polymorphs, provide a rich materials space to study what influences thermodynamic stability and polymorph synthesizability. For example, ZnZrN2 is theoretically predicted at zero Kelvin to have an unusual layered "wurtsalt" ground state crystal structure with compelling optoelectronic properties, but it is unknown whether this structure can be realized experimentally under practical synthesis conditions. Here, we use combinatorial sputtering to synthesize hundreds of ZnxZr1-xNy thin film samples, and find metastable rocksalt-derived or boron-nitride-derived structures rather than the predicted wurtsalt structure. Using a statistical polymorph sampler approach, it is demonstrated that although rocksalt is the least stable polymorph at zero Kelvin, it becomes the most stable polymorph at high effective temperatures similar to those achieved using this sputter deposition method, and thus corroborates experimental results. Additional calculations show that this destabilization of the wurtsalt polymorph is due to configurational entropic and enthalpic effects, and that vibrational contributions are negligible. Specifically, rocksalt- and boron-nitride-derived structures become the most stable polymorphs in the presence of disorder because of higher tolerances to cation cross-substitution and off-stoichiometry than the wurtsalt structure. This understanding of the role of disorder tolerance in the synthesis of competing polymorphs can enable more accurate predictions of synthesizable crystal structures and their achievable material properties.

cond-mat.mtrl-sci↗

arXiv2022

Theoretical insights for Improving the Schottky-barrier Height at the Ga$_2$O$_3$/Pt Interface

In this work we study the Schottky barrier height (SBH) at the junction between $β$-Ga$_2$O$_3$ and platinum, a system of great importance for the next generation of high-power and high-temperature electronic devices. Specifically, we obtain interfacial atomic structures at different orientations using our structure matching algorithm and compute their SBH using electronic structure calculations based on hybrid density functional theory. The orientation and strain of platinum are found to have little impact on the barrier height. In contrast, we find that decomposed water (H.OH), which could be present at the interface from Ga$_2$O$_3$ substrate preparation, has a strong influence on the SBH, in particular in the ($\overline{2}$01) orientation. The SBH can range from $\sim$2 eV for the pristine interface to nearly zero for the full H.OH coverage. This result suggests that SBH of $\sim$2~eV can be achieved for the Ga$_2$O$_3$($\overline{2}$01)/Pt junction using the substrate preparation methods that can reduce the amount of adsorbed water at the interface.

cond-mat.mtrl-sci↗

arXiv2021

Computational Fermi level engineering and doping-type conversion of Ga2O3 via three-step synthesis process

Ga2O3 is being actively explored for high-power and high-temperature electronics, deep-ultraviolet optoelectronics, and other applications. Efficient n-type doping of Ga2O3 has been achieved, but p-type doping faces fundamental obstacles due to compensation, deep acceptor levels, and the polaron transport mechanism of free holes. However, aside from achieving p-type conductivity, plenty of opportunity exists to engineer the position of the Fermi level for improved design of Ga2O3 based devices. We use first-principles defect theory and defect equilibrium calculations to simulate a 3-step growth-annealing-quench synthesis protocol for hydrogen assisted Mg doping in beta-Ga2O3, taking into account the gas phase equilibrium between H2, O2 and H2O, which determines the H chemical potential. We predict Ga2O3 doping-type conversion to a net p-type regime after growth under reducing conditions in the presence of H2 followed by O-rich annealing, which is a similar process to the Mg acceptor activation by H removal in GaN. For equilibrium annealing there is an optimal temperature that maximizes the Ga2O3 net acceptor density for a given Mg doping level, which is further increased in the non-equilibrium annealing scenario without re-equilibration. After quenching to operating temperature, the Ga2O3 Fermi level drops below mid-gap down to about +1.5 eV above the valence band maximum, creating a significant number of uncompensated neutral MgGa0 acceptors. The Fermi level reduction down to +1.5 eV and suppression of free electron density in this doping type converted (NA > ND) Ga2O3 material is of significance and impact for the design of Ga2O3 power electronics devices.

cond-mat.mtrl-sci↗

arXiv2021

Search and Structural Featurization of Magnetically Frustrated Kagomé Lattices

We have searched nearly 40,000 inorganic solids in the Inorganic Crystal Structural Database to identify compounds containing a transition metal or rare earth kagomé sublattice, a geometrically magnetically frustrated lattice, ultimately identifying $\sim$500 materials. A broad analysis of the chemical and structural trends of these materials shows three types of kagomé sheet stacking and several classes of magnetic complexity. Following the search and classification, we rapidly screen the magnetic properties of a subset of the materials using density functional theory to eliminate those that are unlikely to exhibit magnetic frustration. From the results of our computational screening, we rediscover six materials that have previously been explored for their low temperature magnetic behavior, albeit showing symmetry breaking distortions, spin glass behavior, or magnetic ordering. However, all are materials with antiferromagnetic behavior, which we correctly predict. Finally, we also report three materials that appear to be unexplored for their magnetic properties.

cond-mat.mtrl-sci↗

arXiv2020

Minimization of atomic displacements as a guiding principle of the martensitic phase transformation

We present a unifying description for the martensitic transformation of steel that accounts for important experimentally observable features of the transformation namely, the Neumann bands, the interfacial (habit) plane between the transformed and untransformed phases and their orientation relationship (OR). It is obtained through a simple geometric minimization of the total distance traveled by all the atoms from the austenite (FCC or $γ$) phase to the martensite (BCC or $α$) phase, without the need for any explicit energy minimization. Our description unites previously proposed mechanisms but it does not rely on assumptions and experimental knowledge regarding the shear planes and directions, or external adjustable parameters. We show how the Kurdjumov-Sach orientation relationship between the two phases and the $\{225\}_γ$ habit plane, which have both been extensively reported in experiments, naturally emerge from the distance minimization. We also propose an explanation for the occurrence of a different orientation relationship (Pitsch) in thin films.

cond-mat.mtrl-sci↗

arXiv2020

Matching Crystal Structures Atom-to-Atom

Finding an optimal match between two different crystal structures underpins many important materials science problems, including describing solid-solid phase transitions, developing models for interface and grain boundary structures. In this work, we formulate the matching of crystals as an optimization problem where the goal is to find the alignment and the atom-to-atom map that minimize a given cost function such as the Euclidean distance between the atoms. We construct an algorithm that directly solves this problem for large finite portions of the crystals and retrieves the periodicity of the match subsequently. We demonstrate its capacity to describe transformation pathways between known polymorphs and to reproduce experimentally realized structures of semi-coherent interfaces. Additionally, from our findings we define a rigorous metric for measuring distances between crystal structures that can be used to properly quantify their geometric (Euclidean) closeness.

cond-mat.mtrl-sci↗

arXiv2019

Physical descriptor for the Gibbs energy of inorganic crystalline solids and temperature-dependent materials chemistry

The Gibbs energy, G, determines the equilibrium conditions of chemical reactions and materials stability. Despite this fundamental and ubiquitous role, G has been tabulated for only a small fraction of known inorganic compounds, impeding a comprehensive perspective on the effects of temperature and composition on materials stability and synthesizability. Here, we use the SISSO (sure independence screening and sparsifying operator) approach to identify a simple and accurate descriptor to predict G for stoichiometric inorganic compounds with ~50 meV/atom (~1 kcal/mol) resolution, and with minimal computational cost, for temperatures ranging from 300-1800 K. We then apply this descriptor to ~30,000 known materials curated from the Inorganic Crystal Structure Database (ICSD). Using the resulting predicted thermochemical data, we generate thousands of temperature-dependent phase diagrams to provide insights into the effects of temperature and composition on materials synthesizability and stability and to establish the temperature-dependent scale of metastability for inorganic compounds.

cond-mat.mtrl-sci↗

arXiv2018

Ionic vs. van der Waals Layered Materials: Identification and Comparison of Elastic Anisotropy

In this work, we expand the set of known layered compounds to include ionic layered materials, which are well known for superconducting, thermoelectric, and battery applications. Focusing on known ternary compounds from the ICSD, we screen for ionic layered structures by expanding upon our previously developed algorithm for identification of van der Waals (vdW) layered structures, thus identifying over 1,500 ionic layered compounds. Since vdW layered structures can be chemically mutated to form ionic layered structures, we have developed a methodology to structurally link binary vdW to ternary ionic layered materials. We perform an in-depth analysis of similarities and differences between these two classes of layered systems and assess the interplay between layer geometry and bond strength with a study of the elastic anisotropy. We observe a rich variety of anisotropic behavior in which the layering direction alone is not a simple predictor of elastic anisotropy. Our results enable discovery of new layered materials through intercalation or de- intercalation of vdW or ionic layered systems, respectively, as well as lay the groundwork for studies of anisotropic transport phenomena such as sound propagation or lattice thermal conductivity.

cond-mat.mtrl-sci↗