arXiv subjects

Kirill Kovnir

Publications and source records attributed to Kirill Kovnir.

14 recordsLinked to original sources

arXiv2026

Stability and superstructural ordering of alkali-triel-pnictide clathrates A$_8$T$_{27}$Pn$_{19}$

Clathrates are a class of inclusion compounds that offer various useful and surprising phenomena, including superconductivity, thermoelectricity, and the potential for high-density ion storage. Stability conditions within the Alkali-Triel-Pnictide A$_8$T$_{27}$Pn$_{19}$ family of unconventional clathrates are investigated with high-throughput density functional theory calculations, establishing trends in formation energy, structural and electronic properties. Electronic structure calculations and first-principles molecular dynamics simulations show that the ionization potential of guest alkaline atoms strongly influences the stability of electron-exact clathrates and affects their rattler behavior. Targeted reactive synthesis from elemental precursors is attempted, resulting in two novel ternary compounds. However, the targeted clathrate phases are not obtained. Further analysis reveals that the stability of ATPn clathrate compounds containing heavy elements, such as bismuth, depends strongly on spin-orbit effects, which are often neglected in high-throughput studies that compute formation energies. Finally, chemically induced superstructural ordering is described in relation to Wyckoff sites in the prototypical type-I clathrate unit cell.

cond-mat.mtrl-sci↗

arXiv2026

Metallic d-wave altermagnetism in WFeB: a platform for electrically switchable perpendicular spin-splitter response

We report the synthesis and magnetic characterization of WFeB and identify it as a metallic d-wave altermagnet representative of a broader TiNiSi-type family. Neutron diffraction, M\"ossbauer spectroscopy, and magnetometry establish a collinear altermagnetic ordering confirmed by first-principles calculations. The electronic structure shows a nonrelativistic spin splitting of approximately 100 meV, but it also supports a strong spin-splitter transport response. This demonstrates that efficient spin-current generation can occur even with such modest band splitting. Symmetry analysis shows that selected film orientations permit deterministic switching of the N\'eel vector by current-induced staggered torques, enabling electrical control of a perpendicular spin-splitter response. These results establish WFeB and related TiNiSi-type antiferromagnets as a platform for electrically switchable charge-to-spin conversion driven by altermagnetic symmetry.

cond-mat.mtrl-sci↗

arXiv2026

Long photoexcited carrier lifetime in a stable and earth-abundant zinc polyphosphide

Halide perovskites have revolutionized optoelectronics by demonstrating that long carrier lifetime can be achieved in materials processed in relatively uncontrolled environments, whereas conventional inorganic semiconductors typically suffer from short carrier lifetime unless very carefully prepared and postprocessed. Here, we report the discovery of exceptionally long photoexcited carrier lifetime in monoclinic ZnP2, effectively bridging the carrier lifetime gap between direct-gap inorganic semiconductors and halide perovskites. Through computational screening, ZnP2 is identified as a long carrier lifetime semiconductor characterized by an unconventional polyphosphide bonding, combining covalently bonded phosphorus chains and polar-covalent Zn-P tetrahedra. Experimentally, ZnP2 crystals synthesized from low-purity precursors exhibit bright band-to-band photoluminescence at 1.49 eV and carrier lifetimes of up to 1 $\mu$s. Further analysis reveals that the polyphosphide bonding of ZnP2 suppresses the formation of deep intrinsic defects, making it defect resistant. Combined with its remarkable environmental stability, ZnP2 presents a highly promising material for solar absorbers and light emitters. Our work illustrates that underexplored inorganic materials spaces with unusual chemical bonding hold great promise for discovering novel optoelectronic materials.

cond-mat.mtrl-sci↗

arXiv2025

Alloying to Tune the Bandgap of the AM2Pn2 Zintl Compounds

The AM2Pn2 Zintl compounds are a large class of semiconductor materials that have a wide range of bandgaps and are mostly stable in the same crystal structure. Representative compounds BaCd2P2 and CaZn2P2 have recently been found to exhibit high visible light absorption and long carrier lifetime. Here we use high throughput first-principles calculations to study AM2Pn2 alloys for applications as tandem top cell absorbers (i.e., bandgaps around 1.8 eV) and far infrared detector materials (i.e., bandgaps lower than 0.5 eV). Using a first-principles computational screening workflow for assessing stability and electronic structure of alloys, we identify several promising candidates. These include Ca(Cd0.8Mg0.2)2P2 with a suitable direct bandgap for use in tandem top cells on silicon bottom cells and SrCd2(Sb1-xBix)2 for far infrared detectors. We demonstrate that alloys of AM2Pn2 materials can be realized by experimentally synthesizing Ca(Zn0.8Mg0.2)2P2.

cond-mat.mtrl-sci↗

arXiv2025

CaCd$_2$P$_2$: A Visible-Light Absorbing Zintl Phosphide Stable under Photoelectrochemical Water Oxidation

A key bottleneck to solar fuels is the absence of stable and strongly absorbing photoelectrode materials for the oxygen evolution reaction (OER). Modern approaches generally trade off between stable but weakly absorbing materials, such as wide bandgap oxides, or strongly absorbing materials that rely on encapsulation for stability and are weakly catalytic, such as the III-V family of semiconductors. Of interest are materials like transition metal phosphides, such as FeP$_2$, that are known to undergo beneficial in situ surface transformations in the oxidative environment of OER, though stability has remained a primary hurdle. Here we report on CaCd$_2$P$_2$, a Zintl phase visible-light absorber with favorable 1.6 eV bandgap, that we identified using high-throughput computational screening. Using a combination of photoelectrochemical measurements, microscopy, and spectroscopy, we show that CaCd$_2$P$_2$ undergoes a light-stabilized surface transformation that renders it stable under alkaline OER conditions. We also show that the well known OER catalyst CoPi can act as a stable co-catalyst in synergy with the \textit{in-situ} CaCd$_2$P$_2$ surface. The light-induced stabilization that CaCd$_2$P$_2$ displays is in sharp contrast to the photocorrosion commonly observed in visible light-absorbing photoelectrodes. The broader AM$_2$P$_2$ family of Zintl phases offers a significant opportunity to explore stabilizing interface chemistry and re-design the manner in which low-bandgap semiconductors are used for photoelectrochemical energy conversion.

cond-mat.mtrl-sci↗

arXiv2025

Discovery and Synthesis of a Family of Boride Altermagnets

Borides are a rich material family. To push the boundaries of borides' properties and applications into broader fields, we have conducted systematic theoretical and experimental searches for synthesizable phases in ternary borides TM$_2$B$_2$ (T = 3d, M = 4d/5d transition metals). We find that TM$_2$B$_2$ in the FeMo$_2$B$_2$-type and CoW$_2$B$_2$-type structures form a large family of stable/metastable materials of 120 members. Among them, we identify 40 materials with stable magnetic solutions. Further, we discover 11 altermagnets in the FeMo$_2$B$_2$-type structure. So far, boride altermagnets are rare. In these altermagnets, T = Fe or Mn atoms are arranged in parallel T-chains with strong ferromagnetic intrachain couplings and antiferromagnetic interchain couplings. They simultaneously exhibit electronic band spin splitting, typical of ferromagnetism, and zero net magnetization, typical of antiferromagnetism. They also exhibit magnonic band chiral splitting. Both effects originate from the unique altermagnetic symmetries crucially constrained by the nonmagnetic atoms in the structure. Transport properties of relevance to spintronic applications, including the strain-induced spin-splitter effect and anomalous Hall effect, are predicted. An iodine-assisted synthesis method for TM$_2$B$_2$ is developed, using which 7 of the predicted low-energy phases are experimentally synthesized and characterized, including 4 altermagnets. This work expands the realm of borides by offering new opportunities for studying altermagnetism and altermagnons in borides. It also provides valuable insights into the discovery and design of altermagnets. By demonstrating that altermagnets can exist as families sharing a common motif, this work paves a feasible route for discovering altermagnets by elemental substitutions and high-throughput computations.

cond-mat.mtrl-sci↗

arXiv2025

BaCd2P2: a promising impurity-tolerant counterpart of GaAs for photovoltaics

BaCd2P2 (BCP) has been recently identified as a new solar absorber with promising optoelectronic properties. This work demonstrates that, despite having a low precursor purity (98.90% to 99.95%), synthesized BCP samples exhibit a promising photoconductive carrier lifetime up to 300 ns, an implied open-circuit voltage exceeding 1 V, and photoluminescence quantum yield in the order of 0.2%, comparable to a high-purity single-crystalline GaAs wafer. To better understand the underlying mechanisms of BCP's promising properties, its tolerance to intrinsic defects and extrinsic impurities is investigated using first-principles defect modeling and compared with that of the well-studied GaAs. The results show that the nonradiative recombination rates induced by dominant deep-level intrinsic antisite defects are lower in BCP than in GaAs under typical growth conditions. Further exploration of the impact of transition metal impurities in the raw materials used to make BCP and impurities introduced during its synthesis shows that most of these do not form deep-level nonradiative recombination centers. As an impurity-tolerant counterpart of GaAs, BCP demonstrates great potential to improve the cost-to-performance ratio of photovoltaics.

cond-mat.mtrl-sci↗

arXiv2025

Developing a Neural Network Machine Learning Interatomic Potential for Molecular Dynamics Simulations of La-Si-P Systems

While molecular dynamics (MD) is a very useful computational method for atomistic simulations, modeling the interatomic interactions for reliable MD simulations of real materials has been a long-standing challenge. In 2007, Behler and Perrinello first proposed and demonstrated an artificial neural network machine learning (ANN-ML) scheme, opening a new paradigm for developing accurate and efficient interatomic potentials for reliable MD simulation studies of the thermodynamics and kinetics of materials. In this paper, we show that an accurate and transferable ANN-ML interatomic potential can be developed for MD simulations of La-Si-P system. The crucial role of training data in the ML potential development is discussed. The developed ANN-ML potential accurately describes not only the energy versus volume curves for all the known elemental, binary, and ternary crystalline structures in La-Si-P system, but also the structures of La-Si-P liquids with various compositions. Using the developed ANN-ML potential, the melting temperatures of several crystalline phases in La-Si-P system are predicted by the coexistence of solid-liquid phases from MD simulations. While the ANN-ML model systematically underestimates the melting temperatures of these phases, the overall trend agrees with experiment. The developed ANN-ML potential is also applied to study the nucleation and growth of LaP as a function of different relative concentrations of Si and P in the La-Si-P liquid, and the obtained results are consistent with experimental observations.

cond-mat.mtrl-sci↗

arXiv2025

A Map of the Zintl AM2Pn2 Compounds: Influence of Chemistry on Stability and Electronic Structure

The AM2Pn2 (A= Ca, Sr, Ba, Yb, Mg; M= Mn, Zn, Cd, Mg; and Pn=N, P, As, Sb, Bi) family of Zintl phases has been known as thermoelectric materials and has recently gained much attention for highly promising materials for solar absorbers in single junction and tandem solar cells. In this paper we will, from first-principles, explore the entire family of AM2Pn2 compounds in terms of their ground state structure, thermodynamic stability, and electronic structure. We also perform photoluminescence spectroscopy on bulk powder and thin film samples to verify our results, including the first measurements of the bandgaps of SrCd2P2 and CaCd2P2. The AM2Pn2 compounds exhibit broad stability, are mostly isostructural in the CaAl2Si2-type structure (P3m1), and cover a wide range of bandgaps from 0 to beyond 3 eV. This could make them useful for a variety of purposes, for which we propose several candidates, such as CaZn2N2 for tandem top cell solar absorbers and SrCd2Sb2 and CaZn2Sb2 for infrared detectors. By examining the band structures of the AM2Pn2, we find that Mg3Sb2 has the most promise as a thermoelectric material due to several off-{\Gamma} valence band pockets which are unique to it among the compositions studied here.

cond-mat.mtrl-sci↗

arXiv2024

Low-Temperature Synthesis of Stable CaZn$_2$P$_2$ Zintl Phosphide Thin Films as Candidate Top Absorbers

The development of tandem photovoltaics and photoelectrochemical solar cells requires new absorber materials with band gaps in the range of ~1.5-2.3 eV, for use in the top cell paired with a narrower-gap bottom cell. An outstanding challenge is finding materials with suitable optoelectronic and defect properties, good operational stability, and synthesis conditions that preserve underlying device layers. This study demonstrates the Zintl phosphide compound CaZn$_2$P$_2$ as a compelling candidate semiconductor for these applications. We prepare phase pure, 500 nm-thick CaZn$_2$P$_2$ thin films using a scalable reactive sputter deposition process at growth temperatures as low as 100 {\deg}C, which is desirable for device integration. UV-vis spectroscopy shows that CaZn$_2$P$_2$ films exhibit an optical absorptivity of ~10$^4$ cm$^-$$^1$ at ~1.95 eV direct band gap. Room-temperature photoluminescence (PL) measurements show near-band-edge optical emission, and time-resolved microwave conductivity (TRMC) measurements indicate a photoexcited carrier lifetime of ~30 ns. CaZn$_2$P$_2$ is highly stable in both ambient conditions and moisture, as evidenced by PL and TRMC measurements. Experimental data are supported by first-principles calculations, which indicate the absence of low-formation-energy, deep intrinsic defects. Overall, our study should motivate future work integrating this potential top cell absorber material into tandem solar cells.

cond-mat.mtrl-sci↗

arXiv2024

Unveiling a Family of Dimerized Quantum Magnets in Ternary Metal Borides

Dimerized quantum magnets are exotic crystalline materials where Bose-Einstein condensation of magnetic excitations can happen. However, known dimerized quantum magnets are limited to only a few oxides and halides. Here, we unveil 9 dimerized quantum magnets and 11 conventional antiferromagnets in ternary metal borides MTB$_4$ (M = Sc, Y, La, Ce, Lu, Mg, Ca, Al; T = V, Cr, Mn, Fe, Co, Ni). In this type of structure, 3d transition-metal atoms T are arranged in dimers. Quantum magnetism in these compounds is dominated by strong antiferromagnetic interactions between Cr (both Cr and Mn for M = Mg and Ca) atoms within the structural dimers, with much weaker interactions between the dimers. These systems are proposed to be close to a quantum critical point between a disordered singlet spin-dimer phase, with a spin gap, and the ordered conventional Néel antiferromagnetic phase. This new family of dimerized quantum magnets greatly enriches the materials inventory that allows investigations of the spin-gap phase. All the quantum-, conventionally-, and non-magnetic systems identified, together with experimental synthesis methods of a phase suitable for characterization, provide a platform with abundant possibilities to tune the magnetic exchange coupling by doping and study this unconventional type of quantum phase transition. This work opens up new avenues for studying the quantum magnetism of spin dimers in borides and establishes a theoretical workflow for future searches for dimerized quantum magnets in other families or types of materials.

cond-mat.mtrl-sci↗

arXiv2023

Discovery of the Zintl-phosphide BaCd$_{2}$P$_{2}$ as a long carrier lifetime and stable solar absorber

Thin-film photovoltaics offers a path to significantly decarbonize our energy production. Unfortunately, current materials commercialized or under development as thin-film solar cell absorbers are far from optimal as they show either low power conversion efficiency or issues with earth-abundance and stability. Entirely new and disruptive materials platforms are rarely discovered as the search for new solar absorbers is traditionally slow and serendipitous. Here, we use first principles high-throughput screening to accelerate this process. We identify new solar absorbers among known inorganic compounds using considerations on band gap, carrier transport, optical absorption but also on intrinsic defects which can strongly limit the carrier lifetime and ultimately the solar cell efficiency. Screening about 40,000 materials, we discover the Zintl-phosphide BaCd$_{2}$P$_{2}$ as a potential high-efficiency solar absorber. Follow-up experimental work confirms the predicted promises of BaCd$_{2}$P$_{2}$ highlighting an optimal band gap for visible absorption, bright photoluminescence, and long carrier lifetime of up to 30 ns even for unoptimized powder samples. Importantly, BaCd$_{2}$P$_{2}$ does not contain any critical elements and is highly stable in air and water. Our work opens an avenue for a new family of stable, earth-abundant, high-performance Zintl-based solar absorbers. It also demonstrates how recent advances in first principles computation can accelerate the search of photovoltaic materials by combining high-throughput screening with experiment.

cond-mat.mtrl-sci↗

arXiv2023

Prediction of Van Hove singularity systems in ternary borides

A computational search for stable structures among both $α$ and $β$ phases of ternary ATB4 borides (A= Mg, Ca, Sr, Ba, Al, Ga, and Zn, T is 3d or 4d transition elements) has been performed. We found that $α$-ATB4 compounds with A=Mg, Ca, Al, and T=V, Cr, Mn, Fe, Ni, and Co form a family of structurally stable or almost stable materials. These systems are metallic in non-magnetic states and characterized by the formation of the localized molecular-like state of 3d transition metal atom dimers, which leads to the appearance of numerous Van Hove singularities (VHS) in the electronic spectrum. The closeness of these VHS to the Fermi level can be easily tuned by electron doping. For the atoms in the middle of the 3d row (Cr, Mn, and Fe), these VHS led to magnetic instabilities and new magnetic ground states with a weakly metallic or semiconducting nature. The magnetic ground states in these systems appear as an analog of the spin glass state. Experimental attempts to produce MgFeB4 and associated challenges are discussed, and promising directions for further synthetic studies are formulated.

cond-mat.mtrl-sci↗

arXiv2011

Glass-like ordering and spatial inhomogeneity of magnetic structure in Ba3FeRu2O9 : The role of Fe/Ru-site disorder

Several doped 6H hexagonal ruthenates, having the general formula Ba3MRu2O9, have been studied over a significant period of time in order to understand the unusual magnetism of ruthenium metal. However, among them, the M=Fe compound appears different since it is observed that unlike others, the 3d Fe ions and 4d Ru ions can easily exchange their crystallographic positions and as a result many possible magnetic interactions become realizable. The present study involving several experimental methods on this compound establish that the magnetic structure of Ba3FeRu2O9 is indeed very different from all other 6H ruthenates. Local structural study reveals that the possible Fe/Ru-site disorder further extends to create local chemical inhomogeneity, affecting the high temperature magnetism of this material. There is a gradual decrease of 57Fe Mössbauer spectral intensity with decreasing temperature (below 100 K), which reveals that there is a large spread in the magnetic ordering temperatures, corresponding to many spatially inhomogeneous regions. However, finally at about 25 K, the whole compound is found to take up a global glass-like magnetic ordering.

cond-mat.mtrl-sci↗