N. Zarkevich - Academia.edu (original) (raw)

Papers by N. Zarkevich

Cornell University - arXiv, Mar 8, 2018

Polymorphic solids of the same chemical composition can have different atomic structures; in each... more Polymorphic solids of the same chemical composition can have different atomic structures; in each polymorph atoms vibrate around a local potential energy minimum (LPEM). If transformations to other structures have sufficiently high enthalpy barriers, then each polymorph is either stable or metastable; it is stationary and does not spontaneously change with time. But what happens, if those barriers are low? As examples, we consider NiTi shape memory alloy exhibiting a large elastocaloric effect, and selected elemental solids. We suggest a model for dynamically polymorphic solids, where multiple LPEMs are visited by ergodic motion of a single atom. We predict that upon cooling a dynamically polymorphic phase should undergo a symmetry-breaking first-order phase transition, accompanied by a finite change of the lattice entropy. We discuss 3 methods used to calculate phonons in solids with non-harmonic dimpled atomic potentials, and compare theoretical predictions to experiment.

NiTi is the most used shape-memory alloy, nonetheless, a lack of understanding remains regarding ... more NiTi is the most used shape-memory alloy, nonetheless, a lack of understanding remains regarding the associated structures and transitions, including their barriers. Using a generalized solid-state nudge elastic band (GSSNEB) method implemented via density-functional theory, we detail the structural transformations in NiTi relevant to shape memory: those between body-centered orthorhombic (BCO) groundstate and a newly identified stable austenite (“glassy” B2-like) structure, including energy barriers (hysteresis) and intermediate structures (observed as a kinetically limited R-phase), and between martensite variants (BCO orientations). All results are in good agreement with available experiment. We contrast the austenite results to those from the often-assumed, but unstable B2. These highand low-temperature structures and structural transformations provide much needed atomic-scale detail for transitions responsible for NiTi shape-memory effects.

Journal of Applied Physics, 2021

Proper coupling between structural and magnetic transitions is critical for the emergence and con... more Proper coupling between structural and magnetic transitions is critical for the emergence and control of magnetocaloric effects in solids. We examine the influence of minor substitutional doping (replacing Mn by Cr and Al by Sn) and interstitial doping with B on the magnetic, structural, and magnetocaloric properties of recently discovered Mn0.5Fe0.5NiSi0.94Al0.06 alloy exhibiting a giant magnetocaloric effect near room temperature. We demonstrate that magnetocaloric properties of the base compound can be controlled and, in some cases, improved by chemical substitutions. First-principles computations elucidate how small changes in the composition affect properties in this family of compounds and, thus, provide useful guidance for the selection of suitable doping elements for such materials. The magnetic-field-induced entropy change measured for Mn0.5Fe0.5NiSi0.94Al0.06B0.005 is −22 J/kg K near room temperature for the applied magnetic field of 2 T, and it is among the highest known values for this class of materials.

Physical Review B, 2020

Magnetic first-order phase transitions are key for the emergence of functionalities of fundamenta... more Magnetic first-order phase transitions are key for the emergence of functionalities of fundamental and applied significance, including magnetic shape memory as well as magnetostrictive and magnetocaloric effects. Such transitions are usually associated with thermomagnetic hysteresis. We report the observation of a firstorder transition in Pr 2 In from a paramagnetic to a ferromagnetic state at T C = 57 K without a detectable thermomagnetic hysteresis, which is also accompanied by a large magnetocaloric effect. The peculiar electronic structure of Pr 2 In exhibiting a large density of states near the Fermi energy explains the highly responsive magnetic behavior of the material. The magnetic properties of Pr 2 In are reported, including observation of another (second-order) magnetic transition at 35 K.

Acta Materialia, 2018

A first-order magnetoelastic transition (FOMT) is found near the triple point between ferromagnet... more A first-order magnetoelastic transition (FOMT) is found near the triple point between ferromagnetic, antiferromagnetic and paramagnetic phases in the magneto-chemical phase diagram of (Hf1-xNbx)Fe2 Laves phase system. We show that bringing different magnetic states to the edge of stability, both as a function of the chemical composition and under the influence of external stimuli, such as temperature, pressure and magnetic field, is essential to obtain and control FOMTs. Temperature dependent X-ray diffraction experiments reveal a discontinuity in the lattice parameter a and the unit cell volume without the change in the crystal symmetry at the FOMT. Under applied pressure, the transition temperature drastically shifts downward at a remarkable rate of −122 K/GPa. It is this first-order magnetic transition that leads to a negative thermal expansion (NTE) with average ΔV/(VΔT) ≈ −15 × 10−6 K−1 observed over a 90 K broad temperature range, which is uncommon for magnetoelastic NTE materials. Density functional theory calculations and microstructural analyses demonstrate that the unusual broadness of the FOMT originates from phase separation between ferro-and antiferromagnetic phases, which in turn is rooted in partial segregation of Hf and Nb and a peculiar microstructure. This new understanding of the composition-structure-property relationships in transition metal based Laves phases is an essential step toward a better control and more precise tailoring of rich functionalities in this group of material.

Journal of Physics D: Applied Physics, 2017

The high-throughput search paradigm adopted by the newly established caloric materials consortium... more The high-throughput search paradigm adopted by the newly established caloric materials consortium-CaloriCool®-with the goal to substantially accelerate discovery and design of novel caloric materials is briefly discussed. We begin with describing material selection criteria based on known properties, which are then followed by heuristic fast estimates, ab-initio calculations and measurements, all of which has been implemented in a set of automated computational tools. We also demonstrate how theoretical and computational methods serve as a guide for experimental efforts by considering a representative example from the field of magnetocaloric materials Disciplines

Physical Review Letters, 2014

NiTi is the most used shape-memory alloy, nonetheless, a lack of understanding remains regarding ... more NiTi is the most used shape-memory alloy, nonetheless, a lack of understanding remains regarding the associated structures and transitions, including their barriers. Using a generalized solid-state nudge elastic band (GSSNEB) method implemented via density-functional theory, we detail the structural transformations in NiTi relevant to shape memory: those between body-centered orthorhombic (BCO) groundstate and a newly identified stable austenite ("glassy" B2-like) structure, including energy barriers (hysteresis) and intermediate structures (observed as a kinetically limited R-phase), and between martensite variants (BCO orientations). All results are in good agreement with available experiment. We contrast the austenite results to those from the often-assumed, but unstable B2. These high-and low-temperature structures and structural transformations provide much needed atomic-scale detail for transitions responsible for NiTi shape-memory effects.

The Journal of Chemical Physics, 2015

We extend the solid-state nudged elastic band method to handle a non-conserved order parameter-in... more We extend the solid-state nudged elastic band method to handle a non-conserved order parameter-in particular, magnetization, that couples to volume and leads to many observed effects in magnetic systems. We apply this formalism to the well-studied magneto-volume collapse during the pressure-induced transformation in iron-from ferromagnetic body-centered cubic (bcc) austenite to hexagonal close-packed (hcp) martensite. We find a bcc-hcp equilibrium coexistence pressure of 8.4 GPa, with the transition-state enthalpy of 156 meV/Fe at this pressure. A discontinuity in magnetization and coherent stress occurs at the transition state, which has a form of a cusp on the potential-energy surface (yet all the atomic and cell degrees of freedom are continuous); the calculated pressure jump of 25 GPa is related to the observed 25 GPa spread in measured coexistence pressures arising from martensitic and coherency stresses in samples. Our results agree with experiments, but necessarily differ from those arising from drag and restricted parametrization methods having improperly constrained or uncontrolled degrees of freedom.

, OH 45221-Over the last decade many body theory and electronic structure calculations have come ... more , OH 45221-Over the last decade many body theory and electronic structure calculations have come together within the "LDA+DMFT" approach in which dynamical mean field theory (DMFT) provides a frequency dependent self-energy Σ(ω) for electronic structure calculation within the local density approximation (LDA). Here we describe initial results with a new approach which uses the determinant Quantum Monte Carlo method to supply the self energy. This technique has the advantage of providing a momentum dependent Σ(k, ω). However, the fermion sign problem can limit the ability to access the ground state value of the self energy. We present tests of the approach on a model of cuprate superconductors.

A ``toolkit'' for the simulation of alloy thermodynamics has been developed by integratin... more A ``toolkit'' for the simulation of alloy thermodynamics has been developed by integrating first-principles, electronic-structure calculations and the cluster expansion (CE) with Monte Carlo (MC) methods. This Thermal Toolkit (TTK) is aimed at producing reliable thermodynamics of alloys with limited input from the user. Given an alloy, TTK first generates a comprehensive set of structures, automatically submits an electronic-structure calculation to determine the structural energies, stores the structure and its energy in a database, then constructs the CE via the structural inversion method that conforms to a set of mathematical conditions to produce an optimal truncated cluster expansion. Using this optimal CE, a MC code (included in TTK) can be used to calculate thermodynamic properties, such as structural phase diagram (T vs c). We present here example application and functionality of TTK on binary and ternary alloy. The ``Structural Database'' http://data.mse.ui...

Surface Science, 2005

We show that steric repulsion energies between halogen dimers on a passivated Si(001) surface sca... more We show that steric repulsion energies between halogen dimers on a passivated Si(001) surface scale with square of the principle quantum number (or period) n of the halogen, and arise principally from bonding with Si substrate. We exemplify the scaling from previously calculated steric interactions of F, Cl, and Br, predict the interactions for I and At, and then verify the prediction by direct density-functional calculations. From the energetics, we explain the patterning of the halogen-terminated Si(001), for a better understanding of the halogen-roughening process, and predict a crossover to a new vacancy-line defect for large halogens.

Physical Review B, 2014

We propose a reliable and efficient computational method for predicting elastic and thermal expan... more We propose a reliable and efficient computational method for predicting elastic and thermal expansion properties in crystals, particularly complex anisotropic molecular solids, and we apply it to the room-temperature orthorhombic P nma phase of LiBH 4. Using density-functional theory, we find thermal expansion coefficients at finite temperature, and we confirm them by temperature-dependent, in situ x-ray diffraction measurements. We also consider the effects of volume and pressure, as well as energy barriers for BH 4 − rotations and collective motions. Our combined study validates the theory and provides a better understanding of the structural behavior of LiBH 4 .

Journal of Physics: Condensed Matter, 2014

MnBi has attracted much attention in recent years due to its potential as a rare-earth-free perma... more MnBi has attracted much attention in recent years due to its potential as a rare-earth-free permanent magnet material. It is unique because its coercivity increases with increasing temperature, which makes it a good hard phase material for exchange coupling nanocomposite magnets. MnBi phase is difficult to obtain, partly because the reaction between Mn and Bi is peritectic, and partly because Mn reacts readily with oxygen. MnO formation is irreversible and harmful to magnet performance. In this paper, we report our efforts toward developing MnBi permanent magnets. To date, high purity MnBi (>90%) can be routinely produced in large quantities. The produced powder exhibits 74.6 emu g −1 saturation magnetization at room temperature with 9 T applied field. After proper alignment, the maximum energy product (BH) max of the powder reached 11.9 MGOe, and that of the sintered bulk magnet reached 7.8 MGOe at room temperature. A comprehensive study of thermal stability shows that MnBi powder is stable up to 473 K in air.

We study the short- and long- range chemical ordering in hcp bulk Ag_2Al using the Monte Carlo me... more We study the short- and long- range chemical ordering in hcp bulk Ag_2Al using the Monte Carlo method based on a Hamiltonian constructed via structural formation energies from ab initio electronic-structure calculations. We find that the ground-state structure and thermodynamic properties of bulk Ag_2Al is that determined from the X-ray experimental data. We also address the influence of the interface,

Acta Materialia, 2002

We study the short-and long-range chemical ordering in hcp bulk Ag 2 Al using the Monte Carlo met... more We study the short-and long-range chemical ordering in hcp bulk Ag 2 Al using the Monte Carlo method based on a Hamiltonian constructed via structural formation energies from ab initio electronic-structure calculations. We find that the ground-state structure of bulk Ag 2 Al is that proposed from X-ray data, but there are several competing metastable hcp structures. Our results provide the structural and thermodynamic properties of Ag 2 Al, with good agreement to experimental short-range order data (after reprocessing our data according to experimental procedure). We also discuss the influence of the Al:Ag 2 Al interface, coherency strain, and off-stoichiometric disorder on the structure of metastable Ag 2 Al γЈ nano-precipitates in an fcc Al matrix. We show that γЈ precipitates are off-stoichiometric and we provide a new structure that reproduces the observed transmission electron microscopy image while allowing for a distribution of Ag concentrations amongst the precipitates.

Physical Review B, 2016

As titanium is a highly utilized metal for structural light-weighting, its phases, transformation... more As titanium is a highly utilized metal for structural light-weighting, its phases, transformation pathways (transition states), and structures have scientific and industrial importance. Using a proper solid-state nudged elastic band (SS-NEB) method employing two climbing images (C2-NEB) combined with density-function theory (DFT+U) methods for accurate energetics, we detail the pressure-induced α (ductile) to ω (brittle) transformation at the coexistence pressure. We find two transition states along the minimal-enthalpy path (MEP) and discover a metastable body-centered orthorhombic (bco) structure, with stable phonons, a lower density than the endpoint phases, and decreasing stability with increasing pressure.

Physical Review B, 2015

The coexistence pressure of two phases is a well-defined point at fixed temperature. In experimen... more The coexistence pressure of two phases is a well-defined point at fixed temperature. In experiment, however, due to non-hydrostatic stresses and a stress-dependent potential energy barrier, different measurements yield different ranges of pressure with a hysteresis. Accounting for these effects, we propose an inequality for comparison of the theoretical value to a plurality of measured intervals. We revisit decades of pressure experiments on the bcc ↔ hcp transformations in iron, which are sensitive to non-hydrostatic conditions and sample size. From electronic-structure calculations, we find a bcc ↔ hcp coexistence pressure of 8.4 GPa. We construct the equation of state for competing phases under hydrostatic pressure, compare to experiments and other calculations, and address the observed pressure hysteresis and range of onset pressures of the nucleating phase.

APL Mater., 2014

Low-temperature MnBi (hexagonal NiAs phase) exhibits anomalies in the lattice constants (a, c) an... more Low-temperature MnBi (hexagonal NiAs phase) exhibits anomalies in the lattice constants (a, c) and bulk elastic modulus (B) below 100 K, spin reorientation and magnetic susceptibility maximum near 90 K, and, importantly for high-temperature magnetic applications, an increasing coercivity (unique to MnBi) above 180 K. We calculate the total energy and magneto-anisotropy energy (MAE) versus (a, c) using DFT+U methods. We reproduce and explain all the above anomalies. We predict that coercivity and MAE increase due to increasing a, suggesting means to improve MnBi permanent magnets.

Journal of Applied Physics, 2014

MnBi attracts great attention in recent years for its great potential as permanent magnet materia... more MnBi attracts great attention in recent years for its great potential as permanent magnet materials. MnBi phase is difficult to obtain because of the rather drastic peritectic reaction between Mn and Bi. In this paper, we report our effort on synthesizing high purity MnBi compound using conventional powder metallurgical approaches. Neutron diffraction was carried out to investigate the crystal and nuclear structure of the obtained powder. The result shows that the purity of the obtained powder is about 91 wt. % at 300 K, and the magnetic moment of the Mn atom in MnBi lattice is 4.424 and 4.013 l B at 50 K and 300 K, respectively. V

Cornell University - arXiv, Mar 8, 2018

Polymorphic solids of the same chemical composition can have different atomic structures; in each... more Polymorphic solids of the same chemical composition can have different atomic structures; in each polymorph atoms vibrate around a local potential energy minimum (LPEM). If transformations to other structures have sufficiently high enthalpy barriers, then each polymorph is either stable or metastable; it is stationary and does not spontaneously change with time. But what happens, if those barriers are low? As examples, we consider NiTi shape memory alloy exhibiting a large elastocaloric effect, and selected elemental solids. We suggest a model for dynamically polymorphic solids, where multiple LPEMs are visited by ergodic motion of a single atom. We predict that upon cooling a dynamically polymorphic phase should undergo a symmetry-breaking first-order phase transition, accompanied by a finite change of the lattice entropy. We discuss 3 methods used to calculate phonons in solids with non-harmonic dimpled atomic potentials, and compare theoretical predictions to experiment.

NiTi is the most used shape-memory alloy, nonetheless, a lack of understanding remains regarding ... more NiTi is the most used shape-memory alloy, nonetheless, a lack of understanding remains regarding the associated structures and transitions, including their barriers. Using a generalized solid-state nudge elastic band (GSSNEB) method implemented via density-functional theory, we detail the structural transformations in NiTi relevant to shape memory: those between body-centered orthorhombic (BCO) groundstate and a newly identified stable austenite (“glassy” B2-like) structure, including energy barriers (hysteresis) and intermediate structures (observed as a kinetically limited R-phase), and between martensite variants (BCO orientations). All results are in good agreement with available experiment. We contrast the austenite results to those from the often-assumed, but unstable B2. These highand low-temperature structures and structural transformations provide much needed atomic-scale detail for transitions responsible for NiTi shape-memory effects.

Journal of Applied Physics, 2021

Proper coupling between structural and magnetic transitions is critical for the emergence and con... more Proper coupling between structural and magnetic transitions is critical for the emergence and control of magnetocaloric effects in solids. We examine the influence of minor substitutional doping (replacing Mn by Cr and Al by Sn) and interstitial doping with B on the magnetic, structural, and magnetocaloric properties of recently discovered Mn0.5Fe0.5NiSi0.94Al0.06 alloy exhibiting a giant magnetocaloric effect near room temperature. We demonstrate that magnetocaloric properties of the base compound can be controlled and, in some cases, improved by chemical substitutions. First-principles computations elucidate how small changes in the composition affect properties in this family of compounds and, thus, provide useful guidance for the selection of suitable doping elements for such materials. The magnetic-field-induced entropy change measured for Mn0.5Fe0.5NiSi0.94Al0.06B0.005 is −22 J/kg K near room temperature for the applied magnetic field of 2 T, and it is among the highest known values for this class of materials.

Physical Review B, 2020

Magnetic first-order phase transitions are key for the emergence of functionalities of fundamenta... more Magnetic first-order phase transitions are key for the emergence of functionalities of fundamental and applied significance, including magnetic shape memory as well as magnetostrictive and magnetocaloric effects. Such transitions are usually associated with thermomagnetic hysteresis. We report the observation of a firstorder transition in Pr 2 In from a paramagnetic to a ferromagnetic state at T C = 57 K without a detectable thermomagnetic hysteresis, which is also accompanied by a large magnetocaloric effect. The peculiar electronic structure of Pr 2 In exhibiting a large density of states near the Fermi energy explains the highly responsive magnetic behavior of the material. The magnetic properties of Pr 2 In are reported, including observation of another (second-order) magnetic transition at 35 K.

Acta Materialia, 2018

A first-order magnetoelastic transition (FOMT) is found near the triple point between ferromagnet... more A first-order magnetoelastic transition (FOMT) is found near the triple point between ferromagnetic, antiferromagnetic and paramagnetic phases in the magneto-chemical phase diagram of (Hf1-xNbx)Fe2 Laves phase system. We show that bringing different magnetic states to the edge of stability, both as a function of the chemical composition and under the influence of external stimuli, such as temperature, pressure and magnetic field, is essential to obtain and control FOMTs. Temperature dependent X-ray diffraction experiments reveal a discontinuity in the lattice parameter a and the unit cell volume without the change in the crystal symmetry at the FOMT. Under applied pressure, the transition temperature drastically shifts downward at a remarkable rate of −122 K/GPa. It is this first-order magnetic transition that leads to a negative thermal expansion (NTE) with average ΔV/(VΔT) ≈ −15 × 10−6 K−1 observed over a 90 K broad temperature range, which is uncommon for magnetoelastic NTE materials. Density functional theory calculations and microstructural analyses demonstrate that the unusual broadness of the FOMT originates from phase separation between ferro-and antiferromagnetic phases, which in turn is rooted in partial segregation of Hf and Nb and a peculiar microstructure. This new understanding of the composition-structure-property relationships in transition metal based Laves phases is an essential step toward a better control and more precise tailoring of rich functionalities in this group of material.

Journal of Physics D: Applied Physics, 2017

The high-throughput search paradigm adopted by the newly established caloric materials consortium... more The high-throughput search paradigm adopted by the newly established caloric materials consortium-CaloriCool®-with the goal to substantially accelerate discovery and design of novel caloric materials is briefly discussed. We begin with describing material selection criteria based on known properties, which are then followed by heuristic fast estimates, ab-initio calculations and measurements, all of which has been implemented in a set of automated computational tools. We also demonstrate how theoretical and computational methods serve as a guide for experimental efforts by considering a representative example from the field of magnetocaloric materials Disciplines

Physical Review Letters, 2014

NiTi is the most used shape-memory alloy, nonetheless, a lack of understanding remains regarding ... more NiTi is the most used shape-memory alloy, nonetheless, a lack of understanding remains regarding the associated structures and transitions, including their barriers. Using a generalized solid-state nudge elastic band (GSSNEB) method implemented via density-functional theory, we detail the structural transformations in NiTi relevant to shape memory: those between body-centered orthorhombic (BCO) groundstate and a newly identified stable austenite ("glassy" B2-like) structure, including energy barriers (hysteresis) and intermediate structures (observed as a kinetically limited R-phase), and between martensite variants (BCO orientations). All results are in good agreement with available experiment. We contrast the austenite results to those from the often-assumed, but unstable B2. These high-and low-temperature structures and structural transformations provide much needed atomic-scale detail for transitions responsible for NiTi shape-memory effects.

The Journal of Chemical Physics, 2015

We extend the solid-state nudged elastic band method to handle a non-conserved order parameter-in... more We extend the solid-state nudged elastic band method to handle a non-conserved order parameter-in particular, magnetization, that couples to volume and leads to many observed effects in magnetic systems. We apply this formalism to the well-studied magneto-volume collapse during the pressure-induced transformation in iron-from ferromagnetic body-centered cubic (bcc) austenite to hexagonal close-packed (hcp) martensite. We find a bcc-hcp equilibrium coexistence pressure of 8.4 GPa, with the transition-state enthalpy of 156 meV/Fe at this pressure. A discontinuity in magnetization and coherent stress occurs at the transition state, which has a form of a cusp on the potential-energy surface (yet all the atomic and cell degrees of freedom are continuous); the calculated pressure jump of 25 GPa is related to the observed 25 GPa spread in measured coexistence pressures arising from martensitic and coherency stresses in samples. Our results agree with experiments, but necessarily differ from those arising from drag and restricted parametrization methods having improperly constrained or uncontrolled degrees of freedom.

, OH 45221-Over the last decade many body theory and electronic structure calculations have come ... more , OH 45221-Over the last decade many body theory and electronic structure calculations have come together within the "LDA+DMFT" approach in which dynamical mean field theory (DMFT) provides a frequency dependent self-energy Σ(ω) for electronic structure calculation within the local density approximation (LDA). Here we describe initial results with a new approach which uses the determinant Quantum Monte Carlo method to supply the self energy. This technique has the advantage of providing a momentum dependent Σ(k, ω). However, the fermion sign problem can limit the ability to access the ground state value of the self energy. We present tests of the approach on a model of cuprate superconductors.

A ``toolkit'' for the simulation of alloy thermodynamics has been developed by integratin... more A ``toolkit'' for the simulation of alloy thermodynamics has been developed by integrating first-principles, electronic-structure calculations and the cluster expansion (CE) with Monte Carlo (MC) methods. This Thermal Toolkit (TTK) is aimed at producing reliable thermodynamics of alloys with limited input from the user. Given an alloy, TTK first generates a comprehensive set of structures, automatically submits an electronic-structure calculation to determine the structural energies, stores the structure and its energy in a database, then constructs the CE via the structural inversion method that conforms to a set of mathematical conditions to produce an optimal truncated cluster expansion. Using this optimal CE, a MC code (included in TTK) can be used to calculate thermodynamic properties, such as structural phase diagram (T vs c). We present here example application and functionality of TTK on binary and ternary alloy. The ``Structural Database'' http://data.mse.ui...

Surface Science, 2005

We show that steric repulsion energies between halogen dimers on a passivated Si(001) surface sca... more We show that steric repulsion energies between halogen dimers on a passivated Si(001) surface scale with square of the principle quantum number (or period) n of the halogen, and arise principally from bonding with Si substrate. We exemplify the scaling from previously calculated steric interactions of F, Cl, and Br, predict the interactions for I and At, and then verify the prediction by direct density-functional calculations. From the energetics, we explain the patterning of the halogen-terminated Si(001), for a better understanding of the halogen-roughening process, and predict a crossover to a new vacancy-line defect for large halogens.

Physical Review B, 2014

We propose a reliable and efficient computational method for predicting elastic and thermal expan... more We propose a reliable and efficient computational method for predicting elastic and thermal expansion properties in crystals, particularly complex anisotropic molecular solids, and we apply it to the room-temperature orthorhombic P nma phase of LiBH 4. Using density-functional theory, we find thermal expansion coefficients at finite temperature, and we confirm them by temperature-dependent, in situ x-ray diffraction measurements. We also consider the effects of volume and pressure, as well as energy barriers for BH 4 − rotations and collective motions. Our combined study validates the theory and provides a better understanding of the structural behavior of LiBH 4 .

Journal of Physics: Condensed Matter, 2014

MnBi has attracted much attention in recent years due to its potential as a rare-earth-free perma... more MnBi has attracted much attention in recent years due to its potential as a rare-earth-free permanent magnet material. It is unique because its coercivity increases with increasing temperature, which makes it a good hard phase material for exchange coupling nanocomposite magnets. MnBi phase is difficult to obtain, partly because the reaction between Mn and Bi is peritectic, and partly because Mn reacts readily with oxygen. MnO formation is irreversible and harmful to magnet performance. In this paper, we report our efforts toward developing MnBi permanent magnets. To date, high purity MnBi (>90%) can be routinely produced in large quantities. The produced powder exhibits 74.6 emu g −1 saturation magnetization at room temperature with 9 T applied field. After proper alignment, the maximum energy product (BH) max of the powder reached 11.9 MGOe, and that of the sintered bulk magnet reached 7.8 MGOe at room temperature. A comprehensive study of thermal stability shows that MnBi powder is stable up to 473 K in air.

We study the short- and long- range chemical ordering in hcp bulk Ag_2Al using the Monte Carlo me... more We study the short- and long- range chemical ordering in hcp bulk Ag_2Al using the Monte Carlo method based on a Hamiltonian constructed via structural formation energies from ab initio electronic-structure calculations. We find that the ground-state structure and thermodynamic properties of bulk Ag_2Al is that determined from the X-ray experimental data. We also address the influence of the interface,

Acta Materialia, 2002

We study the short-and long-range chemical ordering in hcp bulk Ag 2 Al using the Monte Carlo met... more We study the short-and long-range chemical ordering in hcp bulk Ag 2 Al using the Monte Carlo method based on a Hamiltonian constructed via structural formation energies from ab initio electronic-structure calculations. We find that the ground-state structure of bulk Ag 2 Al is that proposed from X-ray data, but there are several competing metastable hcp structures. Our results provide the structural and thermodynamic properties of Ag 2 Al, with good agreement to experimental short-range order data (after reprocessing our data according to experimental procedure). We also discuss the influence of the Al:Ag 2 Al interface, coherency strain, and off-stoichiometric disorder on the structure of metastable Ag 2 Al γЈ nano-precipitates in an fcc Al matrix. We show that γЈ precipitates are off-stoichiometric and we provide a new structure that reproduces the observed transmission electron microscopy image while allowing for a distribution of Ag concentrations amongst the precipitates.

Physical Review B, 2016

As titanium is a highly utilized metal for structural light-weighting, its phases, transformation... more As titanium is a highly utilized metal for structural light-weighting, its phases, transformation pathways (transition states), and structures have scientific and industrial importance. Using a proper solid-state nudged elastic band (SS-NEB) method employing two climbing images (C2-NEB) combined with density-function theory (DFT+U) methods for accurate energetics, we detail the pressure-induced α (ductile) to ω (brittle) transformation at the coexistence pressure. We find two transition states along the minimal-enthalpy path (MEP) and discover a metastable body-centered orthorhombic (bco) structure, with stable phonons, a lower density than the endpoint phases, and decreasing stability with increasing pressure.

Physical Review B, 2015

The coexistence pressure of two phases is a well-defined point at fixed temperature. In experimen... more The coexistence pressure of two phases is a well-defined point at fixed temperature. In experiment, however, due to non-hydrostatic stresses and a stress-dependent potential energy barrier, different measurements yield different ranges of pressure with a hysteresis. Accounting for these effects, we propose an inequality for comparison of the theoretical value to a plurality of measured intervals. We revisit decades of pressure experiments on the bcc ↔ hcp transformations in iron, which are sensitive to non-hydrostatic conditions and sample size. From electronic-structure calculations, we find a bcc ↔ hcp coexistence pressure of 8.4 GPa. We construct the equation of state for competing phases under hydrostatic pressure, compare to experiments and other calculations, and address the observed pressure hysteresis and range of onset pressures of the nucleating phase.

APL Mater., 2014

Low-temperature MnBi (hexagonal NiAs phase) exhibits anomalies in the lattice constants (a, c) an... more Low-temperature MnBi (hexagonal NiAs phase) exhibits anomalies in the lattice constants (a, c) and bulk elastic modulus (B) below 100 K, spin reorientation and magnetic susceptibility maximum near 90 K, and, importantly for high-temperature magnetic applications, an increasing coercivity (unique to MnBi) above 180 K. We calculate the total energy and magneto-anisotropy energy (MAE) versus (a, c) using DFT+U methods. We reproduce and explain all the above anomalies. We predict that coercivity and MAE increase due to increasing a, suggesting means to improve MnBi permanent magnets.

Journal of Applied Physics, 2014

MnBi attracts great attention in recent years for its great potential as permanent magnet materia... more MnBi attracts great attention in recent years for its great potential as permanent magnet materials. MnBi phase is difficult to obtain because of the rather drastic peritectic reaction between Mn and Bi. In this paper, we report our effort on synthesizing high purity MnBi compound using conventional powder metallurgical approaches. Neutron diffraction was carried out to investigate the crystal and nuclear structure of the obtained powder. The result shows that the purity of the obtained powder is about 91 wt. % at 300 K, and the magnetic moment of the Mn atom in MnBi lattice is 4.424 and 4.013 l B at 50 K and 300 K, respectively. V