Publications by authors named "Igor A Abrikosov"

30 Publications

  • Page 1 of 1

Realization of an Ideal Cairo Tessellation in Nickel Diazenide NiN: High-Pressure Route to Pentagonal 2D Materials.

ACS Nano 2021 Aug 6. Epub 2021 Aug 6.

The Earth and Planets Laboratory, Carnegie Institution for Science, Washington, D.C. 20015, United States.

Most of the studied two-dimensional (2D) materials are based on highly symmetric hexagonal structural motifs. In contrast, lower-symmetry structures may have exciting anisotropic properties leading to various applications in nanoelectronics. In this work we report the synthesis of nickel diazenide NiN which possesses atomic-thick layers comprised of NiN pentagons forming Cairo-type tessellation. The layers of NiN are weakly bonded with the calculated exfoliation energy of 0.72 J/m, which is just slightly larger than that of graphene. The compound crystallizes in the space group of the ideal Cairo tiling (4/) and possesses significant anisotropy of elastic properties. The single-layer NiN is a direct-band-gap semiconductor, while the bulk material is metallic. This indicates the promise of NiN to be a precursor of a pentagonal 2D material with a tunable direct band gap.
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http://dx.doi.org/10.1021/acsnano.1c04325DOI Listing
August 2021

High-Pressure Synthesis of Dirac Materials: Layered van der Waals Bonded BeN_{4} Polymorph.

Phys Rev Lett 2021 Apr;126(17):175501

Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-58183 Linköping, Sweden.

High-pressure chemistry is known to inspire the creation of unexpected new classes of compounds with exceptional properties. Here, we employ the laser-heated diamond anvil cell technique for synthesis of a Dirac material BeN_{4}. A triclinic phase of beryllium tetranitride tr-BeN_{4} was synthesized from elements at ∼85  GPa. Upon decompression to ambient conditions, it transforms into a compound with atomic-thick BeN_{4} layers interconnected via weak van der Waals bonds and consisting of polyacetylene-like nitrogen chains with conjugated π systems and Be atoms in square-planar coordination. Theoretical calculations for a single BeN_{4} layer show that its electronic lattice is described by a slightly distorted honeycomb structure reminiscent of the graphene lattice and the presence of Dirac points in the electronic band structure at the Fermi level. The BeN_{4} layer, i.e., beryllonitrene, represents a qualitatively new class of 2D materials that can be built of a metal atom and polymeric nitrogen chains and host anisotropic Dirac fermions.
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http://dx.doi.org/10.1103/PhysRevLett.126.175501DOI Listing
April 2021

Stabilization of Polynitrogen Anions in Tantalum-Nitrogen Compounds at High Pressure.

Angew Chem Int Ed Engl 2021 Apr 10;60(16):9003-9008. Epub 2021 Mar 10.

The Earth and Planets Laboratory, Carnegie Institution for Science, Washington, DC, 20015, USA.

The synthesis of polynitrogen compounds is of great importance due to their potential as high-energy-density materials (HEDM), but because of the intrinsic instability of these compounds, their synthesis and stabilization is a fundamental challenge. Polymeric nitrogen units which may be stabilized in compounds with metals at high pressure are now restricted to non-branched chains with an average N-N bond order of 1.25, limiting their HEDM performances. Herein, we demonstrate the synthesis of a novel polynitrogen compound TaN via a direct reaction between tantalum and nitrogen in a diamond anvil cell at circa 100 GPa. TaN is the first example of a material containing branched all-single-bonded nitrogen chains [N ] . Apart from that we discover two novel Ta-N compounds: TaN with finite N chains and the incommensurately modulated compound TaN , which is recoverable at ambient conditions.
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http://dx.doi.org/10.1002/anie.202100283DOI Listing
April 2021

Anharmonicity and Ultralow Thermal Conductivity in Lead-Free Halide Double Perovskites.

Phys Rev Lett 2020 Jul;125(4):045701

Theoretical Physics Division, Department of Physics, Chemistry and Biology (FIM), Linköping University, SE-581 83 Linköping, Sweden.

The lead-free halide double perovskite class of materials offers a promising venue for resolving issues related to toxicity of Pb and long-term stability of the lead-containing halide perovskites. We present a first-principles study of the lattice vibrations in Cs_{2}AgBiBr_{6}, the prototypical compound in this class and show that the lattice dynamics of Cs_{2}AgBiBr_{6} is highly anharmonic, largely in regards to tilting of AgBr_{6} and BiBr_{6} octahedra. Using an energy- and temperature-dependent phonon spectral function, we then show how the experimentally observed cubic-to-tetragonal phase transformation is caused by the collapse of a soft phonon branch. We finally reveal that the softness and anharmonicity of Cs_{2}AgBiBr_{6} yield an ultralow thermal conductivity, unexpected of high-symmetry cubic structures.
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http://dx.doi.org/10.1103/PhysRevLett.125.045701DOI Listing
July 2020

Lead-Free Halide Double Perovskite Cs AgBiBr with Decreased Band Gap.

Angew Chem Int Ed Engl 2020 Aug 22;59(35):15191-15194. Epub 2020 Jun 22.

Department of Physics, Chemistry, and Biology (IFM), Linköping University, 58183, Linköping, Sweden.

Environmentally friendly halide double perovskites with improved stability are regarded as a promising alternative to lead halide perovskites. The benchmark double perovskite, Cs AgBiBr , shows attractive optical and electronic features, making it promising for high-efficiency optoelectronic devices. However, the large band gap limits its further applications, especially for photovoltaics. Herein, we develop a novel crystal-engineering strategy to significantly decrease the band gap by approximately 0.26 eV, reaching the smallest reported band gap of 1.72 eV for Cs AgBiBr under ambient conditions. The band-gap narrowing is confirmed by both absorption and photoluminescence measurements. Our first-principles calculations indicate that enhanced Ag-Bi disorder has a large impact on the band structure and decreases the band gap, providing a possible explanation of the observed band-gap narrowing effect. This work provides new insights for achieving lead-free double perovskites with suitable band gaps for optoelectronic applications.
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http://dx.doi.org/10.1002/anie.202005568DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7496408PMC
August 2020

High-Pressure Synthesis of Metal-Inorganic Frameworks Hf N ⋅N , WN ⋅N , and Os N ⋅3 N with Polymeric Nitrogen Linkers.

Angew Chem Int Ed Engl 2020 Jun 8;59(26):10321-10326. Epub 2020 May 8.

Bayerisches Geoinstitut, University of Bayreuth, Universitätstrasse 30, 95440, Bayreuth, Germany.

Polynitrides are intrinsically thermodynamically unstable at ambient conditions and require peculiar synthetic approaches. Now, a one-step synthesis of metal-inorganic frameworks Hf N ⋅N , WN ⋅N , and Os N ⋅3 N via direct reactions between elements in a diamond anvil cell at pressures exceeding 100 GPa is reported. The porous frameworks (Hf N , WN , and Os N ) are built from transition-metal atoms linked either by polymeric polydiazenediyl (polyacetylene-like) nitrogen chains or through dinitrogen units. Triply bound dinitrogen molecules occupy channels of these frameworks. Owing to conjugated polydiazenediyl chains, these compounds exhibit metallic properties. The high-pressure reaction between Hf and N also leads to a non-centrosymmetric polynitride Hf N that features double-helix catena-poly[tetraz-1-ene-1,4-diyl] nitrogen chains [-N-N-N=N-] .
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http://dx.doi.org/10.1002/anie.202002487DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7317814PMC
June 2020

Stabilization of point-defect spin qubits by quantum wells.

Nat Commun 2019 12 6;10(1):5607. Epub 2019 Dec 6.

Wigner Research Centre for Physics, PO Box 49, H-1525, Budapest, Hungary.

Defect-based quantum systems in wide bandgap semiconductors are strong candidates for scalable quantum-information technologies. However, these systems are often complicated by charge-state instabilities and interference by phonons, which can diminish spin-initialization fidelities and limit room-temperature operation. Here, we identify a pathway around these drawbacks by showing that an engineered quantum well can stabilize the charge state of a qubit. Using density-functional theory and experimental synchrotron X-ray diffraction studies, we construct a model for previously unattributed point defect centers in silicon carbide as a near-stacking fault axial divacancy and show how this model explains these defects' robustness against photoionization and room temperature stability. These results provide a materials-based solution to the optical instability of color centers in semiconductors, paving the way for the development of robust single-photon sources and spin qubits.
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http://dx.doi.org/10.1038/s41467-019-13495-6DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6898666PMC
December 2019

High-pressure synthesis of ultraincompressible hard rhenium nitride pernitride Re(N)(N) stable at ambient conditions.

Nat Commun 2019 Jul 5;10(1):2994. Epub 2019 Jul 5.

Bayerisches Geoinstitut, University of Bayreuth, Universitätstraβe 30, 95440, Bayreuth, Germany.

High-pressure synthesis in diamond anvil cells can yield unique compounds with advanced properties, but often they are either unrecoverable at ambient conditions or produced in quantity insufficient for properties characterization. Here we report the synthesis of metallic, ultraincompressible (K = 428(10) GPa), and very hard (nanoindentation hardness 36.7(8) GPa) rhenium nitride pernitride Re(N)(N). Unlike known transition metals pernitrides Re(N)(N) contains both pernitride N and discrete N anions, which explains its exceptional properties. Re(N)(N) can be obtained via a reaction between rhenium and nitrogen in a diamond anvil cell at pressures from 40 to 90 GPa and is recoverable at ambient conditions. We develop a route to scale up its synthesis through a reaction between rhenium and ammonium azide, NHN, in a large-volume press at 33 GPa. Although metallic bonding is typically seen incompatible with intrinsic hardness, Re(N)(N) turned to be at a threshold for superhard materials.
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http://dx.doi.org/10.1038/s41467-019-10995-3DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6611777PMC
July 2019

High Pressure Investigation of the S-N System up to the Megabar Range: Synthesis and Characterization of the SN Solid.

Inorg Chem 2019 Jul 25;58(14):9195-9204. Epub 2019 Jun 25.

Material Physics and Technology at Extreme Conditions, Laboratory of Crystallography , University of Bayreuth , 95440 Bayreuth , Germany.

Sulfur and nitrogen represent one of the most studied inorganic binary systems at ambient pressure on account of their large wealth of metastable exotic ring-like compounds. Under high pressure conditions, however, their behavior is unknown. Here, sulfur and nitrogen were compressed in a diamond anvil cell up to about 120 GPa and laser-heated at regular pressure intervals in an attempt to stabilize novel sulfur-nitrogen compounds. Above 64 GPa, an orthorhombic (space group ) SN compound was synthesized and characterized by single-crystal and powder X-ray diffraction as well as Raman spectroscopy. It is shown to adopt a CaCl-type structure-hence it is isostructural, isomassic, and isoelectronic to CaCl-type SiO-comprised of SN octahedra. Complementary theoretical calculations were performed to provide further insight into the physicochemical properties of SN, notably its equation of state, the bonding type between its constitutive elements, and its electronic density of states. This new solid is shown to be metastable down to about 20 GPa, after which it spontaneously decomposes into S and N. This investigation shows that despite the many metastable S-N compounds existing at ambient conditions, none of them are formed by pressure.
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http://dx.doi.org/10.1021/acs.inorgchem.9b00830DOI Listing
July 2019

Penta- and hexa-coordinated beryllium and phosphorus in high-pressure modifications of CaBePO.

Nat Commun 2019 Jun 26;10(1):2800. Epub 2019 Jun 26.

Bayerisches Geoinstitut, University of Bayreuth, 95440, Bayreuth, Germany.

Beryllium oxides have been extensively studied due to their unique chemical properties and important technological applications. Typically, in inorganic compounds beryllium is tetrahedrally coordinated by oxygen atoms. Herein based on results of in situ single crystal X-ray diffraction studies and ab initio calculations we report on the high-pressure behavior of CaBePO, to the best of our knowledge the first compound showing a step-wise transition of Be coordination from tetrahedral (4) to octahedral (6) through trigonal bipyramidal (5). It is remarkable that the same transformation route is observed for phosphorus. Our theoretical analysis suggests that the sequence of structural transitions of CaBePO is associated with the electronic transformation from predominantly molecular orbitals at low pressure to the state with overlapping electronic clouds of anions orbitals.
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http://dx.doi.org/10.1038/s41467-019-10589-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6594954PMC
June 2019

Phase stability and electronic structure of iridium metal at the megabar range.

Sci Rep 2019 Jun 20;9(1):8940. Epub 2019 Jun 20.

Departamento de Física Aplicada-ICMUV, Universitat de València, MALTA Consolider Team, Edificio de Investigación, C/Dr. Moliner 50, 46100 Burjassot, Valencia, Spain.

The 5d transition metals have attracted specific interest for high-pressure studies due to their extraordinary stability and intriguing electronic properties. In particular, iridium metal has been proposed to exhibit a recently discovered pressure-induced electronic transition, the so-called core-level crossing transition at the lowest pressure among all the 5d transition metals. Here, we report an experimental structural characterization of iridium by x-ray probes sensitive to both long- and short-range order in matter. Synchrotron-based powder x-ray diffraction results highlight a large stability range (up to 1.4 Mbar) of the low-pressure phase. The compressibility behaviour was characterized by an accurate determination of the pressure-volume equation of state, with a bulk modulus of 339(3) GPa and its derivative of 5.3(1). X-ray absorption spectroscopy, which probes the local structure and the empty density of electronic states above the Fermi level, was also utilized. The remarkable agreement observed between experimental and calculated spectra validates the reliability of theoretical predictions of the pressure dependence of the electronic structure of iridium in the studied interval of compressions.
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http://dx.doi.org/10.1038/s41598-019-45401-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6586894PMC
June 2019

High-Pressure Synthesis of a Nitrogen-Rich Inclusion Compound ReN ⋅x N with Conjugated Polymeric Nitrogen Chains.

Angew Chem Int Ed Engl 2018 Jul 19;57(29):9048-9053. Epub 2018 Jun 19.

Bayerisches Geoinstitute, University of Bayreuth, Universitätstrasse 30, 95440, Bayreuth, Germany.

A nitrogen-rich compound, ReN ⋅x N , was synthesized by a direct reaction between rhenium and nitrogen at high pressure and high temperature in a laser-heated diamond anvil cell. Single-crystal X-ray diffraction revealed that the crystal structure, which is based on the ReN framework, has rectangular-shaped channels that accommodate nitrogen molecules. Thus, despite a very high synthesis pressure, exceeding 100 GPa, ReN ⋅x N is an inclusion compound. The amount of trapped nitrogen (x) depends on the synthesis conditions. The polydiazenediyl chains [-N=N-] that constitute the framework have not been previously observed in any compound. Ab initio calculations on ReN ⋅x N provide strong support for the experimental results and conclusions.
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http://dx.doi.org/10.1002/anie.201805152DOI Listing
July 2018

Cerium oxide nanoparticles with antioxidant capabilities and gadolinium integration for MRI contrast enhancement.

Sci Rep 2018 05 3;8(1):6999. Epub 2018 May 3.

Division of Molecular Surface Physics and Nanoscience, Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83, Linköping, Sweden.

The chelating gadolinium-complex is routinely used as magnetic resonance imaging (MRI) -contrast enhancer. However, several safety issues have recently been reported by FDA and PRAC. There is an urgent need for the next generation of safer MRI-contrast enhancers, with improved local contrast and targeting capabilities. Cerium oxide nanoparticles (CeNPs) are designed with fractions of up to 50% gadolinium to utilize the superior MRI-contrast properties of gadolinium. CeNPs are well-tolerated in vivo and have redox properties making them suitable for biomedical applications, for example scavenging purposes on the tissue- and cellular level and during tumor treatment to reduce in vivo inflammatory processes. Our near edge X-ray absorption fine structure (NEXAFS) studies show that implementation of gadolinium changes the initial co-existence of oxidation states Ce and Ce of cerium, thereby affecting the scavenging properties of the nanoparticles. Based on ab initio electronic structure calculations, we describe the most prominent spectral features for the respective oxidation states. The as-prepared gadolinium-implemented CeNPs are 3-5 nm in size, have r-relaxivities between 7-13 mM s and show clear antioxidative properties, all of which means they are promising theranostic agents for use in future biomedical applications.
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http://dx.doi.org/10.1038/s41598-018-25390-zDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5934375PMC
May 2018

Thermal expansion of quaternary nitride coatings.

J Phys Condens Matter 2018 Apr 20;30(13):135901. Epub 2018 Feb 20.

Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden. Materials Modeling and Development Laboratory, National University of Science and Technology 'MISIS', 119049 Moscow, Russia.

The thermal expansion coefficient of technologically relevant multicomponent cubic nitride alloys are predicted using the Debye model with ab initio elastic constants calculated at 0 K and an isotropic approximation for the Grüneisen parameter. Our method is benchmarked against measured thermal expansion of TiN and TiAl N as well as against results of molecular dynamics simulations. We show that the thermal expansion coefficients of TiX Al N (X  =  Zr, Hf, Nb, V, Ta) solid solutions monotonously increase with the amount of alloying element X at all temperatures except for Zr and Hf, for which they instead decrease for [Formula: see text].
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http://dx.doi.org/10.1088/1361-648X/aab0b8DOI Listing
April 2018

Hybrid-DFT  +  V method for band structure calculation of semiconducting transition metal compounds: the case of cerium dioxide.

J Phys Condens Matter 2017 Nov;29(45):454002

Department of Physics, Chemistry and Biology, Linköping University, SE-581 83 Linköping, Sweden. Wigner Research Centre for Physics, Hungarian Academy of Sciences, PO Box 49, H-1525, Budapest, Hungary.

Hybrid functionals' non-local exchange-correlation potential contains a derivative discontinuity that improves on standard semi-local density functional theory (DFT) band gaps. Moreover, by careful parameterization, hybrid functionals can provide self-interaction reduced description of selected states. On the other hand, the uniform description of all the electronic states of a given system is a known drawback of these functionals that causes varying accuracy in the description of states with different degrees of localization. This limitation can be remedied by the orbital dependent exact exchange extension of hybrid functionals; the hybrid-DFT  +  V method (Ivády et al 2014 Phys. Rev. B 90 035146). Based on the analogy of quasi-particle equations and hybrid-DFT single particle equations, here we demonstrate that parameters of hybrid-DFT  +  V functional can be determined from approximate theoretical quasi-particle spectra without any fitting to experiment. The proposed method is illustrated on the charge self-consistent electronic structure calculation for cerium dioxide where itinerant valence states interact with well-localized 4f atomic like states, making this system challenging for conventional methods, either hybrid-DFT or LDA  +  U, and therefore allowing for a demonstration of the advantages of the proposed scheme.
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http://dx.doi.org/10.1088/1361-648X/aa8b93DOI Listing
November 2017

On the interaction of toxic Heavy Metals (Cd, Hg, Pb) with graphene quantum dots and infinite graphene.

Sci Rep 2017 06 21;7(1):3934. Epub 2017 Jun 21.

Department of Physics, Chemistry and Biology, Linköping University, SE-58183, Linköping, Sweden.

The promise of graphene and its derivatives as next generation sensors for real-time detection of toxic heavy metals (HM) requires a clear understanding of behavior of these metals on the graphene surface and response of the graphene to adsorption events. Our calculations herein were focused on the investigation of the interaction between three HMs, namely Cd, Hg and Pb, with graphene quantum dots (GQDs). We determine binding energies and heights of both neutral and charged HM ions on these GQDs. The results show that the adsorption energy of donor-like physisorbed neutral Pb atoms is larger than that of either Cd or Hg. In contrast to the donor-like behavior of elemental HMs, the chemisorbed charged HM species act as typical acceptors. The energy barriers to migration of the neutral adatoms on GQDs are also estimated. In addition, we show how the substitution of a carbon atom by a HM adatom changes the geometric structure of GQDs and hence their electronic and vibrational properties. UV-visible absorption spectra of HM-adsorbed GQDs vary with the size and shape of the GQD. Based on our results, we suggest a route towards the development of a graphene-based sensing platform for the optical detection of toxic HMs.
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http://dx.doi.org/10.1038/s41598-017-04339-8DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5479795PMC
June 2017

Synthesis of TiAuC, TiAuC and TiIrC by noble metal substitution reaction in TiSiC for high-temperature-stable Ohmic contacts to SiC.

Nat Mater 2017 08 1;16(8):814-818. Epub 2017 May 1.

Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden.

The large class of layered ceramics encompasses both van der Waals (vdW) and non-vdW solids. While intercalation of noble metals in vdW solids is known, formation of compounds by incorporation of noble-metal layers in non-vdW layered solids is largely unexplored. Here, we show formation of TiAuC and TiAuC phases with up to 31% lattice swelling by a substitutional solid-state reaction of Au into TiSiC single-crystal thin films with simultaneous out-diffusion of Si. TiIrC is subsequently produced by a substitution reaction of Ir for Au in TiAuC. These phases form Ohmic electrical contacts to SiC and remain stable after 1,000 h of ageing at 600 °C in air. The present results, by combined analytical electron microscopy and ab initio calculations, open avenues for processing of noble-metal-containing layered ceramics that have not been synthesized from elemental sources, along with tunable properties such as stable electrical contacts for high-temperature power electronics or gas sensors.
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http://dx.doi.org/10.1038/nmat4896DOI Listing
August 2017

High-Fidelity Bidirectional Nuclear Qubit Initialization in SiC.

Phys Rev Lett 2016 Nov 23;117(22):220503. Epub 2016 Nov 23.

Wigner Research Centre for Physics, Hungarian Academy of Sciences, PO Box 49, H-1525 Budapest, Hungary.

Dynamic nuclear polarization (DNP) is an attractive method for initializing nuclear spins that are strongly coupled to optically active electron spins because it functions at room temperature and does not require strong magnetic fields. In this Letter, we theoretically demonstrate that DNP, with near-unity polarization efficiency, can be generally realized in weakly coupled electron spin-nuclear spin systems. Furthermore, we theoretically and experimentally show that the nuclear spin polarization can be reversed by magnetic field variations as small as 0.8 Gauss. This mechanism offers new avenues for DNP-based sensors and radio-frequency free control of nuclear qubits.
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http://dx.doi.org/10.1103/PhysRevLett.117.220503DOI Listing
November 2016

Lattice Vibrations Change the Solid Solubility of an Alloy at High Temperatures.

Phys Rev Lett 2016 Nov 8;117(20):205502. Epub 2016 Nov 8.

Department of Physics, Chemistry, and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden.

We develop a method to accurately and efficiently determine the vibrational free energy as a function of temperature and volume for substitutional alloys from first principles. Taking Ti_{1-x}Al_{x}N alloy as a model system, we calculate the isostructural phase diagram by finding the global minimum of the free energy corresponding to the true equilibrium state of the system. We demonstrate that the vibrational contribution including anharmonicity and temperature dependence of the mixing enthalpy have a decisive impact on the calculated phase diagram of a Ti_{1-x}Al_{x}N alloy, lowering the maximum temperature for the miscibility gap from 6560 to 2860 K. Our local chemical composition measurements on thermally aged Ti_{0.5}Al_{0.5}N alloys agree with the calculated phase diagram.
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http://dx.doi.org/10.1103/PhysRevLett.117.205502DOI Listing
November 2016

Highly Efficient Free Energy Calculations of the Fe Equation of State Using Temperature-Dependent Effective Potential Method.

J Phys Chem A 2016 Nov 21;120(43):8761-8768. Epub 2016 Oct 21.

Department of Physics, Chemistry and Biology (IFM), Linköping University , SE-58183 Linköping, Sweden.

Free energy calculations at finite temperature based on ab initio molecular dynamics (AIMD) simulations have become possible, but they are still highly computationally demanding. Besides, achieving simultaneously high accuracy of the calculated results and efficiency of the computational algorithm is still a challenge. In this work we describe an efficient algorithm to determine accurate free energies of solids in simulations using the recently proposed temperature-dependent effective potential method (TDEP). We provide a detailed analysis of numerical approximations employed in the TDEP algorithm. We show that for a model system considered in this work, hcp Fe, the obtained thermal equation of state at 2000 K is in excellent agreement with the results of standard calculations within the quasiharmonic approximation.
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http://dx.doi.org/10.1021/acs.jpca.6b08633DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5199118PMC
November 2016

Computer simulations of glasses: the potential energy landscape.

J Phys Condens Matter 2015 Jul 3;27(29):293201. Epub 2015 Jul 3.

Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83, Linköping, Sweden.

We review the current state of research on glasses, discussing the theoretical background and computational models employed to describe them. This article focuses on the use of the potential energy landscape (PEL) paradigm to account for the phenomenology of glassy systems, and the way in which it can be applied in simulations and the interpretation of their results. This article provides a broad overview of the rich phenomenology of glasses, followed by a summary of the theoretical frameworks developed to describe this phenomonology. We discuss the background of the PEL in detail, the onerous task of how to generate computer models of glasses, various methods of analysing numerical simulations, and the literature on the most commonly used model systems. Finally, we tackle the problem of how to distinguish a good glass former from a good crystal former from an analysis of the PEL. In summarising the state of the potential energy landscape picture, we develop the foundations for new theoretical methods that allow the ab initio prediction of the glass-forming ability of new materials by analysis of the PEL.
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http://dx.doi.org/10.1088/0953-8984/27/29/293201DOI Listing
July 2015

Electrically and mechanically tunable electron spins in silicon carbide color centers.

Phys Rev Lett 2014 May 5;112(18):187601. Epub 2014 May 5.

Institute for Molecular Engineering, University of Chicago, Chicago, Illinois 60637, USA and Center for Spintronics and Quantum Computation, University of California, Santa Barbara, Santa Barbara, California 93106, USA.

The electron spins of semiconductor defects can have complex interactions with their host, particularly in polar materials like SiC where electrical and mechanical variables are intertwined. By combining pulsed spin resonance with ab initio simulations, we show that spin-spin interactions in 4H-SiC neutral divacancies give rise to spin states with a strong Stark effect, sub-10(-6) strain sensitivity, and highly spin-dependent photoluminescence with intensity contrasts of 15%-36%. These results establish SiC color centers as compelling systems for sensing nanoscale electric and strain fields.
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http://dx.doi.org/10.1103/PhysRevLett.112.187601DOI Listing
May 2014

Multiple π-bands and Bernal stacking of multilayer graphene on C-face SiC, revealed by nano-Angle Resolved Photoemission.

Sci Rep 2014 Feb 24;4:4157. Epub 2014 Feb 24.

Department of Physics, Chemistry and Biology, Linköping University, SE-58183 Linköping, Sweden.

Only a single linearly dispersing π-band cone, characteristic of monolayer graphene, has so far been observed in Angle Resolved Photoemission (ARPES) experiments on multilayer graphene grown on C-face SiC. A rotational disorder that effectively decouples adjacent layers has been suggested to explain this. However, the coexistence of μm-sized grains of single and multilayer graphene with different azimuthal orientations and no rotational disorder within the grains was recently revealed for C-face graphene, but conventional ARPES still resolved only a single π-band. Here we report detailed nano-ARPES band mappings of individual graphene grains that unambiguously show that multilayer C-face graphene exhibits multiple π-bands. The band dispersions obtained close to the K-point moreover clearly indicate, when compared to theoretical band dispersion calculated in the framework of the density functional method, Bernal (AB) stacking within the grains. Thus, contrary to earlier claims, our findings imply a similar interaction between graphene layers on C-face and Si-face SiC.
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http://dx.doi.org/10.1038/srep04157DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3932481PMC
February 2014

Pressure-induced hydrogen bond symmetrization in iron oxyhydroxide.

Phys Rev Lett 2013 Oct 24;111(17):175501. Epub 2013 Oct 24.

School of Physics and Astronomy, Tel Aviv University, 69978, Tel Aviv, Israel.

Under high pressures the hydrogen bonds were predicted to transform from a highly asymmetric soft O-H···O to a symmetric rigid configuration in which the proton lies midway between the two oxygen atoms. Despite four decades of research on hydroxyl containing compounds, pressure induced hydrogen bond symmetrization remains elusive. Following single crystal x-ray diffraction, Mössbauer and Raman spectroscopy measurements supported by ab initio calculations, we report the H-bonds symmetrization in iron oxyhydroxide, FeOOH, resulting from the Fe(3+) high-to-low spin crossover at above 45 GPa.
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http://dx.doi.org/10.1103/PhysRevLett.111.175501DOI Listing
October 2013

Phase Stability and Elasticity of TiAlN.

Materials (Basel) 2011 Sep 15;4(9):1599-1618. Epub 2011 Sep 15.

Department of Physics, Chemistry, and Biology (IFM), Linköping University, Linköping 58183, Sweden.

We review results of recent combined theoretical and experimental studies of TiAlN, an archetypical alloy system material for hard-coating applications. Theoretical simulations of lattice parameters, mixing enthalpies, and elastic properties are presented. Calculated phase diagrams at ambient pressure, as well as at pressure of 10 GPa, show a wide miscibility gap and broad region of compositions and temperatures where the spinodal decomposition takes place. The strong dependence of the elastic properties and sound wave anisotropy on the Al-content offers detailed understanding of the spinodal decomposition and age hardening in TiAlN alloy films and multilayers. TiAlN/TiN multilayers can further improve the hardness and thermal stability compared to TiAlN since they offer means to influence the kinetics of the favorable spinodal decomposition and suppress the detrimental transformation to w-AlN. Here, we show that a 100 degree improvement in terms of w-AlN suppression can be achieved, which is of importance when the coating is used as a protective coating on metal cutting inserts.
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http://dx.doi.org/10.3390/ma4091599DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5448856PMC
September 2011

Electron-deficient and polycenter bonds in the high-pressure γ-B28 phase of boron.

Phys Rev Lett 2011 May 25;106(21):215502. Epub 2011 May 25.

Laboratory of Crystallography, University of Bayreuth, 95440 Bayreuth, Germany.

The peculiar bonding situation in γ boron is characterized on the basis of an experimental electron-density distribution which is obtained by multipole refinement against low-temperature single-crystal x-ray diffraction data. A topological analysis of the electron-density distribution reveals one-electron-two-center bonds connecting neighboring icosahedral B(12) clusters. A unique polar-covalent two-electron-three-center bond between a pair of atoms of an icosahedral cluster and one atom of the interstitial B(12) dumbbell explains the observed charge separation in this high-pressure high-temperature polymorph of boron.
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http://dx.doi.org/10.1103/PhysRevLett.106.215502DOI Listing
May 2011

Origin of the anomalous piezoelectric response in wurtzite Sc(x)Al(1-x)N alloys.

Phys Rev Lett 2010 Apr 2;104(13):137601. Epub 2010 Apr 2.

Department of Physics, Chemistry and Biology (IFM), Linköping University, SE-581 83 Linköping, Sweden.

The origin of the anomalous, 400% increase of the piezoelectric coefficient in Sc(x)Al(1-x)N alloys is revealed. Quantum mechanical calculations show that the effect is intrinsic. It comes from a strong change in the response of the internal atomic coordinates to strain and pronounced softening of C33 elastic constant. The underlying mechanism is the flattening of the energy landscape due to a competition between the parent wurtzite and the so far experimentally unknown hexagonal phases of the alloy. Our observation provides a route for the design of materials with high piezoelectric response.
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http://dx.doi.org/10.1103/PhysRevLett.104.137601DOI Listing
April 2010

Questionable collapse of the bulk modulus in CrN.

Nat Mater 2010 Apr;9(4):283-4; author reply 284

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http://dx.doi.org/10.1038/nmat2722DOI Listing
April 2010

Optimization of ionic conductivity in doped ceria.

Proc Natl Acad Sci U S A 2006 Mar 14;103(10):3518-21. Epub 2006 Feb 14.

Applied Materials Physics, Department of Materials Science and Engineering, Royal Institute of Technology, SE-100 44 Stockholm, Sweden.

Oxides with the cubic fluorite structure, e.g., ceria (CeO2), are known to be good solid electrolytes when they are doped with cations of lower valence than the host cations. The high ionic conductivity of doped ceria makes it an attractive electrolyte for solid oxide fuel cells, whose prospects as an environmentally friendly power source are very promising. In these electrolytes, the current is carried by oxygen ions that are transported by oxygen vacancies, present to compensate for the lower charge of the dopant cations. Ionic conductivity in ceria is closely related to oxygen-vacancy formation and migration properties. A clear physical picture of the connection between the choice of a dopant and the improvement of ionic conductivity in ceria is still lacking. Here we present a quantum-mechanical first-principles study of the influence of different trivalent impurities on these properties. Our results reveal a remarkable correspondence between vacancy properties at the atomic level and the macroscopic ionic conductivity. The key parameters comprise migration barriers for bulk diffusion and vacancy-dopant interactions, represented by association (binding) energies of vacancy-dopant clusters. The interactions can be divided into repulsive elastic and attractive electronic parts. In the optimal electrolyte, these parts should balance. This finding offers a simple and clear way to narrow the search for superior dopants and combinations of dopants. The ideal dopant should have an effective atomic number between 61 (Pm) and 62 (Sm), and we elaborate that combinations of Nd/Sm and Pr/Gd show enhanced ionic conductivity, as compared with that for each element separately.
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http://dx.doi.org/10.1073/pnas.0509537103DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1450116PMC
March 2006
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