Publications by authors named "Björn Alling"

8 Publications

  • Page 1 of 1

Mysterious SiB: Identifying the Relation between α- and β-SiB.

ACS Omega 2019 Nov 1;4(20):18741-18759. Epub 2019 Nov 1.

Department of Materials and Environmental Chemistry, Stockholm University, S-10691 Stockholm, Sweden.

Binary silicon boride SiB has been reported to occur in two forms, as disordered and nonstoichiometric α-SiB , which relates to the α-rhombohedral phase of boron, and as strictly ordered and stoichiometric β-SiB. Similar to other boron-rich icosahedral solids, these SiB phases represent potentially interesting refractory materials. However, their thermal stability, formation conditions, and thermodynamic relation are poorly understood. Here, we map the formation conditions of α-SiB and β-SiB and analyze their relative thermodynamic stabilities. α-SiB is metastable (with respect to β-SiB and Si), and its formation is kinetically driven. Pure polycrystalline bulk samples may be obtained within hours when heating stoichiometric mixtures of elemental silicon and boron at temperatures 1200-1300 °C. At the same time, α-SiB decomposes into SiB and Si, and optimum time-temperature synthesis conditions represent a trade-off between rates of formation and decomposition. The formation of stable β-SiB was observed after prolonged treatment (days to weeks) of elemental mixtures with ratios Si/B = 1:1-1:4 at temperatures 1175-1200 °C. The application of high pressures greatly improves the kinetics of SiB formation and allows decoupling of SiB formation from decomposition. Quantitative formation of β-SiB was seen at 1100 °C for samples pressurized to 5.5-8 GPa. β-SiB decomposes peritectoidally at temperatures between 1250 and 1300 °C. The highly ordered nature of β-SiB is reflected in its Raman spectrum, which features narrow and distinct lines. In contrast, the Raman spectrum of α-SiB is characterized by broad bands, which show a clear relation to the vibrational modes of isostructural, ordered BP. The detailed composition and structural properties of disordered α-SiB were ascertained by a combination of single-crystal X-ray diffraction and Si magic angle spinning NMR experiments. Notably, the compositions of polycrystalline bulk samples (obtained at ≤ 1200 °C) and single crystal samples (obtained from Si-rich molten Si-B mixtures at > 1400 °C) are different, SiB and SiB, respectively. The incorporation of Si in the polar position of B icosahedra results in highly strained cluster units. This disorder feature was accounted for in the refined crystal structure model by splitting the polar position into three sites. The electron-precise composition of α-SiB is SiB and corresponds to the incorporation of, on average, two Si atoms in each B icosahedron. Accordingly, α-SiB constitutes a mixture of BSi and BSi clusters. The structural and phase stability of α-SiB were explored using a first-principles cluster expansion. The most stable composition at 0 K is SiB, which however is unstable with respect to the decomposition β-SiB + Si. Modeling of the configurational and vibrational entropies suggests that α-SiB only becomes more stable than β-SiB at temperatures above its decomposition into SiB and Si. Hence, we conclude that α-SiB is metastable at all temperatures. Density functional theory electronic structure calculations yield band gaps of similar size for electron-precise α-SiB and β-SiB, whereas α-SiB represents a p-type conductor.
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http://dx.doi.org/10.1021/acsomega.9b02727DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6854836PMC
November 2019

Anomalous Phonon Lifetime Shortening in Paramagnetic CrN Caused by Spin-Lattice Coupling: A Combined Spin and Ab Initio Molecular Dynamics Study.

Phys Rev Lett 2018 Sep;121(12):125902

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

We study the mutual coupling of spin fluctuations and lattice vibrations in paramagnetic CrN by combining atomistic spin dynamics and ab initio molecular dynamics. The two degrees of freedom are dynamically coupled, leading to nonadiabatic effects. Those effects suppress the phonon lifetimes at low temperature compared to an adiabatic approach. The dynamic coupling identified here provides an explanation for the experimentally observed unexpected temperature dependence of the thermal conductivity of magnetic semiconductors above the magnetic ordering temperature.
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http://dx.doi.org/10.1103/PhysRevLett.121.125902DOI Listing
September 2018

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

Transition-metal-nitride-based thin films as novel energy harvesting materials.

J Mater Chem C Mater 2016 May 19;4(18):3905-3914. Epub 2016 Feb 19.

Thin Film Physics Division , Linköping University , IFM , 581 83 Linköping , Sweden . Email: Max-Planck-Institut für Eisenforschung GmbH , D-40237 Düsseldorf , Germany.

The last few years have seen a rise in the interest in early transition-metal and rare-earth nitrides, primarily based on ScN and CrN, for energy harvesting by thermoelectricity and piezoelectricity. This is because of a number of important advances, among those the discoveries of exceptionally high piezoelectric coupling coefficient in (Sc,Al)N alloys and of high thermoelectric power factors of ScN-based and CrN-based thin films. These materials also constitute well-defined model systems for investigating thermodynamics of mixing for alloying and nanostructural design for optimization of phase stability and band structure. These features have implications for and can be used for tailoring of thermoelectric and piezoelectric properties. In this highlight article, we review the ScN- and CrN-based transition-metal nitrides for thermoelectrics, and drawing parallels with piezoelectricity. We further discuss these materials as a models systems for general strategies for tailoring of thermoelectric properties by integrated theoretical-experimental approaches.
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http://dx.doi.org/10.1039/c5tc03891jDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4894070PMC
May 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

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

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
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