arXiv subjects

Björn Alling

Publications and source records attributed to Björn Alling.

25 records · Page 2Linked to original sources

arXiv2016

Finite temperature elastic constants of paramagnetic materials within the disordered local moment picture from ab initio molecular dynamics calculations

We present a theoretical scheme to calculate the elastic constants of magnetic materials in the high-temperature paramagnetic state. Our approach is based on a combination of disordered local moments picture and ab initio molecular dynamics (DLM-MD). Moreover, we investigate a possibility to enhance the efficiency of the simulations of elastic properties using recently introduced method: symmetry imposed force constant temperature dependent effective potential (SIFC-TDEP). We have chosen cubic paramagnetic CrN as a model system. This is done due to its technological importance and its demonstrated strong coupling between magnetic and lattice degrees of freedom. We have studied the temperature dependent single-crystal and polycrystalline elastic constants of paramagentic CrN up to 1200 K. The obtained results at T= 300 K agree well with the experimental values of polycrystalline elastic constants as well as Poisson ratio at room temperature. We observe that the Young's modulus is strongly dependent on temperature, decreasing by ~14% from T=300 K to 1200 K. In addition we have studied the elastic anisotropy of CrN as a function of temperature and we observe that CrN becomes substantially more isotropic as the temperature increases. We demonstrate that the use of Birch law may lead to substantial errors for calculations of temperature induced changes of elastic moduli. The proposed methodology can be used for accurate predictions of mechanical properties of magnetic materials at temperatures above their magnetic order-disorder phase transition.

cond-mat.mtrl-sci↗

arXiv2016

Efficient and accurate determination of lattice-vacancy diffusion coefficients via non equilibrium ab initio molecular dynamics

We revisit the color-diffusion algorithm [P. C. Aeberhard et al., Phys. Rev. Lett. 108, 095901 (2012)] in nonequilibrium ab initio molecular dynamics (NE-AIMD), and propose a simple efficient approach for the estimation of monovacancy jump rates in crystalline solids at temperatures well below melting. Color-diffusion applied to monovacancy migration entails that one lattice atom (colored-atom) is accelerated toward the neighboring defect-site by an external constant force F. Considering bcc molybdenum between 1000 and 2800 K as a model system, NE-AIMD results show that the colored-atom jump rate k_{NE} increases exponentially with the force intensity F, up to F values far beyond the linear-fitting regime employed previously. Using a simple model, we derive an analytical expression which reproduces the observed k_{NE}(F) dependence on F. Equilibrium rates extrapolated by NE-AIMD results are in excellent agreement with those of unconstrained dynamics. The gain in computational efficiency achieved with our approach increases rapidly with decreasing temperatures, and reaches a factor of four orders of magnitude at the lowest temperature considered in the present study.

cond-mat.mtrl-sci↗

arXiv2015

Anharmonicity changes the solid solubility of an alloy at high temperatures

We have developed 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 anharmonic contribution 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 K 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.

cond-mat.mtrl-sci↗

arXiv2014

The role of N defects in paramagnetic CrN at finite temperatures from first-principles

Simulations of defects in paramagnetic materials at high temperature constitute a formidable challenge to solid state theory due to the interaction of magnetic disorder, vibrations, and structural relaxations. CrN is a material where these effects are particularly large due to a strong magneto-lattice coupling and a tendency for deviations from the nominal 1:1 stoichiometry. In this work we present a first-principles study of nitrogen vacancies and nitrogen interstitals in CrN at elevated temperature. We report on formation energetics, the geometry of interstital nitrogen dimers, and impact on the electronic structure caused by the defects. We find a vacancy formation energy of 2.28 eV with a small effect of temperature, a formation energy for N interstitial in the form of a $\big<111\big>$ oriented split-bond of 3.77 eV with an increase to 3.97 at 1000 K. Vacancies are found to add three electrons while split bond interstitial adds one electron to the conduction band. The band gap of defect-free CrN is smeared out due to vibrations, though it is difficult to draw conclusion about the exact temperature at which the band gap closes from our calculations. However, it is clear that at 900 K there is a non-zero density of electronic states at the Fermi level. At 300 K our results indicate a boarder case were the band-gap is about to close.

cond-mat.str-el↗

arXiv2014

Vibrational free energy and phase stability of paramagnetic and antiferromagnetic CrN from ab-initio molecular dynamics

We present a theoretical first-principles method to calculate the free energy of a magnetic system in its high-temperature paramagnetic phase, including vibrational, electronic, and magnetic contributions. The method for calculating free energies is based on ab-initio molecular dynamics and combines a treatment of disordered magnetism using disordered local moments molecular dynamics (DLM-MD) with the temperature dependent effective potential (TDEP) method to obtain the vibrational contribution to the free energy. We illustrate the applicability of the method by obtaining the anharmonic free energy for the paramagnetic cubic and the antiferromagnetic orthorhombic phases of chromium nitride. The influence of lattice dynamics on the transition between the two phases is demonstrated by constructing the temperature-pressure phase diagram.

cond-mat.mtrl-sci↗

arXiv2011

Equation of state of paramagnetic CrN from ab initio molecular dynamics

Equation of state for chromium nitride has been debated in the literature in connection with a proposed collapse of its bulk modulus following the pressure induced transition from the paramagnetic cubic phase to the antiferromagnetic orthorhombic phase [F. Rivadulla et al., Nat Mater 8, 974 (2009); B. Alling et al., Nat Mater 9, 283 (2010)]. Experimentally the measurements are complicated due to the low transition pressure, while theoretically the simulation of magnetic disorder represent a major challenge. Here a first-principles method is suggested for the calculation of thermodynamic properties of magnetic materials in their high temperature paramagnetic phase. It is based on ab-initio molecular dynamics and simultaneous redistributions of the disordered but finite local magnetic moments. We apply this disordered local moments molecular dynamics method to the case of CrN and simulate its equation of state. In particular the debated bulk modulus is calculated in the paramagnetic cubic phase and is shown to be very similar to that of the antiferromagnetic orthorhombic CrN phase for all considered temperatures.

cond-mat.mtrl-sci↗

arXiv2010

Origin of the anomalous piezoelectric response in wurtzite Sc$_x$Al$_{1-x}$N alloys

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 C$_{33}$ 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.

cond-mat.mtrl-sci↗