<i>Ab initio</i>study of AlN: Anisotropic thermal expansion, phase diagram, and high-temperature rocksalt to wurtzite phase transition

Schmerler, Steve; Kortus, Jens

doi:10.1103/physrevb.89.064109

Cited by 35 publications

(37 citation statements)

References 47 publications

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“…Our method uses Born-Oppenheimer molecular dynamics (BOMD) to more effectively sample the potential energy surface. A general problem with the QHA is the appearance of negative branches in the phonon dispersion at large volumes [12], as a result of the shift in the equilibrium volume as a function of temperature; BOMD does not suffer from this problem.…”

Section: Methodsmentioning

confidence: 99%

“…Calculations within the quasiharmonic approximation (QHA) [9][10][11][12][13] placed the transition pressure between 12.9 GPa [11] and 17 GPa [9], depending on the details of the calculation. They also suggest that the rocksalt structure is stabilized by temperature, but the extent of this effect cannot be extrapolated from the QHA, which breaks down at high temperatures due to anharmonic effects; Schmerler and Kortus [12] demonstrate that the temperature limit is approximately 2200 K.…”

Section: Introductionmentioning

confidence: 99%

“…If the pressure is increased along an isotherm, c/a increases, but if the temperature is increased along an isobar, c/a shows the opposite behavior and decreases-this can cause problems if only zero-temperature structures are used for QHA calculations [12]. In practice, this problem can be overcome by interpolating c/a as a function of temperature and volume.…”

Section: Introductionmentioning

confidence: 99%

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Impact of anharmonic effects on the phase stability, thermal transport, and electronic properties of AlN

et al. 2016

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Wurtzite aluminium nitride (AlN) is a technologically important wide-band-gap semiconductor with an unusually high thermal conductivity, used in optical applications and as a heatsink substrate. Explaining many of its properties depends on an accurate description of its lattice dynamics, which have thus far only been captured in the quasiharmonic approximation. In this work, we show that anharmonic effects have a considerable impact on its phase stability and transport properties, since they are much stronger in the rocksalt phase. We construct a theoretical pressure-temperature phase diagram of AlN, demonstrating that the rocksalt phase is stabilized by increasing temperature, with respect to the wurtzite phase. We recover the thermal conductivity of the wurtzite phase (320 Wm −1 K −1 under ambient conditions) and compute the hitherto unknown thermal conductivity of the rocksalt phase (81 Wm −1 K −1 ). We also show that the electronic band gap decreases with temperature. These findings provide further evidence that anharmonic effects cannot be ignored in simulations of materials intended for high-temperature applications.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Impact of anharmonic effects on the phase stability, thermal transport, and electronic properties of AlN

et al. 2016

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“…The phase diagram has been computed in the quasiharmonic approximation, but the pressure and temperature range is limited by dynamical instability at at large (zero temperature) cell volumes. 10,22 TDEP calculations are therefore ideal for predicting its properties. The reconstructed phase diagram (figure 9.1) shows the effect of anharmonicity at high temperatures: the region in which the wurtzite phase is stable closes, and the rocksalt phase becomes thermodynamically stable at around 3200 K. The melting point of AlN is unknown, but is estimated at around 3000-3500 K, so it is unclear whether this phase transition will occur.…”

Section: 2 1 a L U M I N I U M N I T R I D Ementioning

confidence: 99%

“…16 Like other group III nitrides, AlN has been shown both experimentally and theoretically to undergo a pressure-induced phase transition from wurtzite to rocksalt at low temperatures, 17,18 although its high temperature behaviour is less clear due to the failure of the quasiharmonic approximation. 19,20,21,22,23 The rocksalt phase is metastable under ambient conditions, and can therefore only be synthesised in the form of thin films, for which it is difficult to measure the thermal conductivity. By reconstructing the pressure-temperature phase diagram, I demonstrate that the rocksalt phase is stabilised by temperature at lower pressure due to anharmonicity.…”

Section: Introductionmentioning

confidence: 99%

Vibrations in solids : From first principles lattice dynamics to high temperature phase stability

Shulumba¹

2015

Linköping Studies in Science and Technology. Dissertations

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In this thesis I introduce a new method for calculating the temperature dependent vibrational contribution to the free energy of a substitutionally disordered alloy that accounts for anharmonicity at high temperatures. This method exploits the underlying crystal symmetries in an alloy to make the calculations tractable. The validity of this approach is demonstrated by constructing the phase diagram via direct minimization of the Gibbs free energy of a notoriously awkward and technologically important system, Ti 1-x Al x N. The vibrational entropy including anharmonic effects is shown to be large and comparable to the configurational entropy at high temperatures, and with its inclusion, the theoretical miscibility gap of Ti 1-x Al x N is reduced from 6560 K to 2860 K, in line with atom probe experiments. A similar treatment of Zr 1-x Al x N and Hf 1-x Al x N alloys suggests that mass disorder has a minimal effect on phase stability compared with chemical ordering. My method is also capable of demonstrating that Hf 1-x Al x N, which is dynamically unstable at room temperature, is stabilised at high temperatures. Moreover I develop a new method of computing temperature dependent elastic constants for alloys from their phonon spectra, and show that for Ti 1-x Al x N, the elastic anisotropy is found to increase with temperature, helping to explain the spinodal decomposition.The effects of lattice dynamics on phase stability, mechanical, magnetic and transport properties on other materials are also examined. Four specific systems are discussed in detail. Firstly, in the case of CrN, lattice vibrations are shown to decrease the antiferromagnetic to paramagnetic phase transition temperature from 500 K to 380 K, in line with experimental evidence. Secondly, a temperature/pressure induced phase transition in AlN becomes much more facile than in the quasiharmonic approximation, and the thermal conductivity of the rocksalt phase is shown to be much lower than that of the wurtzite phase, as a result of the increased anharmonicity in the rocksalt structure. Thirdly, the temperature dependence of elastic constants of TiN becomes more isotropic as the temperature increases. Finally, iron carbides are evaluated as potentially important phases at the Earth's core; specifically, calculating the Gibbs free energy of a recently discovered orthorhombic phase of Fe 7 C 3 demonstrates that it is not stable relative to the known hexagonal phase at extreme pressure and temperatures.V Popular science summary Solid materials can be described using a simple mathematical model from the theory of lattice dynamics. In a solid, atoms are arranged in a regular and repeating structure (a lattice), and the forces between them can essentially be modelled as springs. Despite the simplicity of this model, it is possible to derive a surprising amount of information on materials from it. One can determine which arrangements of atoms, or phases, form the stable microscopic structures that we encounter in the real world, their mechanical properti...

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Conceptual Inorganic Materials Discovery – A Road Map

Jansen

2015

Advanced Materials

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Synthesis of novel solids, which is a pivotal starting point in innovative materials research, is markedly impeded by the lack of predictability. A conception is presented that enables syntheses of new materials to be rationally planned. The approach is based on the atomic configuration space, and the potential energies associated to the atomic arrangements. Each minimum of the respective hyperspace of potential energy corresponds to a chemical compound capable of existence. Thus the whole realm of known and not-yet-known chemical compounds is represented in virtuo on that energy landscape. From this view it follows further that the full sets of the corresponding materials' properties are pre-determined. Within the scope of the "Energy Landscape Concept of Chemical Matter" presented, targets for synthesis are identified in a rational manner by searching the underlying potential energy landscapes for (meta)stable candidates computationally. Subsequently, the gained information are transferred to finite temperatures, which enables phase diagrams to be calculated, including metastable manifestations of matter, from first principles. The subsequent steps in materials discovery, e.g., assessing the properties and the impact of defects on the performance of the solids predicted are addressed briefly. The approach presented is complete and physically consistent; its feasibility has been proven and validated experimentally.

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Ab initiostudy of AlN: Anisotropic thermal expansion, phase diagram, and high-temperature rocksalt to wurtzite phase transition

Cited by 35 publications

References 47 publications

Impact of anharmonic effects on the phase stability, thermal transport, and electronic properties of AlN

Impact of anharmonic effects on the phase stability, thermal transport, and electronic properties of AlN

Vibrations in solids : From first principles lattice dynamics to high temperature phase stability

Conceptual Inorganic Materials Discovery – A Road Map

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