Lattice dynamics of anharmonic solids from first principles
TL;DR: In this paper, the authors have made adjustments to existing frameworks and developed a qualitatively new method, the high temperature effective potential method, which is a general theory and is proven on a number of model systems.
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Abstract: In the search of clean and efficient energy sources intermediate temperature solid oxide fuel cells are among the prime candidates. What sets the limit of their efficiency is the solid electrolyte. A promising material for the electrolyte is ceria. This thesis aims to improve the characteristics of these electrolytes and help provide thorough physical understanding of the processes involved. This is realised using first principles calculations. The class of methods based on density functional theory generally ignores temperature effects. To accurately describe the intermediate temperature characteristics I have made adjustments to existing frameworks and developed a qualitatively new method. The new technique, the high temperature effective potential method, is a general theory. The validity is proven on a number of model systems. Other subprojects include low-dimensional segregation effects, adjustments to defect concentration formalism and optimisations of ionic conductivity.
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Figures
FIG. 1. (Color online) Difference between the free energy (solid line, left axis) and ||D11(R11)|| (dashed line, right axis), the Frobenius norm of the nearest neighbor interaction derived from increasingly longer simulations for bcc Li at T = 300 K. As a reference, the results obtained from simulations with 25 000 time steps are used, which are considered to give the converged results (0.43 eV/Å2 for ||D11(R11)||). 
FIG. 4. (Color online) Calculated bcc-hcp phase diagram for Zr. The calculated stability fields for low-temperature hcp and high-temperature bcc phases are given in light blue (light grey) and red (dark grey) colors, respectively. The black lines show the experimental phase diagram,32 which also includes solid ω and liquid phases, not considered in simulations. 
FIG. 3. (Color online) Phonon dispersion relations for bcc Zr. Solid lines correspond to calculations at 1300 K, dashed lines denote the quasiharmonic results, and symbols represent experimental values from Heiming et al.30 (circles) and Stassis et al.31 (filled circles). The dotted vertical line is at q = ( 23 , 23 , 23 ). The observed softening at this point, experimental as well as theoretical, is important for the bcc to ω phase transition.
Citations
Temperature dependent effective potential method for accurate free energy calculations of solids
TL;DR: In this article, the authors developed a thorough and accurate method of determining anharmonic free energies based on ab initio molecular dynamics and map a model Hamiltonian to the fully anharmoric free energy.
A perspective on conventional high-temperature superconductors at high pressure: Methods and materials
TL;DR: In this paper, the authors provide an up-to-date compendium of the available results on superconducting hydrides and explain how the synergy of different methodologies led to extraordinary discoveries in the field.
461
Unified theory of thermal transport in crystals and glasses
TL;DR: In this paper, a unified theory for the conduction of heat in materials is derived and shown to account for both the limiting regimes of periodic crystals and aperiodic glasses, respectively, in anharmonic crystals or harmonic glasses, while also covering the intermediate regimes where both effects are relevant.
404
Anharmonic free energies and phonon dispersions from the stochastic self-consistent harmonic approximation: Application to platinum and palladium hydrides
TL;DR: In this paper, a stochastic implementation of the self-consistent harmonic approximation valid to treat anharmonicity at any temperature in the nonperturbative regime is presented.
394
Temperature-dependent effective third-order interatomic force constants from first principles
Olle Hellman,Igor A. Abrikosov +1 more
TL;DR: In this paper, the temperature-dependent effective potential (TDEP) method is generalized beyond pair interactions and the second and third-order force constants are determined consistently from ab initio molecular molecular simulations.
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