Olle Hellman

Lattice dynamics of anharmonic solids from first principles

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.

Temperature dependent effective potential method for accurate free energy calculations of solids

We have developed a thorough and accurate method of determining anharmonic free energies. The technique is based in ab initio molecular dynamics and map a model Hamiltonian to the fully anharmonic ...

Phonon Self-Energy and Origin of Anomalous Neutron Scattering Spectra in SnTe and PbTe Thermoelectrics

The anharmonic lattice dynamics of rock-salt thermoelectric compounds SnTe and PbTe are investigated with inelastic neutron scattering (INS) and first-principles calculations. The experiments show that, surprisingly, although SnTe is closer to the ferroelectric instability, phonon spectra in PbTe exhibit a more anharmonic character. This behavior is reproduced in first-principles calculations of the temperature-dependent phonon self-energy. Our simulations reveal how the nesting of phonon dispersions induces prominent features in the self-energy, which account for the measured INS spectra and their temperature dependence. We establish that the phase space for three-phonon scattering processes, combined with the proximity to the lattice instability, is the mechanism determining the complex spectrum of the transverse-optic ferroelectric mode.

Phonon thermal transport in Bi 2 Te 3 from first principles

We present first-principles calculations of the thermal and thermal transport properties of

Phonon thermal transport inBi2Te3from first principles

{\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}

Temperature-dependent effective third-order interatomic force constants from first principles

that combine an ab initio molecular dynamics (AIMD) approach to calculate interatomic force constants (IFCs) along with a full iterative solution of the Peierls-Boltzmann transport equation for phonons. The newly developed AIMD approach allows determination of harmonic and anharmonic interatomic forces at each temperature, which is particularly appropriate for highly anharmonic materials such as

Thermal conductivity in PbTe from first principles

{\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}

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

. The calculated phonon dispersions, heat capacity, and thermal expansion coefficient are found to be in good agreement with measured data. The lattice thermal conductivity,

Two-Step Phase Transition in SnSe and the Origins of its High Power Factor from First Principles

{\ensuremath{\kappa}}_{l}

Quenching of bcc-Fe from high to room temperature at high-pressure conditions : a molecular dynamics simulation

, calculated using the AIMD approach nicely matches measured values, showing better agreement than the