Petros Souvatzis

Entropically Stabilized Local Dipole Formation in Lead Chalcogenides

A Hot Dipole In a ferroelectric material, there is an alignment of local electric dipole moments that produces a net overall electric polarization. This state is accompanied by a decrease in symmetry, which can be restored by heating above a critical temperature. In contrast, through a combination of theory and experiments, Božin et al. (p. 1660) now show that with increasing temperature, rock-salt–structured lead telluride and lead sulfide go through a phase transition from a high symmetry phase to a low symmetry phase with an associated dipole moment. Paradoxically, the dipoles are stabilized in the disordered state at high temperature, even though the undistorted structure has lower internal energy. Upon heating, lead telluride and lead sulfide show the formation of a less symmetric, dipolar structure. We report the observation of local structural dipoles that emerge from an undistorted ground state on warming, in contrast to conventional structural phase transitions in which distortions emerge on cooling. Using experimental and theoretical probes of the local structure, we demonstrate this behavior in binary lead chalcogenides, which were believed to adopt the ideal, undistorted rock-salt structure at all temperatures. The behavior is consistent with a simple thermodynamic model in which the emerging dipoles are stabilized in the disordered state at high temperature due to the extra configurational entropy despite the fact that the undistorted structure has lower internal energy. Our findings shed light on the anomalous electronic and thermoelectric properties of the lead chalcogenides. Similar searches may show that the phenomenon is more widespread.

Entropy driven stabilization of energetically unstable crystal structures explained from first principles theory

Conventional methods to calculate the thermodynamics of crystals evaluate the harmonic phonon spectra and therefore do not work in frequent and important situations where the crystal structure is unstable in the harmonic approximation, such as the body-centered cubic (bcc) crystal structure when it appears as a high-temperature phase of many metals. A method for calculating temperature dependent phonon spectra self-consistently from first principles has been developed to address this issue. The method combines concepts from Borns interatomic self-consistent phonon approach with first principles calculations of accurate interatomic forces in a supercell. The method has been tested on the high-temperature bcc phase of Ti, Zr, and Hf, as representative examples, and is found to reproduce the observed high-temperature phonon frequencies with good accuracy.

Dynamical stability of body center cubic iron at the Earth’s core conditions

Here, using self-consistent ab initio lattice dynamical calculations that go beyond the quasiharmonic approximation, we show that the high-pressure high-temperature bcc-Fe phase is dynamically stable. In this treatment the temperature-dependent phonon spectra are derived by exciting all the lattice vibrations, in which the phonon–phonon interactions are considered. The high-pressure and high-temperature bcc-Fe phase shows standard bcc-type phonon dispersion curves except for the transverse branch, which is overdamped along the high symmetry direction Γ-N, at temperatures below 4,500 K. When lowering the temperature down to a critical value TC, the lattice instability of the bcc structure is reached. The pressure dependence of this critical temperature is studied at conditions relevant for the Earth’s core.

Anomalous thermal expansion in alpha-titanium.

We provide a complete quantitative explanation for the anisotropic thermal expansion of hcp Ti at low temperature. The observed negative thermal expansion along the c axis is reproduced theoretically by means of a parameter free theory which involves both the electron and phonon contributions to the free energy. The thermal expansion of titanium is calculated and found to be negative along the c axis for temperatures below approximately 170 K, in good agreement with observations. We have identified a saddle point van Hove singularity near the Fermi level as the main reason for the anisotropic thermal expansion in alpha-titanium.

First principles molecular dynamics without self-consistent field optimization

We present a first principles molecular dynamics approach that is based on time-reversible extended Lagrangian Born-Oppenheimer molecular dynamics [A. M. N. Niklasson, Phys. Rev. Lett. 100, 123004 (2008)] in the limit of vanishing self-consistent field optimization. The optimization-free dynamics keeps the computational cost to a minimum and typically provides molecular trajectories that closely follow the exact Born-Oppenheimer potential energy surface. Only one single diagonalization and Hamiltonian (or Fockian) construction are required in each integration time step. The proposed dynamics is derived for a general free-energy potential surface valid at finite electronic temperatures within hybrid density functional theory. Even in the event of irregular functional behavior that may cause a dynamical instability, the optimization-free limit represents a natural starting guess for force calculations that may require a more elaborate iterative electronic ground state optimization. Our optimization-free dynamics thus represents a flexible theoretical framework for a broad and general class of ab initio molecular dynamics simulations.

Anomalous Thermal Expansion inα-Titanium

We provide a complete quantitative explanation for the anisotropic thermal expansion of hcp Ti at low temperature. The observed negative thermal expansion along the c axis is reproduced theoretically by means of a parameter free theory which involves both the electron and phonon contributions to the free energy. The thermal expansion of titanium is calculated and found to be negative along the c axis for temperatures below approximately 170 K, in good agreement with observations. We have identified a saddle point van Hove singularity near the Fermi level as the main reason for the anisotropic thermal expansion in alpha-titanium.

Dynamical stabilization of the body centered cubic phase in lanthanum and thorium by phonon-phonon interaction.

A recently developed self-consistent ab initio lattice dynamical method has been applied to the high temperature body centered cubic (bcc) phase of La and Th, which are dynamically unstable at low temperatures. The bcc phase of these metals is found to be stabilized by phonon-phonon interactions. The calculated high temperature phonon frequencies for La are found to be in good agreement with the corresponding experimental data.

Temperature-driven α-to-β phase transformation in Ti, Zr and Hf from first-principles theory combined with lattice dynamics

Lattice dynamical methods used to predict phase transformations in crystals typically deal with harmonic phonon spectra and are therefore not applicable in important situations where one of the competing crystal structures is unstable in the harmonic approximation, such as the bcc structure involved in the hcp-to-bcc martensitic phase transformation in Ti, Zr and Hf. Here we present an expression for the free energy that does not suffer from such shortcomings, and we show by self-consistent ab initio lattice dynamical calculations (SCAILD), that the critical temperature for the hcp-to-bcc phase transformation in Ti, Zr and Hf, can be effectively calculated from the free-energy difference between the two phases. This opens up the possibility to study quantitatively, from first-principles theory, temperature-induced phase transitions.

Dynamical stabilization of cubic ZrO 2 by phonon-phonon interactions: Ab initio calculations

Cubic zirconia exhibits a soft phonon mode (X{sup -}{sub 2}), which becomes dynamically unstable at low temperatures. Previous ab initio invest.igations into the temperature-induced stabilization of the soft mode treated it as an independent anharmonic oscillator. Calculations presented here, using the self consistent ab initio lattice dynamical (SCAILD) method to evaluate the phonons at 2570 K, show that the soft mode should not be treated independently of other phonon modes. Phonon-phonon interactions stabilize the X{sup -}{sub 2} mode. Furthermore, the effective potential experienced by the mode takes on a quadratic form.

Lattice dynamics and chemical bonding in Sb2Te3 from first-principles calculations.

Pressure effects on the lattice dynamics and the chemical bonding of the three-dimensional topological insulator, Sb2Te3, have been studied from a first-principles perspective in its rhombohedral phase. Where it is possible to compare, theory agrees with most of the measured phonon dispersions. We find that the inclusion of relativistic effects, in terms of the spin-orbit interaction, affects the vibrational features to some extend and creates large fluctuations on phonon density of state in high frequency zone. By investigations of structure and electronic structure, we analyze in detail the semiconductor to metal transition at ∼2 GPa followed by an electronic topological transition at a pressure of ∼4.25 GPa.