Coupled polaron-phonon effects on Seebeck coefficient and lattice conductivity of B13C2from first principles

Abstract

Anomalous temperature-independent behavior of the Seebeck coefficient and lattice thermal conductivity of B13C2 are explained through polaron and phonon evolutions found using ab-initio molecular dynamics (AIMD). Analyses of lattice dynamics show that the unique icosahedron structures dominate the optical phonon modes and CBC inter-icosahedral bonds dominate the local acoustic vibration. We identify that the temperature-induced Jahn-Teller distortion and electron-phonon coupling in icosahedron structures create the small polarons (i.e., charge trapping and phonon softening). We also verify that large-displacement chain atoms scatter heat-conducting phonons. Using equilibrium and non-equilibrium AIMD methods (including entropy and energy analyses), we predict the Seebeck coefficient and its components, and the lattice thermal conductivity, and find good agreement with experiments. Softened and localized phonons make significant vibrational contribution to the Seebeck coefficient and allow for an amorphous-like lattice thermal conductivity.

Lattice thermal conductivity of UO2 using ab-initio and classical molecular dynamics

“…We also predict the charge (including surface polarons) and photon contributions to the thermal transport in UO 2 and find they are small in the defect-free structures and for temperatures below 1500 K. The polaron hopping model can be examined by the charge and bond analyses of the DFT results. [75][76][77] Despite the ionic bonds and the polarization effect of the UO 2 structure, in our DFT results for the bulk, pristine crystalline structure we do not observe any polaron formation, over the temperature range of 0 to 2000 K. Since the pristine UO 2 structure is a highly stable, homogeneous symmetric structure, we find no charge localization (or trapping) and structural distortion even at high temperatures. To explore such charge localization using the DFT, we use the stoichiometric and net-neutral UO 2 slab models consisting of 72 atoms with the vacuum gap of 10 Å to minimize the interactions between the slabs 78 with a simple termination of (111) surface (having the lowest surface energy 79,80 ).…”

Dynamical conductivity of boron carbide: heavily damped plasma vibrations

“…This qualitative relation between structural defects and electronic properties is meanwhile widely accepted, although it is in fundamental contrast to the theoretical band structure calculations performed so far [17][18][19][20][21][22][23]; determining boron carbide, represented by the hypothetical, energetically most favorable structure (B 12 )CBC, to be metallic. Merely Bylander et al [24] obtained completely filled valence bands on the likewise hypothetical structure (B 11 C)CBC, which is outside the homogeneity range of boron carbide.…”

Theoretical study of the structure of boron carbideB13C2

“…Emin and coworkers proposed a small bipolaron hopping mechanism [1,40,[45][46][47][48][49]. This model is still under debate [3,[50][51][52].…”