arXiv subjects

Stefan W. Kycia

Publications and source records attributed to Stefan W. Kycia.

4 recordsLinked to original sources

arXiv2024

Role of filler lanthanide ions on lattice dynamics of phosphide skutterudites RFe$_4$P$_{12}$ (R = La, Ce, and Pr) from first principles

Phosphide skutterudites primarily show promise for thermoelectric applications due to their chemical stability at high temperatures and relatively low cost. Ion doping and band gap engineering have been used to enhance their typically poor thermoelectric performance, opening avenues for practical applications. Herein, we report a comparative lattice dynamics study on the impact of filler and temperature on the structural and vibrational properties of RFe$_4$P$_{12}$ (R = La, Ce, and Pr) skutterudites. Calculations are performed within the quasi-harmonic approximation, and the results are critically compared against experimental data and other \textit{ab initio} calculations. We found gaps between the heat-carrying acoustic and optical modes, a-o gaps, of approximately 4, -2, and 0.01\,cm$^{-1}$ for La, Ce, and Pr compounds, respectively. These results suggest a filler-induced reduction in the a-o gap is attributed to the softening of the optical modes instead of the conventionally considered upward shift of acoustic modes proposed in the \textit{rattling} scenario. The distinct softening of the optical modes is rationalized by the stiffening of chemical bonds between the filler and host lattice.

cond-mat.mtrl-sci↗

arXiv2023

Proper usage of Scherrer's and Guinier's formulas in X-ray analysis of size distribution in systems of monocrystalline CeO2 nanoparticles

Small-angle X-ray scattering (SAXS) and X-ray diffraction (XRD) techniques are widely used as analytical tools in the optimization and control of nanomaterial synthesis processes. In crystalline nanoparticle systems with size distribution, the discrepant size values determined by using SAXS and XRD still lacks a well-established description in quantitative terms. To address fundamental questions, the isolated effect of size distribution is investigated by SAXS and XRD simulation in polydisperse systems of virtual nanoparticles. It quantitatively answered a few questions, among which the most accessible and reliable size values and what they stand for regarding the size distribution parameters. When a finite size distribution is introduced, the two techniques produce differing results even in perfectly crystalline nanoparticles. Once understood, the deviation in resulting size values can, in principle, resolve two parameters size distributions of crystalline nanoparticles. To demonstrate data analysis procedures in light of this understanding, XRD and SAXS experiments were carried out on a series of powder samples of cubic ceria nanoparticles. Besides changes in the size distribution related to the synthesis parameters, proper comparison of XRD and SAXS results revealed particle-particle interaction effects underneath the SAXS intensity curves. It paves the way for accurate and reliable methodologies to assess size, size dispersion, and degree of crystallinity in synthesized nanoparticles.

cond-mat.mtrl-sci↗

arXiv2020

Morphology Control in van der Waals Epitaxy of Bismuth Telluride Topological Insulators

Bismuth telluride have regained significant attention as a prototype of topological insulator. Thin films of high quality have been investigated as a basic platform for novel spintronic devices. Low mobility of bismuth and high desorption coefficient of telluride compose a scenario where growth parameters have drastic effects on structural and electronic properties of the films. Recently [J. Phys. Chem. C 2019, 123, 24818-24825], a detailed investigation has been performed on the dynamics of defects in epitaxial films of this material, revealing the impact of film/substrate lattice misfit on the films' lateral coherence. Very small lattice misfit (<0.05%) are expected to have no influence on quality of epitaxial system with atomic layers weakly bonded to each other by van der Waals forces, contrarily to what was observed. In this work, we investigate the correlation between lattice misfit and size and morphology of the film crystalline domains. Three-dimensional reciprocal-space maps of film Bragg reflections obtained with synchrotron X-rays are used to visualize the spatial conformation of the crystallographic domains through film thickness, while atomic force microscopy images provide direct information of the domains morphology at the film surface.

cond-mat.mtrl-sci↗

arXiv2020

Phonon Scattering Mechanism in Thermoelectric Materials Revised via Resonant X-ray Dynamical Diffraction

Engineering of thermoelectric materials requires an understanding of thermal conduction by lattice and electronic degrees of freedom. Filled skutterudites denote a large family of materials suitable for thermoelectric applications where reduced lattice thermal conduction attributed to localized low-frequency vibrations (rattling) of filler cations inside large cages of the structure. In this work, a multi-wavelength method of exploiting X-ray dynamical diffraction in single crystals of CeFe$_4$P$_{12}$ is presented and applied to resolve the atomic amplitudes of vibrations. The results suggest that the vibrational dynamics of the whole filler-cage system is the actual active mechanism behind the optimization of thermoelectric properties.

cond-mat.mtrl-sci↗