10 research outputs found
Predicting ground-state configurations and electronic properties of the thermoelectric clathrates BaAlSi and SrAlSi
The structural and electronic properties of the clathrate compounds
BaAlSi and SrAlSi are studied from
first principles, considering an Al content between 6 and 16. Due to the
large number of possible substitutional configurations we make use of a special
iterative cluster-expansion approach, to predict ground states and
quasi-degenerate structures in a highly efficient way. These are found from a
simulated annealing technique where millions of configurations are sampled. For
both compounds, we find a linear increase of the lattice constant with the
number of Al substituents, confirming experimental observations for
BaAlSi. Also the calculated bond distances between
high-symmetry sites agree well with experiment for the full compositional
range. For being below 16, all configurations are metallic for both
materials. At the charge-balanced composition (), the substitutional
ordering leads to a metal-semiconductor transition, and the ground states of
BaAlSi and SrAlSi exhibit indirect
Kohn-Sham band gaps of 0.36 and 0.30 eV, respectively, while configurations
higher in energy are metals. The finding of semiconducting behavior is a
promising result in view of exploiting these materials in thermoelectric
applications.Comment: 9 figure
CELL: a Python package for cluster expansion with a focus on complex alloys
We present the Python package CELL, which provides a modular approach to the
cluster expansion (CE) method. CELL can treat a wide variety of substitutional
systems, including one-, two-, and three-dimensional alloys, in a general
multi-component and multi-sublattice framework. It is capable of dealing with
complex materials comprising several atoms in their parent lattice. CELL uses
state-of-the-art techniques for the construction of training data sets, model
selection, and finite-temperature simulations. The user interface consists of
well-documented Python classes and modules
(http://sol.physik.hu-berlin.de/cell/). CELL also provides visualization
utilities and can be interfaced with virtually any ab initio package,
total-energy codes based on interatomic potentials, and more. The usage and
capabilities of CELL are illustrated by a number of examples, comprising a
Cu-Pt surface alloy with oxygen adsorption, featuring two coupled binary
sublattices, and the thermodynamic analysis of its order-disorder transition;
the demixing transition and lattice-constant bowing of the Si-Ge alloy; and an
iterative CE approach for a complex clathrate compound with a parent lattice
consisting of 54 atoms
Investigation of the Pd(1−x)Znx alloy phase diagram using ab initio modelling approaches
The identification of the stable phases in alloy materials is challenging because composition affects the structural stability of different intermediate phases. Computational simulation, via multiscale modelling approaches, can significantly accelerate the exploration of phase space and help to identify stable phases. Here, we apply such new approaches to understand the complex phase diagram of binary alloys of PdZn, with the relative stability of structural polymorphs considered through application of density functional theory coupled with cluster expansion (CE). The experimental phase diagram has several competing crystal structures, and we focus on three different closed-packed phases that are commonly observed for PdZn, namely the face-centred cubic (FCC), body-centred tetragonal (BCT) and hexagonal close packed (HCP), to identify their respective stability ranges. Our multiscale approach confirms a narrow range of stability for the BCT mixed alloy, within the Zn concentration range from 43.75% to 50%, which aligns with experimental observations. We subsequently use CE to show that the phases are competitive across all concentrations, but with the FCC alloy phase favoured for Zn concentrations below 43.75%, and that the HCP structure favoured for Zn-rich concentrations. Our methodology and results provide a platform for future investigations of PdZn and other close-packed alloy systems with multiscale modelling techniques
Investigation of the Pd (1− x ) Zn x alloy phase diagram using ab initio modelling approaches
The identification of the stable phases in alloy materials is challenging because composition affects the structural stability of different intermediate phases. Computational simulation, via multiscale modelling approaches, can significantly accelerate the exploration of phase space and help to identify stable phases. Here, we apply such new approaches to understand the complex phase diagram of binary alloys of PdZn, with the relative stability of structural polymorphs considered through application of density functional theory coupled with cluster expansion (CE). The experimental phase diagram has several competing crystal structures, and we focus on three different closed-packed phases that are commonly observed for PdZn, namely the face-centred cubic (FCC), body-centred tetragonal (BCT) and hexagonal close packed (HCP), to identify their respective stability ranges. Our multiscale approach confirms a narrow range of stability for the BCT mixed alloy, within the Zn concentration range from 43.75% to 50%, which aligns with experimental observations. We subsequently use CE to show that the phases are competitive across all concentrations, but with the FCC alloy phase favoured for Zn concentrations below 43.75%, and that the HCP structure favoured for Zn-rich concentrations. Our methodology and results provide a platform for future investigations of PdZn and other close-packed alloy systems with multiscale modelling techniques
Multi-Chip Integration by Photonic Wire Bonding: Connecting Surface and Edge Emitting Lasers to Silicon Chips
We demonstrate coupling of surface and edge emitting InP lasers to silicon photonic chips using photonic wire bonding. We confirm that back-reflections from the silicon chip do not deteriorate the linewidth of the lasers
Electronic transport properties of thermoelectric materials with a focus on clathrate compounds
Thermoelektrische Bauelemente ermöglichen die Erzeugung von Elektrizität aus überschüssiger Wärme, wie sie in großen Mengen in Geräten und Prozessen entsteht. Effiziente Thermoelektrika benötigen eine hohe thermoelektrische Gütezahl, die durch elektronische und thermische Transporteigenschaften der Materialien bestimmt wird. Die Dissertation untersucht zunächst die elektronischen Transporteigenschaften zweier hochaktueller thermoelektrischer Materialien, des Schichtsystems SnSe und einer komplexen Klathrat-Legierung. Deren theoretische Beschreibung benötigt unterschiedliche Methoden, die während dieses Dissertationsprojektes implementiert, erweitert oder entwickelt wurden. Die Temperaturabhängigkeit der Leitfähigkeit von SnSe wurde mittels der Boltzmann-Transportmethode in Relaxationszeitnäherung untersucht. Wir zeigen, dass nur bei gleichzeitiger Einbeziehung von thermischer Ausdehnung des Kristallgitters und Elektron-Phonon-Streuprozessen eine gute Übereinstimmung mit Experimenten erreicht wird. Die Eigenschaften des Typ-I-Klathrats Ba8AlxSi46-x sind sowohl von der Stöchiometrie als auch von der Al-Konfiguration, d.h. der Anordnung der Al-Atome im Wirtsgitter, abhängig. Für x=16 wurde der Grundzustand als hableitend bestimmt, während Konfigurationen mit höheren Energien metallisch sind. Wir erhalten eine zuverlässige Beschreibung der elektronischen, strukturellen und Transporteigenschaften von Ba8AlxSi46-x bei endlichen Temperaturen durch Mittlungen über Konfigurationen. Mittels einer neu entwickelten Methode zur Berechnung der temperaturabhängigen effektiven Bandstruktur von Legierungen beobachten wir ein temperaturbedingtes Schließen der Bandlücke bei x=16, was mit einem Phasenübergang von partieller Ordnung zu Unordnung bei 582K einher geht. Basierend auf Gedächtnisfunktions-Modellen präsentieren wir ferner eine neue Ab-initio-Methode zur Berechnung der elektrischen Leitfähigkeit von Festkörpern mit einem Unordungspotential beliebiger Kopplungsstärke.Thermoelectric devices convert heat into electricity, thus enabling the reuse of waste heat produced by all kinds of engines. To make this conversion process profitable, materials with a high thermoelectric figure of merit, ZT, are demanded. ZT depends on electronic and thermal transport properties. In this thesis, we study the electronic transport properties of two emerging thermoelectric materials, the layered material SnSe and a complex type-I clathrate alloy. Their reliable description requires different methodologies, that has been implemented, extended, or developed during this PhD project. For SnSe, the temperature dependence of the conductivity and the Seebeck coefficient is studied using the Boltzmann transport approach in the relaxation time approximation. We show that only by simultaneously accounting for thermal lattice expansion and electron-phonon coupling, a good agreement with experiment is reached. The properties of the type-I clathrate Ba8AlxSi46-x are determined, on the one hand, by its composition, and, on the other hand, by the configuration, i.e., the arrangement of the Al atoms in the host lattice. At the charge-compensated composition x=16, the ground-state configuration is found to be semiconducting, while configurations higher in energy are metallic. We obtain a realistic description of the electronic, structural, and transport properties of Ba8AlxSi46-x at finite temperature by using configurational thermodynamic averages. From a newly developed method to compute the finite-temperature effective band structure of alloys, we observe a temperature-driven closing of the band gap for x=16, which is concomitant with a partial order-disorder phase transition at 582K. We further present a novel ab initio memory-function approach for solids that enables the calculation of the electrical conductivity of solids in a disorder potential at arbitrary coupling strength. An application of the developed formalism is demonstrated with the example of sodium