Energy Landscape Concepts for Chemical Systems under Extreme Conditions

Traditionally, energy landscape studies of chemical systems deal with an isolated system with no interaction with the environment except possibly non-zero pressure and temperature. This changes drastically, if we consider materials under extreme conditions, since now the interaction with the environment plays a central role. In this work, we present extensions and generalizations of the energy landscape paradigm to chemical systems that strongly interact with their environments. The focus is on the general concepts involved, where we discuss the way to incorporate general external fields, e.g., mechanical stresses, electric and magnetic fields, and fluxes, e.g., electric and thermal currents, and analyze the issue of time-dependent energy landscapes. Finally, possible applications of energy landscape concepts in a variety of chemical and physical systems in strong contact with the environment are discussed, and first examples of landscape studies of materials under extreme conditions are given

Hafnium Carbide: Prediction of Crystalline Structures and Investigation of Mechanical Properties

Hafnium carbide (HfC) is a refractory compound known for its exceptional mechanical, thermal, and electrical properties. This compound has gained significant attention in materials science and engineering due to its high melting point, extreme hardness, and excellent thermal stability. This study presents crystal structure prediction via energy landscape explorations of pristine hafnium carbide supplemented by data mining. Apart from the well-known equilibrium rock salt phase, we predict eight new polymorphs of HfC. The predicted HfC phases appear in the energy landscape with known structure types such as the WC type, NiAs type, 5-5 type, sphalerite (ZnS) type, TlI type, and CsCl type; in addition, we predict two new structure types denoted as ortho_HfC and HfC_polytype, respectively. Moreover, we have investigated the structural characteristics and mechanical properties of hafnium carbide at the DFT level of computation, which opens diverse applications in various technological domains

Modelling structure and properties of amorphous silicon boron nitride ceramics

Silicon boron nitride is the parent compound of a new class of high-temperature stable amorphous ceramics constituted of silicon, boron, nitrogen, and carbon, featuring a set of properties that is without precedent, and represents a prototypical random network based on chemical bonds of predominantly covalent character. In contrast to many other amorphous materials of technological interest, a-Si3B3N7 is not produced via glass formation, i.e. by quenching from a melt, the reason being that the binary components, BN and Si3N4, melt incongruently under standard conditions. Neither has it been possible to employ sintering of μm-size powders consisting of binary nitrides BN and Si3N4. Instead, one employs the so-called sol-gel route starting from single component precursors such as TADB ((SiCl3)NH(BCl2)). In order to determine the atomic structure of this material, it has proven necessary to simulate the actual synthesis route.Many of the exciting properties of these ceramics are closely connected to the details of their amorphous structure. To clarify this structure, it is necessary to employ not only experimental probes on many length scales (X-ray, neutron- and electron scattering; complex NMR experiments; IR- and Raman scattering), but also theoretical approaches. These address the actual synthesis route to a-Si3B3N7, the structural properties, the elastic and vibrational properties, aging and coarsening behaviour, thermal conductivity and the metastable phase diagram both for a-Si3B3N7 and possible silicon boron nitride phases with compositions different from Si3N4: BN = 1 : 3. Here, we present a short comprehensive overview over the insights gained using molecular dynamics and Monte Carlo simulations to explore the energy landscape of a-Si3B3N7, model the actual synthesis route and compute static and transport properties of a-Si3BN7

Zinc oxide: Connecting theory and experiment

Zinc oxide (ZnO) is a material with a great variety of industrial applications including high heat capacity, thermal conductivity and temperature stability. Clearly, it would be of great importance to find new stable and/or metastable modifications of zinc oxide, and investigate the influence of pressure and/or temperature on these structures, and try to connect theoretical results to experimental observations. In order to reach this goal, we performed several research studies, using modern theoretical methods. We have predicted possible crystal structures for ZnO using simulated annealing (SA), followed by investigations of the barrier structure using the threshold algorithm (TA). Finally, we have performed calculations using the prescribed path algorithm (PP), where connections between experimental structures on the energy landscape, and in particular transition states, were investigated in detail. The results were in good agreement with previous theoretical and experimental observations, where available, and we have found several additional (meta)stable modifications at standard, elevated and negative pressures. Furthermore, we were able to gain new insight into synthesis conditions for the various ZnO modifications and to connect our results to the actual synthesis and transformation routes

Identification of promising chemical systems for the synthesis of new materials structure types: An ab initio minimization data mining approach

In this research we performed data exploring for binary compounds with elements from groups V, IV-VI, and III-VII, with the goal to identify chemical systems where the recently proposed “5-5” structure type might be experimentally accessible. Among others, TlF, SnO, SnS, SnSe, GeS, GeSe, PbO, PbS, ZnO and ZnS, were chosen for the study. For each of these systems, a local optimization on ab initio level with the LDA functional was performed for the 5-5 structure type, plus other experimentally observed and theoretically proposed structure types, for comparison. Afterwards, the results were combined with earlier theoretical work involving the 5-5 structure in the earth alkaline metal oxides and the alkali metal halides. As a result, we suggest the GeSe and the ZnO systems as the most suitable ones for synthesizing the 5-5 structure type

Band Gap Engineering of Newly Discovered ZnO/ZnS Polytypic Nanomaterials

We report on a new class of ZnO/ZnS nanomaterials based on the wurtzite/sphalerite architecture with improved electronic properties. Semiconducting properties of pristine ZnO and ZnS compounds and mixed ZnO1−xSx nanomaterials have been investigated using ab initio methods. In particular, we present the results of our theoretical investigation on the electronic structure of the ZnO1−xSx (x = 0.20, 0.25, 0.33, 0.50, 0.60, 0.66, and 0.75) nanocrystalline polytypes (2H, 3C, 4H, 5H, 6H, 8H, 9R, 12R, and 15R) calculated using hybrid PBE0 and HSE06 functionals. The main observations are the possibility of alternative polytypic nanomaterials, the effects of structural features of such polytypic nanostructures on semiconducting properties of ZnO/ZnS nanomaterials, the ability to tune the band gap as a function of sulfur content, as well as the influence of the location of sulfur layers in the structure that can dramatically affect electronic properties. Our study opens new fields of ZnO/ZnS band gap engineering on a multi-scale level with possible applications in photovoltaics, light-emitting diodes, laser diodes, heterojunction solar cells, infrared detectors, thermoelectrics, or/and nanostructured ceramics

Crystal Structure Prediction of the Novel Cr2SiN4 Compound via Global Optimization, Data Mining, and the PCAE Method

A number of studies have indicated that the implementation of Si in CrN can significantly improve its performance as a protective coating. As has been shown, the Cr-Si-N coating is comprised of two phases, where nanocrystalline CrN is embedded in a Si3N4 amorphous matrix. However, these earlier experimental studies reported only Cr-Si-N in thin films. Here, we present the first investigation of possible bulk Cr-Si-N phases of composition Cr2SiN4. To identify the possible modifications, we performed global explorations of the energy landscape combined with data mining and the Primitive Cell approach for Atom Exchange (PCAE) method. After ab initio structural refinement, several promising low energy structure candidates were confirmed on both the GGA-PBE and the LDA-PZ levels of calculation. Global optimization yielded six energetically favorable structures and five modifications possible to be observed in extreme conditions. Data mining based searches produced nine candidates selected as the most relevant ones, with one of them representing the global minimum in the Cr2SiN4. Additionally, employing the Primitive Cell approach for Atom Exchange (PCAE) method, we found three more promising candidates in this system, two of which are monoclinic structures, which is in good agreement with results from the closely related Si3N4 system, where some novel monoclinic phases have been predicted in the past

Predicting Feasible Modifications of Ce2ON2 Using a Combination of Global Optimization and Data Mining

Using a combination of global optimization and data mining, we identify feasible modifications of an ionic Ce-O-N ceramic compound, with composition Ce2ON2, that should at least be metastable at T = 0 K. The energy landscape of Ce2ON2 has been explored for various pressures using empirical potentials followed by ab initio level optimizations, and a multitude of structure candidates has been analyzed. The structure of the energetically lowest modification among these candidates at standard pressure, α-Ce2ON2, is predicted to be similar to the AlCo2Pr2 structure type

Computational discovery of new modifications in scandium oxychloride (ScOCl) using a multi-methodological approach

Scandium oxychloride (ScOCl) has recently become of interest as an advanced material with possible applications in solid oxide fuel cells, photocatalysis, and electronic devices, as are oxyhalides of various transition metals. In the present study, crystal structure prediction has been utilized to fully investigate the energy landscape of ScOCl. A multi-methodological approach has been used consisting of a combination of two search methods, where the final structure optimization has been performed on ab initio level using DFT-LDA and hybrid PBE0 functionals. The experimentally observed α-ScOCl phase has been found as well as several additional structure candidates at high pressures and/or temperatures. A successful synthesis of these novel ScOCl modifications would have the potential for extending the scientific, technological and industrial applications of ScOCl

Energy Landscapes 2019

This article summarizes the presentations delivered at the Energy Landscapes Conference held in Belgrade, Serbia, from 26 to 30 August 2019. The focus of the conference was on the present state of the art in theoretical energy landscape approaches, and their applications in the fields of chemistry, physics, biology, and materials science in general. The presentations were organized around some of the hot topics, such as applications from spectroscopy to the solid-state, folding and misfolding of proteins, DNA and RNA, multiscale modeling, materials under extreme pressure/temperature conditions, designing landscapes for self-assembly and multifunctional systems, landscapes for machine learning, atomic, molecular, colloidal and nanoalloy clusters