DORI Reveals the Influence of Noncovalent Interactions on Covalent Bonding Patterns in Molecular Crystals Under Pressure

The study of organic molecular crystals under high pressure provides fundamental insight into crystal packing distortions and reveals mechanisms of phase transitions and the crystallization of polymorphs. These solid-state transformations can be monitored directly by analyzing electron charge densities that are experimentally obtained at high pressure. However, restricting the analysis to the featureless electron density does not reveal the chemical bonding nature and the existence of intermolecular interactions. This shortcoming can be resolved by the use of the DORI (density overlap region indicator) descriptor, which is capable of simultaneously detecting both covalent patterns and noncovalent interactions from electron density and its derivatives. Using the biscarbonyt[14]annulene crystal under pressure as an example, we demonstrate how DORI can be exploited on experimental electron densities to reveal and monitor changes in electronic structure patterns resulting from molecular compression. A novel approach based on a flood-fill-type algorithm is proposed for analyzing the topology of the DORI isosurface. This approach avoids the arbitrary selection of DORI isovalues and provides an intuitive way to assess how compression packing affects covalent bonding in organic solids.

Kintetic modelling of natural gas auto-ignition under engine-like conditions

Daniel Favrat, Stefan Heyne, Jan-Bernhard Vos

The modelling of natural gas combustion with detailed as well as reduced kinetic mechanisms has been subject to many publications already. Most of the mechanisms are validated at higher temperatures and ambient pressures. Only recently - in parallel to a gain of interest in the homogeneous charge compression ignition (HCCI) engine concept - there are more efforts trying to reproduce the low temperature chemistry, and in particular autoignition of natural gas under engine-like conditions. This work is part of a project on stationary natural gas engines equipped with unscavanged prechambers. It has been demonstrated that it is possible to reduce the emissions below the Swiss limits both with natural gas and biogas with the spark ignition prechamber, while keeping high efficiencies of around 38 %. We are now aiming at converting the prechamber from spark ignition to autoignition leading to longer maintenance intervals in the case of stationary natural gas engines and to a probable cleaner combustion - HCCI like combustion inside the prechamber and faster combustion in the main chamber due to an earlier arrival of the flame front - compared to prechamber spark ignition. In order to be able to model the fluid dynamics and the chemistry inside the cylinder, a good chemical model is one of the most important aspects. To obtain an appropriate mechanism - by validating and/or optimising existing mechanisms - experiments have been conducted in the rapid compression machine facility at the University of Lille with different mixtures of methane, ethane and propane. The relative air-to-fuel ratio λ varied from 1 to 1.6, the temperature and pressure at the end of compression varied from 850 to 925 K and 13 to 21 bar respectively. The compression ratio was constantly kept at 9.2 for all experiments and the final temperature and pressure were defined by varying the charge and the composition of the inert gases (Ar/N2). The compression time was between 50 and 60 ms for all experiments. For modelling the ignition delay a zero-dimensional model assuming homogeneity was used, based on the measured pressure and calculated core temperature at the end of compression of each experiment. The chemical solver package used for the simulations is CHEMKIN. Different mechanisms (PRF, GRI3.0, Konnov, …) have been tested. Most of the mechanisms are mainly validated at higher temperature ranges and therefore their applicability to ignition delay prediction in our conditions is questionable. Several attempts to adjust the GRI mechanism (version 1.2) to the lower temperature regime exist, whereof we chose the UBCMech and Petersen mechanism (GRI+RAMEC) to test on our simulations. Both mechanisms initially were developed based on pure methane mixtures while the UBCMech recently was extended to model natural gas as well. The experimentally obtained ignition delays vary from 40 ms at high pressures and temperatures to more than 300 ms for the lower regime of the experimental space. For the ignition delays up to 60 ms the best fit was obtained by a reduced version of PRF accounting only for the C1 to C3 compounds. Konnov’s mechanism and UBCMech are under predicting the experimental ignition delays while the GRI3.0 mechanism (with and without RAMEC sub module) over predicts the ignition delay for the natural gas mixtures. It has to be mentioned that the GRI3.0+RAMEC mechanism as well as the initial mechanism of Petersen (GRI1.2+RAMEC) on the other hand under predict the experimental ignition delay for pure methane mixtures. This indicates that the low temperature chemistry for methane that has been added in the RAMEC sub module is enhancing ignition too strong. Replacing part of methane in pure methane mixtures by equivalent amounts of ethane and propane - know as ignition promoters - lead to longer ignition delays in the simulations. But as the GRI3.0+RAMEC mechanism (without nitrogen chemistry) is the smallest of all tested mechanisms considering number of species and reactions, it still is an interesting option for coupling it to a computational fluid dynamics code as we are planning to do. We though performed a sensitivity analysis to obtain a set of the most dominating reactions and used a multi-objective optimisation algorithm to optimize the reaction rates within their limits of uncertainty. The results show a good fit for the short ignition delays of natural gas mixtures but still an under prediction of the methane delays. For the longer ignition delays the assumption of a constant core temperature during the post compression phase becomes questionable and the under prediction of all mechanisms of the experimental delays is an obvious consequence. A heat loss model predicting the temperature evolution has to be implemented in the simulations to properly simulate the ignition delays.

2006

Infrared- and Raman-Spectroscopy Measurements of a Transition in the Crystal Structure and a Closing of the Energy Gap of BiTeI under Pressure

Ana Akrap, Gabriel Albert Autes, Dirk Van der Marel, Oleg Yazyev

BiTeI is a giant Rashba spin splitting system, in which a noncentrosymmetric topological phase has recently been suggested to appear under high pressure. We investigated the optical properties of this compound, reflectivity and transmission, under pressures up to 15 GPa. The gap feature in the optical conductivity vanishes above p similar to 9 GPa and does not reappear up to at least 15 GPa. The plasma edge, associated with intrinsically doped charge carriers, is smeared out through a phase transition at 9 GPa. Using high-pressure Raman spectroscopy, we follow the vibrational modes of BiTeI, providing additional clear evidence that the transition at 9 GPa involves a change of crystal structure. This change of crystal structure possibly inhibits the high-pressure topological phase from occurring.

American Physical Society2014

Structural properties of organometalic halide perovskite and their application in photodetectors

Anastasiia Glushkova

Hybrid organometallic halide perovskites have been intensively investigated in the past years as highly efficient light harvesters for various optoelectronic applications for both sensing and emitting light. However, many open questions remain regarding the crystallization process, environmental impact and implementation of the material into electrical circuits. The central theme of this PhD dissertation is the structural characterization of hybrid organicinorganic perovskites and device fabrication based on these materials. The focus is on methylammoniumlead iodide, CH3NH3PbI3 as one of the most efficient photovoltaic materials. External factors such as temperature, pressure, humidity, etc. affect the materials properties. Thus, it is crucial to study their influence for further applications. In this thesis, it will be shown, that under high pressure (20 GPa) inert gases as Ar and Ne form high-pressure-induced compounds with CH3NH3PbI3. For Ne pressure transmitting media such high-pressure transformation is reversible and the Ne-incorporated compoundis even stable at ambient conditions after decompression. We show that when applying repeated phase transitions during thermal cycling around both 330 and 160 K, CH3NH3PbI3 does not return to the initial state. Instead, at 330 K, the crystal stabilizes an incommensurately modulated tetragonal phase with successive transitions. On the other hand, performing thermal cycling around 160 K generates an increase in the concentration of domains of different phases. In addition, we studied the influence of thermal treatment on the crystal structure in the absence of any phase transition. For that reason, the lower dimensional compound of NH3CH2CH2NH3PbI4 was synthesized and characterized before and after annealing process. We observe and discuss a correlation between photoconductivity and increased disorder. In this work a novel approach of aerosol jet printing deposition of CH3NH3PbI3 has been developed. Making use of intermediate phases of the crystallization process, this deposition method enables the creation of 3D structures of organic-inorganic perovskites on various surfaces. This technique was successfully used in the fabrication of heterostructures based on CH3NH3PbI3 and graphene. Due to large trap-assisted photogain, these heterostructures are very promising for photoconductors. Taking into account the strong X-ray stopping power of the high atomic number Pb and I, devices based on CH3NH3PbI3/graphene heterostructures are excellent for X-ray detection. Such X-ray detectors demonstrate a record high sensitivity value of 2.2 x10^8 uC/(Gy cm^2).