Lattice energy | EPFL Graph Search

In chemistry, the lattice energy is the energy change upon formation of one mole of a crystalline ionic compound from its constituent ions, which are assumed to initially be in the gaseous state. It is a measure of the cohesive forces that bind ionic solids. The size of the lattice energy is connected to many other physical properties including solubility, hardness, and volatility. Since it generally cannot be measured directly, the lattice energy is usually deduced from experimental data via the Born–Haber cycle. Lattice energy and lattice enthalpy The concept of lattice energy was originally applied to the formation of compounds with structures like rocksalt (NaCl) and sphalerite (ZnS) where the ions occupy high-symmetry crystal lattice sites. In the case of NaCl, lattice energy is the energy change of the reaction : Na+ (g) + Cl− (g) → NaCl (s) which amounts to −786 kJ/mol. Some chemistry textbooks as well as the widely used CRC Handbook o

Lattice Energy as Emerging General Parameter Describing Ionic Organic Semiconducting Materials Properties

Donatas Gesevicius

Cyanine dyes represent a class of ionic functional materials used by a broad heterogeneous science community contain-ing biologists, chemists, physicists and materials scientists. Their diverse application opportunities are ranging from fluorescence markers, nonlinear optics, CD-R data storage, photography and organic electronics. Therefore the development of cyanine dyes and their applications in chemistry, engineering, medicine and pharmacology is growing continuously. A typical leitmotif of a cyanine dye contains two nitrogen heterocyclic rings joined by a conjugated chain of carbon atoms. Due to the synthetic approach, quaternary salt precursors are required resulting in ionic compounds. Such chromophores are positively charged and require a counter anion to maintain the overall charge neutrality. Several reports were presenting certain anions as efficiency enhancers in cyanine dye based organic electronic devices. Despite their long presence in science, there is a lack of knowledge behind the mechanisms causing these efficiency enhancements. Furthermore, in the field of organic electronics, the processability of cyanine dyes towards electronic devices was restricted to solution-based methods. In the present thesis work the three main challenges in cyanine dye research, namely: efficient ion exchange procedures, mechanisms behind organic electronics figures of merit enhancement as well as the introduction of physical vapour deposition method have been addressed. An efficient halide-for-anion exchange method with the potential for industry scale up was introduced. This allowed modifying a cationic cyanine chromophore with organic or inorganic anions yielding six salts. The following investigations of these six anions by either varying organic moieties or negative charge delocalisation paved the way towards a generalized concept describing the semiconducting properties of this cationic cyanine chromophore. The lattice energy was established as a universal parameter describing cyanine salt semiconducting properties on a universal energy scale. Such decoupling of an ionic structure from its Lewis formula allows future predictions of semiconducting properties which can be widened to all organic ionic functional materials. Furthermore weakly coordinating anion influence on the volatility of a cyanine chromophore established physical vapour deposition as thin film fabrication method. Using co-evaporation of the cyanine dye and the fullerene C60 a suitable bulk heterojunction morphology was obtained. This resulted in the fabrication of the first fully vacuum deposited cyanine dye bulk heterojunction device.

EPFL2018

Numerical investigations of 3D CFTs using conformal bootstrap methods

Marten Jan Reehorst

Conformal field theories (CFTs) play a very significant role in modern physics, appearing in such diverse fields as particle physics, condensed matter and statistical physics and in quantum gravity both as the string worldsheet theory and through the AdS/CFT correspondence. In recent years major breakthroughs have been made in solving these CFTs through a method called numerical conformal bootstrap. This method uses consistency conditions on the CFT data in order to find and constrain conformal field theories and obtain precise measurements of physical observables. In this thesis we apply the conformal bootstrap to study among others the O(2)- and the ARP^3- models in 3D. In the first chapter we extend the conventional scalar numerical conformal bootstrap to a mixed system of correlators involving a scalar field charged under a global U(1) symmetry and the associated conserved spin-1 current J. The inclusion of a conserved spinning operator is an important advance in the numerical bootstrap program. Using numerical bootstrap techniques we obtain bounds on new observables not accessible in the usual scalar bootstrap. Concentrating on the O(2) model we extract rigorous bounds on the three-point function coefficient of two currents and the unique relevant scalar singlet, as well as those of two currents and the stress tensor. Using these results, and comparing with a quantum Monte Carlo simulation of the O(2) model conductivity, we give estimates of the thermal one-point function of the relevant singlet and the stress tensor. We also obtain new bounds on operators in various sectors. In the second chapter we investigate the existence of a second-order phase transition in the ARP^3 model. This model has a global O(4) symmetry and a discrete Z_2 gauge symmetry. It was shown by a perturbative renormalization group analysis that its Landau-Ginzburg-Wilson effective description does not have any stable fixed point, thus disallowing a second-order phase transition. However, it was also shown that lattice simulations contradict this, finding strong evidence for the existence of a second-order phase transition. In this chapter we apply conformal bootstrap methods to the correlator of four scalars t transforming in the traceless symmetric representation of O(4) in order to investigate the existence of this second order phase transition. We find various features that stand out in the region predicted by the lattice data. Moreover, under reasonable assumptions a candidate island can be isolated. We also apply a mixed t-s bootstrap setup in which this island persists. In addition we study the kink-landscape for all representations appearing in the t times t OPE for general N. Among others, we find a new family of kinks in the upper-bound on the dimension of the first scalar operator in the "Box" and "Hook" representations.

EPFL2020

Electronic and structural characterization of barrier-type amorphous aluminium oxide

Francesco Ambrosio, Claudia Cancellieri, Fabio Evangelisti, Roland Hauert, Raheleh Partovi Nia, Alfredo Pasquarello, Patrik Schmutz

The structural, electronic, and dielectric properties of barrier -type anodic Al oxides prepared with various thicknesses have been investigated. The planar and cross morphologies as well as the surface charge distribution were investigated as a function of the voltage applied during growth, i.e. the thickness. Three distinct anodizing potential domains can be clearly identified. A transition in ionic mobilities induces, at anodizing voltage higher than 100 V, the built-up of space charge region in the anodic films. A correlated increased oxide density with growing anodization potential is consistently pointed out by Volta Potential Difference and XPS/Auger measurements. The full dielectric response of the oxides, including both the lattice and electronic components, has been accessed by electrochemical impedance spectroscopy and Auger analysis. The lattice contribution of the dielectric constant is found to be strongly affected by the growth voltage, as a clear increase in the lattice component of the dielectric constant is observed, reaching values close to those of bulk crystalline Al2O3 structures. At variance, the electronic contribution is only slightly affected. The results have been compared to dielectric constants calculated for atomistic amorphous models within the framework of density functional theory. It is shown that amorphous Al2O3 models of higher density lead to dielectric constants that could explain the trend observed in theseries of barrier-type anodic oxides prepared in this work. (C) 2016 Elsevier Ltd. All rights reserved.