Vibrational modes and lattice distortion of a nitrogen-vacancy center in diamond from first-principles calculations

We investigate vibrational properties and lattice distortion of negatively charged nitrogen-vacancy (NV) center in diamond. Using the first-principles electronic structure calculations, we show that the presence of NV center leads to appearance of a large number of quasilocalized vibrational modes (qLVMs) with different degree of localization. The vibration patterns and the symmetries of the qLVMs are presented and analyzed in detail for both ground and excited orbital states of the NV center. We find that in the high-symmetry ($C_{3v}$) excited orbital state a pair of degenerate qLVMs becomes unstable, and the stable excited state has lower ($C_{1h}$) symmetry. This is a direct indication of the Jahn-Teller effect, and our studies suggest that dynamical Jahn-Teller effect in the weak coupling regime takes place. We have also performed a detailed comparison of our results with the available experimental data on the vibrations involved in optical emission/absorption of the NV centers. We have directly demonstrated that, among other modes, the qLVMs crucially impact the optical properties of the NV centers in diamond, and identified the most important groups of qLVMs. Our results are important for deeper understanding of the optical properties and the orbital relaxation associated with lattice vibrations of the NV centers.Comment: 10 RevTeX pages, 10 EPS figure

Theory of layered-oxide cathode degradation in Li-ion batteries by oxidation-induced cation disorder

Disorder-driven degradation phenomena, such as structural phase transformations and surface reconstructions, can significantly reduce the lifetime of Li-ion batteries, especially those with nickel-rich layered-oxide cathodes. We develop a general free energy model for layered-oxide ion-intercalation materials as a function of the degree of disorder, which represents the density of defects in the host crystal. The model accounts for defect core energies, long-range dipolar electrostatic forces, and configurational entropy of the solid solution. In the case of nickel-rich oxides, we hypothesize that nickel with a high concentration of defects is driven into the bulk by electrostatic forces as oxidation reactions at the solid-electrolyte interface reduce nickel and either evolve oxygen gas or oxidize the organic electrolyte at high potentials (>4.4V vs. Li/Li+). The model is used in battery cycling simulations to describe the extent of cathode degradation when using different voltage cutoffs, in agreement with experimental observations that lower-voltage cycling can substantially reduce cathode degradation. The theory provides a framework to guide the development of cathode compositions, coatings and electrolytes to enhance rate capability and enhance battery lifetime. The general theory of cation-disorder formation may also find applications in electrochemical water treatment and ion separations, such as lithium extraction from brines, based on competitive ion intercalation in battery materials

Liquid Crystal-Solid Interface Structure at the Antiferroelectric-Ferroelectric Phase Transition

Total Internal Reflection (TIR) is used to probe the molecular organization at the surface of a tilted chiral smectic liquid crystal at temperatures in the vicinity of the bulk antiferroelectric-ferroelectric phase transition. Data are interpreted using an exact analytical solution of a real model for ferroelectric order at the surface. In the mixture T3, ferroelectric surface order is expelled with the bulk ferroelectric-antiferroelectric transition. The conditions for ferroelectric order at the surface of an antiferroelectric bulk are presented

A mathematical model for a house integrated with an elevated Chinese Kang heating system

Chinese kang, a potentially energy-efficient domestic heating system in China, uses high thermal mass to store surplus heat from the stove during cooking and releases it later for space heating. In this paper a preliminary mathematical model is developed for a House Integrated with an Elevated Kang system (HIEK). This model considers the transient thermal behaviors of building envelope, kang system and indoor air. The macroscopic approach is used to model the thermal and airflow process for the elevated kang system. The numerical method for solving the resultant non-linear equations of HIEK is proposed and implemented. The HIEK model is preliminarily evaluated using the measured data from a field survey, and agreement is reasonably good. This model can be used to predict the indoor air temperature for multi-zone HIEK by inputting the basic parameters like geometry, physical properties of building and kang. The model can be easily extended for investigating the thermal performance of a kang system and its influence on indoor thermal environment and building energy consumption. Finally, suggestions for incorporating the kang model into existing building simulation tools are also described.published_or_final_versio

A new understanding of the effect of filler minerals on the precipitation of synthetic C–S–H

The filler effect is the most important physical mechanism of mineral admixtures in the early hydration of cement whose chemical properties greatly affect the precipitation of C–S–H. In this study, calcite, strontianite, magnesite, dolomite, quartz, whewellite and whitlockite were selected as the fillers. The morphology and reaction kinetics of synthetic C–S–H precipitated on the surfaces of different fillers were studied via electron microscopy observations and electrical conductivity and ion concentration measurements. The precipitation rate of C–S–H has a positive correlation with the affinity of Ca2+ for adsorption on the fillers, which can be explained by the nucleation barrier of C–S–H. Extremely ordered honeycomb-like morphology of the C–S–H is found on calcite and strontianite surfaces, while less regular leaf-like or honeycomb-like C–S–H is found on whewellite and whitlockite. The ordered C–S–H pattern is related to the lattice cleavage of the ionic compound filler. In the case of quartz, C–S–H prefers growth along the tangential direction, which is quite different from the normal-direction growth on ionic compounds. The in-plane growth of C–S–H on quartz is believed to be induced by a layer of loosely physically adsorbed Ca2+