Static polarizability of molecular materials: environmental and vibrational contributions

Modeling the dielectric behavior of molecular materials made up of large pi-conjugated molecules is an interesting and complex task. Here we address linear polarizabilities, and the related dielectric constant, of molecular crystals and aggregates made up of closed-shell pi-conjugated molecules with either a non-polar or largely polar ground-state, and also examine the behavior of mixed-valence (or charge-transfer) organic salts. We recognize important collective phenomena due to supramolecular interactions in materials with large molecular polarizabilities, and underline large vibrational contributions to the polarizability in materials with largely delocalized electrons.Comment: 18 pages, including 9 figure

Effect of dispersion interactions on the properties of LiF in condensed phases

Classical molecular dynamics simulations are performed on LiF in the framework of the polarizable ion model. The overlap-repulsion and polarization terms of the interaction potential are derived on a purely non empirical, first-principles basis. For the dispersion, three cases are considered: a first one in which the dispersion parameters are set to zero and two others in which they are included, with different parameterizations. Various thermodynamic, structural and dynamic properties are calculated for the solid and liquid phases. The melting temperature is also obtained by direct coexistence simulations of the liquid and solid phases. Dispersion interactions appear to have an important effect on the density of both phases and on the melting point, although the liquid properties are not affected when simulations are performed in the NVT ensemble at the experimental density.Comment: 8 pages, 5 figure

Non-additive electronic polarizabilities of ionic liquids: charge delocalization effects

Electronic charge delocalization on the molecular backbones of ionic liquid-forming ions substantially impacts their molecular polarizabilities. Density functional theory calculations of polarizabilities and volumes of many cations and anions are reported and applied to yield refractive indices of 1216 ionic liquids. A novel expression for the precise estimation of the molecular volumes of the ionic liquids from simulation data is also introduced, adding quadratic corrections to the usual sum of atomic volumes. Our significant findings include i) that the usual assumption of uniform, additive atomic polarizabilities is challenged when highly mobile electrons in conjugated systems are present, and ii) that cations with conjugated large carbon chains can be used together with anions for the design of ionic liquids with very high refractive indices. A novel relation for the polarizability volume is reported together with a refractive index map made up of the studied ionic liquidsThis work was supported by Ministerio de Economia y Competitividad (MINECO) and FEDER Program through the project MAT2017-89239-C2-1-P; Xunta de Galicia and FEDER (ED431D 2017/06, ED431E2018/08, GRC 508 ED431C 2020/10). C. D. R. F. thanks the support of Xunta de Galicia through the grant ED481A-2018/032. We also thank the Centro de Supercomputacion de Galicia (CESGA) facility, Santiago de Compostela, Galicia, Spain, for providing the computational resources employed in this workS

Ga+, In+ and Tl+ Impurities in Alkali Halide Crystals: Distortion Trends

A computational study of the doping of alkali halide crystals (AX: A = Na, K; X = Cl, Br) by ns2 cations (Ga+, In+ and Tl+) is presented. Active clusters of increasing size (from 33 to 177 ions) are considered in order to deal with the large scale distortions induced by the substitutional impurities. Those clusters are embedded in accurate quantum environments representing the surrounding crystalline lattice. The convergence of the distortion results with the size of the active cluster is analyced for some selected impurity systems. The most important conclusion from this study is that distortions along the (100) and (110) crystallographic directions are not independent. Once a reliable cluster model is found, distortion trends as a function of impurity, alkali cation and halide anion are identified and discussed. These trends may be useful when analycing other cation impurities in similar host lattices.Comment: LaTeX file. 7 pages and 2 pictures. Accepted for publication in J. Chem. Phy

Relative energetics and structural properties of zirconia using a self-consistent tight-binding model

We describe an empirical, self-consistent, orthogonal tight-binding model for zirconia, which allows for the polarizability of the anions at dipole and quadrupole levels and for crystal field splitting of the cation d orbitals. This is achieved by mixing the orbitals of different symmetry on a site with coupling coefficients driven by the Coulomb potentials up to octapole level. The additional forces on atoms due to the self-consistency and polarizabilities are exactly obtained by straightforward electrostatics, by analogy with the Hellmann-Feynman theorem as applied in first-principles calculations. The model correctly orders the zero temperature energies of all zirconia polymorphs. The Zr-O matrix elements of the Hamiltonian, which measure covalency, make a greater contribution than the polarizability to the energy differences between phases. Results for elastic constants of the cubic and tetragonal phases and phonon frequencies of the cubic phase are also presented and compared with some experimental data and first-principles calculations. We suggest that the model will be useful for studying finite temperature effects by means of molecular dynamics.Comment: to be published in Physical Review B (1 march 2000

An experimental and computational research on the optical properties of ionic liquids

Este proxecto céntrase no estudo do efecto que a dopaxe química ou física exerce sobre as propiedades ópticas dun conxunto de líquidos iónicos. A dopaxe física con determinadas sales de metais dos grupos IA e IIA da lugar á formación de microestruturas similares ás observadas en cristais líquidos desordeados e facilmente detectables mediante microscopía de polarización. Mediante aliñamento mecánico ou inducido por campo eléctrico pódense ordear. Estas estruturas ordeadas presentan índices de refracción que dependen da dirección que poden ser caracterizados. A birrefrinxencia e o intervalo de temperatura no que o material se comporta como un cristal líquido iónico depende do metal do sal engadido. A adición de metais de transición tanto en proporción estequiométrica como non estequiométrica, pode dar lugar a líquidos iónicos con altos índices de refracción, termocromáticos, luminiscentes ou cunha resposta non lineal elevada ante campos electromagnéticos intensos

Lattice dynamics and phase transitions in Sn₂P₂S(Se)₆ ferroelectric crystals

Polarizable ion model was used in describing the lattice dynamics in the Sn₂P₂S₆ and Sn₂P₂Se₆ ferroelectric crystals and their solid solutions. There has been obtained a good agreement of the calculated phonon spectra with the experimental data of Raman and Brillouin spectroscopies, neutron scattering and ultrasound experiments. Paraelectric phase instability has been modelled by negative charge transferring from the tin cations to the chalcogen ions, and by increasing the polarizabilities of the latter. The ferroelectric phase has been stabilized by tin atom displacement from centrosymmetric positions and a partial reversal recharging between tin and chalcogen ions. The effective charges decreasing by moving from Sn₂P₂S₆ to Sn₂P₂Se₆ compounds determine the shift of condensation wave vector of soft optic phonons in paraelectric phase from q = 0 in sulfur compound to q > 0 in selenium compound. Such a change of dynamical instability character is connected with the appearance of incommensurate phase in Sn₂P₂(SexS₁₋x)₆ solid solution under x > 0.28. The “two-mode” instability emerged as possible in the vicinity of Lifshitz point – there have been modelled the condensation of soft optic mode with q ≈ 0 and mixed optic and transversal acoustic phonons near q > 0. This fact requires further experimental and theoretical investigationsМодель поляризованих iонiв застосована для опису динамiки ґратки сеґнетоелектричних кристалiв Sn₂P₂S₆ та Sn₂P₂Se₆, а також їх твердих розчинiв. Отримано добре узгодження розрахованих фононних спектрiв з даними спектроскопiї комбiнацiйного та мандельштам–брiллюенiвського розсiювання, ультразвукових дослiджень, розсiювання нейтронiв. Нестабiльнiсть параелектричної фази моделюється перенесенням негативного заряду з катiонiв олова на атоми халькогенiв, та збiльшенням поляризованостi останнiх. Сеґнетоелектрична фаза стабiлiзується зсувом атомiв олова з центросиметричних позицiй та частковою зворотною перезарядкою мiж iонами халькогену та олова. Зменшення iонних зарядiв при переходi вiд Sn₂P₂S₆ до Sn₂P₂Se₆ визначає зсув хвильового вектора конденсацiї м’яких оптичних фононiв параелектричної фази вiд q = 0 в сульфiднiй сполуцi до q > 0 в селенiднiй сполуцi. Така змiна характеру динамiчної нестабiльностi пов’язана з виникненням неспiвмiрної фази в твердих розчинах Sn₂P₂(SexS₁₋x)₆ при x > 0.28. Виявлена можливiсть “двомодової” нестабiльностi в околi точки Лiфшиця — тут моделюється конденсацiя м’якої оптичної моди при q ≈ 0 та змiшаних оптичних та поперечних акустичних фононiв при q > 0, що потребує подальших експериментальних i теоретичних дослiджень