Paper EM-WeM5
Study of Cation Exchange and Transport in Crystalline Solids Through Density Functional Theory Calculations

Wednesday, November 1, 2017, 9:20 am, Room 14

Ion transport is closely tied to chemical transformations of crystalline ionic solids such as impurity doping and ion exchange. In order to introduce heteroatoms and other impurities into a crystal it is necessary to displace ions, create vacancies, and fill vacancies many times to achieve a significant degree of transformation. Thus ionic mobility plays a deterministic role both as to how fast a transformation occurs and the achievable extent of doping/exchange in a given system. For example exchange of Cd²⁺ for Cu⁺ in CdSe nanocrystals occurs rapidly and full conversion is ultimately reached. However replacing Cd²⁺ for Hg²⁺ does not reach completion and proceeds more slowly. A full explanation for how these two processes differ must address the mobility difference between Hg²⁺ and Cu⁺ through the Se^2- sublattice.

As a separate example: fast ion conduction relies on there being extensive ion transport in a solid. Such transport could be facilitated by cooperative effects, low (<k_bT) activation energy barriers against ion migration, or a combination of these and other factors. An understanding for how ionic mobilities differ and how they might be manipulated or enhanced will guide the design of materials such as nanoscale semiconductors for applications in optoelectronics, solid state battery electrolytes, and thermoelectrics. Specifically, control over superionic phase transitions can make new materials available for applications in solid state electrolytes by permitting new superionic conductors to be realized at working conditions as opposed to at elevated temperatures.

The approach is to employ density functional theory (DFT) electronic structure calculations for the elucidation of ion conduction trends in Li_xCu_2-xSe. The Li_xCu_2-xSe alloys vary in their fraction of Li⁺ or Cu⁺ cations both of which are mobile and thus apt for a study into tuning ion mobility. Total energies from DFT calculations for a large set of Li_xCu_2-xSe structures permit the identification of factors that promote or inhibit ion mobility. By varying both the cation locations and cell parameters the energetic penalties associated with pushing cations from their relaxed positions into interstitial sites can be obtained. Coupled with a exploration of the Li_xCu_2-xSe compositional range, the DFT energies reveal the best alloys for cation transport that have the lowest energetic penalties and smallest activation barriers. Complementary calculations in which the simulation cell volume is increased or decreased study the effects of tensile and compressive strain respectively; the calculated energies inform how strain and pressure can be used to control ion conduction.