First principles calculations of behavior of lithium and interaction with helium in molybdenum

Abstract

The engineering design of liquid lithium plasma-facing components has been proposed and practiced as an effective solution to the critical issues resulted from interactions between plasma- and wall materials in tokamak experiments. Various defects have been found in this setting but are largely unknown about their formation mechanisms and effects on the wall materials’ properties. We elect to study behavior of impurity Li atoms in a molybdenum (a typical wall material in tokamak systems) matrix, and interaction of a Li atom with He or additional Li atoms in the molybdenum matrix, as well as electronic and elastic properties of these molybdenum-based solid solutions through ab initio density-functional-theory calculations. According to our results, a single Li atom prefers to occupy a substitutional site in the molybdenum matrix, and there is a strong binding energy between it and He atoms at octahedral interstitial sites, or other Li atoms at tetrahedral interstitial sites. Such attraction is expected to lead to formation of aggregations of the types LiHem and Lin+1. The electronic properties of these molybdenum-based solid solutions are discussed by analyzing electron density of states and Bader charges. Bulk modulus, shear modulus, Young’s modulus, Poisson ratio, and anisotropic factor of the molybdenum-based solid solutions are evaluated from elastic constants using Voigt-Reuss-Hill average scheme. An Ashby map of the ratio of Young’s modulus to density versus the ratio of bulk modulus to shear modulus is constructed to show the inverse correlation between stiffness and ductility.