Experimental and ab initio study of the Ag-Li system for energy storage and high-temperature solders

The standard enthalpies of formation of the Ag30Li70 and Ag20Li80 gamma phases (cubic, sg. 1-43 m) were measured using the solution calorimetry method; the standard enthalpy of formation of the Ag10Li90 alloy, corresponding to a hypothetical gamma phase, was determined using the same method. All the measured and literature values of the standard enthalpies of formation of the Ag-Li intermetallic phases were compared with the data calculated using ab initio for stable and unstable phases and Miedema's model. The XRD pattern for a sample with Ag30Li70 was obtained and it was verified that it contained the gamma phase, as expected. Moreover, the structure of the gamma-phase for the Ag4Li9 disordered phase was minimized and the vibrational heat capacity at constant volume and the thermal coefficient of linear expansion were calculated using ab initio methods. The simulated XRD pattern of the latter phase was compared with the experimental pattern. It was determined that only one gamma phase, with a homogeneity range not wider than 0.69 <= x(Li) <= 0.73, is stable for T >= 298 K. Furthermore, besides AgLi-beta with a homogeneity range of 0.50 <= x(Li) <= 0.60 at 298 K, a cubic Ag15Li49-beta is stable for x(Li) = 0.77. The entropy term stabilizes the cubic structure of the AgLi-beta which can be tetragonal at 298 K. The enthalpies and Gibbs energies of formation were calculated at 298, 320, 425, and 600 K. A new "map" of the phases in equilibrium at different temperatures is proposed. (C) 2019 Elsevier B.V. All rights reserved.