Structural stability of quaternary ACuFeS2 (A = Li, K) phases: a computational approach

At ambient conditions, the quaternary sulfides LiCuFeS(2) and KCuFeS(2) present totally different crystal structures: while LiCuFeS(2) crystallizes in a trigonal CaAl(2)Si(2)-type structure, a tetragonal ThCr(2)Si(2)-like structure is found for KCuFeS(2). In this work, we present a computational study describing first the changes in the structural preference of the ACuFe(2) phases as a function of the alkali ion and second, the structural stability of the CuFeS(2) phases obtained by electrochemical removal of the alkali cations from the two ACuFeS(2) compounds. A high copper mobility is found to be responsible for the observed metastability of the layered trigonal CuFeS(2) phase obtained by delithiation of LiCuFeS(2). In contrast, the tetragonal CuFeS(2) structure obtained removing potassium from KCuFeS(2) is predicted to be stable, both from the kinetic and thermodynamic points of view. The possibility of stabilizing mixed Li(x)Cu(1-x)FeS(2) phases with a ThCr(2)Si(2)-type structure and the mobility of lithium in these is also explored.