First-order order-disorder transition in lithium ferrospinel

Abstract

The order-disorder transition in lithium ferrospinel (LFS) is studied in terms of the Landau theory of phase transitions. By analysing the ordering and the structural changes, two order parameters and corresponding Landau free energy are constructed. It is shown that: (i) the ordering or the Li ions and the structural changes occur simultaneously; (ii) the structural change is associated with softening of a phonon mode with k=(1,1,0) 2 pi /a; (iii) the transition is of the first order due to the occurrence of the cubic terms in the Landau potential. Assuming a weak coupling between the two order parameters the transition temperature, the hysteresis range and the values of the order parameters are estimated.