A possible re-entrant phase transition at the (001) surface in L12 structure

The relative stability of the different terminations of the (001) surface of A3B compounds with the L12 structure is investigated as a function of both the temperature and the relevant energetic parameters. This is performed using the Ising model and a mean-field approximation. For a strong tendency to A surface segregation, the termination of the A3B compound is an almost pure A surface plane in the whole range of temperatures corresponding to the bulk ordered state. Conversely, for a large tendency to B segregation, the stability of the mixed AB termination is predicted. However, between these two domains, a complex behavior is predicted. One or even two transitions, leading to a re-entrant behavior, can occur as a function of the temperature between the pure A and the mixed AB termination, all these transitions being located in the bulk ordered state. It is suggested that Ir3V is a good candidate to exhibit such a rather surprising behavior. We also investigate the influence of some deviation with respect to the stoichiometry. A transition from the mixed AB to the pure A termination is predicted when the bulk concentration in A element increases.