  The generalized gradient approximation (GGA) scheme in the first-principles
calculations are used to study the effect of L21 and XA ordering on the phase
stability, half-metallicity and magnetism of Co2FeAl (CFA) Heusler alloy.
Various possible hypothetical structures: L21-I, L21-II, XA-I, and XA-II were
prepared under the conventional L21 and inverse XA phases by altering the
atomic occupancies at their Wyckoff sites. It is found that the XA-II phase of
CFA is the most stable phase energetically among all the structures. The
electronic structure calculations without U show the presence of half-metallic
(HM) ground state only in L21-1 structure and the other structures are found to
be metallic. However, the electronic structures of CFA are significantly
modified in the presence of U, although the total magnetic moments per cell
remained the same and consistent with the Slater-Pauling (SP) rule. The
metallic ground states of CFA in L21-II and XA-II structures are converted into
the half-metallic ground states in presence of U but remained the same
(metallic) in XA-I structure. The results indicate that the electronic
structures are not only dependent on the L21 and XA ordering of the atoms but
also depend on the choice of U values. So experiments may only verify the
superiority of GGA+U to GGA.
