Abstract
CrAl shows semiconductor-like behavior which has been attributed to a combination of antiferromagnetism and chemical ordering of the Cr and Al atoms on the bcc sublattice. This article presents a detailed theoretical and experimental study of the chemical ordering in CrAl. Using density functional theory within the Korringa-Kohn-Rostoker (KKR) formalism, we consider five possible structures with the CrAl stoichiometry: a bcc solid solution, two-phase C11 , off-stoichiometric C11 CrAl, D0 CrAl, and X-phase CrAl. The calculations show that the chemically ordered, rhombohedrally distorted X-phase structure has the lowest energy of those considered and should, therefore, be the ground state found in nature, while the D0 structure has the highest energy and should not occur. While KKR calculations of the X phase indicate a pseudogap in the density of states, additional calculations using a full potential linear muffin-tin orbital approach and a plane-wave technique show a narrow band gap. Experimentally, thin films of CrAl were grown and the concentration, growth temperature, and substrate were varied systematically. The peak resistivity (2400 -cm) is found for films with , grown epitaxially on a 300 C MgO substrate. At this , a transition between nonmetallic and metallic behavior occurs at a growth temperature of about 400 C, which is accompanied by a change in chemical ordering from X phase to C11 CrAl. These results clarify the range of possible structures for CrAl and the relationship between chemical ordering and electronic transport behavior.
- Received 16 February 2012
DOI:https://doi.org/10.1103/PhysRevB.86.085120
©2012 American Physical Society