Electronic Structure Calculations for Cr 1 - xAlx

Cr3Al shows semiconducting behavior that has not been explained previously. We used density functional theory calculations, performed at the Cornell NanoScale Facility to study the effect of chemical ordering and magnetism on the semiconducting behavior. The calculations show that the proposed chemically ordered Cr3Al structure in the Cr1-xAlx phase diagram is the lowest energy structure of those considered. In addition, the band structure shows a pseudogap, consistent with experimentally observed transport properties. Antiferromagnetic ordering is also shown to be crucial for formation of the pseudogap. These results suggest that chemical ordering and antiferromagnetism work together to cause the unexpected semiconducting behavior in Cr3Al. Summary of Research: Alloys and compounds made of metallic elements are generally expected to be metallic, and indeed most are. However, some such compounds are semiconducting or semimetallic, such as RuAl2 and Fe2VAl [1]. In theory, any compound with an even number of valence electrons in the primitive unit cell can be semiconducting because the electrons can completely fill the valence band. Transition metals usually have several overlapping bands at the Fermi energy (EF) so even in compounds with an even number of electrons typically several bands are partially filled. For an intermetallic compound to be semiconducting, hybridization must shift the bands in a fortuitous way, leaving a gap at EF. When intermetallic compounds do have a gap at EF, they are the subject of significant study. The gap can be exploited for applications, for example, intermetallic semiconductors are attractive for thermoelectric devices due to their typically small gaps and large Seebeck coefficients (ex. ZrNiSn) [2]. In magnetic compounds, the gap is asymmetric with spin; if a gap occurs at EF for one spin but not the other, the result is a half metal (ex. Co2MnAl) [3]. Half­metals are important for spintronics applications such as spin transistors and non­ volatile logic. Cr1­xAlx, where x = 0.15­0.26, shows semiconducting behavior that has not been explained until now. A maximum resistivity of 3600 μΩ­cm occurs, with a negative temperature coefficient of resistivity [4]. A large Hall coefficient and a small electronic specific heat is observed [4,5], all hallmarks of semiconducting behavior. In addition, Cr1­xAlx is antiferromagnetic for x = 0.0­0.50. The maximum resistivity and Hall coefficient and minimum electronic specific heat all occur around x = 0.25, with a plateau in the magnetic susceptibility at that point [6], suggesting an ordered compound Cr3Al is responsible for the behavior. Our previous results included photoemission measurements showing a small semiconducting gap or pseudogap (around 95 meV) at EF in a Cr0.80Al0.20 thin film. Our density functional theoretical results showed that a disordered Cr0.80Al0.20 alloy in theory should show a pseudogap in the density of states, however the pseudogap is not as deep as expected [7]. This led us to further calculations which carefully consider the effect of possible chemical ordering and of the antiferromagnetism on the electronic structure. DFT calculations were done on the Intel cluster at the Cornell NanoScale Facility using the AkaiKKR code, a full­potential DFT Green’s function approach based on the Korringa­Kohn­Rostoker multiple­scattering technique [8­10]. The scalar relativistic approximation was used and disorder in the bcc solid solution was treated using the coherent potential approximation (CPA) [11,12]. The generalized gradient approximation (GGA) was used to approximate the exchange­correlation energy [13]. The atoms in Cr3Al occupy the sites of a bcc lattice, like Cr. We performed density functional theoretical calculations to