Electronic structure of intermetallic LiTl and NaTl

The charge distribution, Knight shift Kₛ, and total energy E of intermetallic LiTl and NaTl have been calculated by the relativistic augmented-plane-wave method. For the B32-phase NaTl it is found that the valence-electron bands are separated by a band gap. The lower valence bands are built up by the 6s electrons of Tl forming covalent s-like bands within the diamondlike Tl sublattice. These 6s bands give a negative contribution to Kₛ(Tl) of -0.52% due to the exchange polarization effect. The valence bands above the 6s bands are sp bands of predominantly metallic character. In the B2-phase LiTl all valence bands overlap and the covalent character of the lower valence bands is much less pronounced than in NaTl. Theoretically the following values for Kₛ are found: Kₛ(LiTl) = 0.003%, Kₛ(LiTl)=1.0%, Kₛ(NaTl) = 0.002%, and Kₛ(NaTl)=-0.14%. The agreement with the experimental results is good besides Kₛ(NaTl) [Kₛ(expt., NaTl)=-0.96%]. From the shape of the electron bands near the Fermi surface one could conclude that the difference between Kₛ(theor., NaTl) and Kₛ(expt. NaTl) is caused by a diamagnetic contribution to Kₛ. To obtain some insight in the stability of Zintl phases (B32 structure), the energy difference {Δ}E=E(B2)-E(B32) is calculated. One obtains {Δ}E(LiTl) =-0.15 eV per atom and {Δ}E(NaTl)=+0.005 eV per atom. The physical mechanisms which lead to the different signs in {Δ}E are discussed.