Coloured intermetallic compounds LiCu2Al and LiCu2Ga

The Li-containing intermetallic compounds LiCu2Al and LiCu2Ga were prepared by induction heating. Powder Xray diffraction revealed cubic structures for these compounds: Li0.5CuAl0.5, CsCl-type (Pm (3) over barm, Z = 1, a = 2.9283 (2) angstrom); LiCu2Ga, Heusler or Cu2MnAl-type (Fm (3) over barm, Z = 4, a = 5.8617 (4) angstrom). Solid-state( 7)Li NMR spectra confirmed that the Li atoms are situated on a single site rather than being disordered over multiple sites. Optical reflectance spectra show absorption edges in the visible region consistent with the appearance of colour in these compounds: red for LiCu2Al, yellow for LiCu2Ga. Band structure calculations revealed the presence of completely filled Cu 3d states, and suggest that interband transitions from the top of this narrow band (located around 2.5 eV below the Fermi level) to empty bands are responsible for the colour.