BONDING AND STRUCTURE OF TLGASE2

The bonding and structure of TlGaSe2 was investigated by tight-binding calculations with an extended Huckel Hamiltonian. The bonding between Tl and Se is found to be reasonably covalent, and the region around the Fermi level consists primarily of Tl 6s states antibonding to Se lone pairs. Two structural deformations have been proposed for the ferroelectricity associated with TlGaSe2. One hypothesis involves a D2d squashing motion of the Ga4Se4 adamantane unit. We find no evidence for a double-well potential, and the energy required for deformation is quite steep. On the other hand, a soft, double-well potential exists for the Tl atoms to slide away from a trigonal prismatic to a (3 + 3) environment. This in turn reinforces Tl-Tl bonding. The electronic factors that create this distortion are discussed and analyzed with the aid of a molecular and solid-state model.