Abstract
High yields of the novel BaHg2Tl2 are achieved from reactions of the appropriate cast alloys at ~ 400 �C. (Isotypic SrHg2Tl2 also exists.) The tetragonal barium structure (P42/mnm, a = 10.417, c = 4.952 �) was refined from both single crystal X-ray and neutron powder diffraction data in order to ensure the atom site assignments although distances and atom site potentials all also supportive of the results. The Hg and Tl network atoms are distinctive in their functions and bonding. Parallel chains of Hg hexagons and of Tl tetrahedra along c are constructed of like polyhedra that share opposed like edges, and these are in turn interconnected by Hg�Tl bonds. Overall, Tl�Tl bonds per cell exceed Hg�Hg by 20:12 although these are ~1:2 in bonding according to the −ICOHP (≈ overlap population) values. Barium is bound within a close 15-atom polyhedron, 12 of which are the more electronegative Hg. LMTO-ASA calculations show that scalar relativistic effects are particularly important for Hg (5d-6s mixing in Hg�Hg and Hg�Tl bonding), whereas relatively separate (lower) 5d and 6s states of Tl are more important in Tl�Tl interactions, appropriate to the 6s2 6p1 ground state for the atom. The highest occupied states define a dominantly 6p band, and the phase is metallic and Pauli-like paramagnetic. Binary active metal � mercury systems that have been studied theoretically all have distinctly higher electron populations per mercury.