Element lines: bonding in the ternary gold polyphosphides, Au(2)MP(2) with M = Pb, Tl, or Hg.

We present a theoretical study of the electronic structures of Au(2)PbP(2), Au(2)TlP(2), and Au(2)HgP(2). Structurally, these compounds contain a framework of condensed Au(2)P(6) and Au(4)P(6) rings forming parallel channels, which are filled by lead, thallium, or mercury atoms. Given the linear coordination of the Au atoms and the existence of zigzag -[P-P-P]- singly-bonded chains in them, these materials present us with a rare instance of approximately linear, one-dimensional, and zerovalent element (Pb, Tl, or Hg) chains with a variable electron count. The Pb-Pb, Tl-Tl, and Hg-Hg element lines in these structures have somewhat longer bond lengths than their respective single bond lengths or their separations in (calculated) isolated chains. Yet, the zerovalent element interactions are prominent and are responsible for the metallicity of some of these materials. In the calculations, both Au(2)PbP(2) and Au(2)TlP(2) emerge as metallic, whereas Au(2)HgP(2) is a semiconductor. The isolated element chains do not undergo a pairing (Peierls) distortion nor do the chains in the three-dimensional ternaries. The small barrier for mobility of the Pb, Tl, and Hg atoms along the chain axis may explain the large thermal parameters observed in the crystal structures along the chain axis (and the need to use fractional occupations of several positions along the chain). These ternaries may show one-dimensional liquidlike behavior under some conditions.