Ultralow thermal conductivity and thermally-deactivated electrical transport in a 1D silver array with alternating δ-bonds.

We report the synthesis of a (TMA)AgBr 2 (TMA = tetramethylammonium) crystal, which comprises inorganic anionic chains of -(AgBr 2 ) ∝ - stabilized by columnar stacks of organic TMA cations with a periodic arrangement of shorter and longer Ag(i)⋯Ag(i) bonds, even though all the Ag(i) ions are chemically equivalent. The presence of two chemically non-equivalent bridging Br ions is attributed to the primary cause of such an unusual arrangement, as clearly visualized in the charge density plot of (TMA)AgBr 2 extracted from the theoretical calculations based on density functional theory. Remarkably, we identified from the orbital-projected density of states the existence of alternate δ-like bonding involving d xy orbitals of 4d 10 Ag(i), which was attributed to the cause for ultralow thermal conductivity and thermally-deactivated electrical transport in (TMA)AgBr 2 . Barring the energetics, our observations on the existence of a δ-bond will shed new light in understanding the nature of metal-metal chemical bonding and its unprecedented implications.