Structure of liquid As: A Peierls distortion in a liquid

A neutron-diffraction experiment ({λ}=0.704 A{}) has been performed for the first time on liquid arsenic above the triple point. The coordination number (Z=3) does not change through the semimetal-to-semiconductor transition on melting. This is in sharp contrast to the elemental semiconductors (Si,Ge) that undergo a semiconductor-to-metal transition upon melting, accompanied by an increase (4{→}6.4) of the coordination number. The nonmetallic behavior of As, which has a half-filled p band, can be understood as a Peierls distortion in the p-bonding mechanism (three p-p bonds are reinforced; the others are considerably weakened).