Pulsed nuclear quadrupole resonance (NQR) experiments have been performed on three forms of elemental arsenic: amorphous ($a$), rhombohedral ($\mathrm{rh}$), and orthorhombic ($or$). The temperature dependence of the spin-lattice relaxation time ($T_1$) provides evidence for the existence of disorder (tunneling) modes in $a$-As. It is found that the NQR line shape of $a$-As is highly asymmetric, and this asymmetry is attributed to a distribution of dihedral angles in $a$-As. The observed NQR frequencies indicate that the amount of bonding $s$ admixture is different in each material ($\mathrm{rh}$-As: 3%, $or$-As: 7%, $a$-As: 10%). Simple calculations in comparison with x-ray results indicate that the bonding configurations in these three forms of arsenic do not necessarily correspond to maximum overlap of bonding orbitals on adjacent atoms.

• Received 14 April 1980

DOI:https://doi.org/10.1103/PhysRevB.22.3903