Study of local structural order in glassy arsenic selenide using nuclear quadrupole resonance

Nuclear quadrupole resonance (NQR) experiments were performed on amorphous As_xSe_1−x to study its local structural order. At concentrations x ≥ 0.45 local regions of different composition begin to form. Measurements of the spin echo intensity were used to identify these regions formed by arsenic atoms bound to zero, two, or three selenium atoms. The NQR spectral lineshapes and the longitudinal relaxation times suggest that these local regions have more ordered structures as compared to the homogeneous samples with low arsenic contents. The bonding in As _xSe_1−x is governed by preferential bonding (chemical ordering) between arsenic and selenium at arsenic concentrations x ≤ 0.40, however the bonding for higher arsenic concentrations, x ≥ 0.45 is governed by both chemical ordering and statistical effects. From x ≥ 0.45, the NQR spin-echo spectra become narrow and peaks corresponding to different numbers of As-As bonds appear that may reveal arsenic-rich small clusters are forming in the network to release the local strains. Measurements of the spin-lattice (longitudinal) relaxation times suggest that the rigidity of amorphous As _xSe_1−x dramatically decreases for arsenic concentrations x > 0.40. This is in contradiction to the prediction of mean field theory. Therefore, mean field theory cannot be applied to amorphous As_xSe _1−x with x ≥ 0.45. The existence of small concentration of crystalline inclusions in amorphous As_0.60Se_0.40 is also suggested.