Structure of Arsenic Selenide Glasses Studied by NMR: Selenium Chain Length Distributions and the Flory Model

International audienceFive homogeneous arsenic selenide glasses with target compositions As 2 Se 3 , AsSe 2 , AsSe 3 , AsSe 4.5 , and AsSe 6 were studied quantitatively by 77 Se Carr−Purcell−Meiboom−Gill magic-angle spinning NMR and transmission electron microscopy−energy-dispersive X-ray spectroscopy. The entire set of NMR spectra is simultaneously fitted with six distinct environments taking into account the effect of first and second neighbors on the position of the 77 Se resonance. The selenium chains are bound at each end to trivalent arsenic atoms, and the chain length distribution can be modeled with the Flory theory, which is well-known in polymer science and is used here for the first time to model the probability of finding each selenium environment in a selenide glass. No arsenic homopolar bond is detected in our experiments. ■ INTRODUCTION Chalcogenide glasses exhibit a wide range of physical properties such as infrared transparency, high refractive indices, and reversible amorphous-to-crystal transitions and can be easily shaped into optical devices. 1−6 Among them, the arsenic selenide glasses As x Se 1−x are considered to be a promising family because glassy As 2 Se 3 is a good candidate for all-optical switching 7 or for use as a mid-infrared laser source; 8 in addition, arsenic selenide glasses can be used for optic fibers. 9 Recent studies also investigated the possibility of preparing these glasses using microwave heating. 10 Numerous attempts were made to draw a link between the changes in the physical properties of arsenic selenide and the evolution of its molecular structure as the arsenic content varies, both at room temperature 11,12 and when the temperature is increased, 13 during aging of the glass, 14 or when irradiated with a laser. 15 To gain some insight into the arsenic selenide glass structures, recent studies relied on molecular dynamics 16−18 combined with anomalous X-ray scattering 19 or 77 Se solid-state NMR 20−23 to characterize the environments and connectivity of selenium and arsenic atoms. Many of these studies hint toward the existence of a small amount of As−As homopolar bond 12,16 with tetravalent arsenic atoms linked to two arsenic and two selenium atoms. 19 Moreover, 77 Se NMR spectroscopy can quantify three distinct selenium environments (selenium atoms linked to two, one, or zero arsenic atoms) and shows that there is some disorder in the distribution of the lengths of the selenium chains that link arsenic atoms together, as opposed to what is inferred in the chain-crossing model (i.e., when the selenium chains are of similar lengths). 20,23 Interestingly, it was suggested that the Flory model, 24 which describes the distribution of chain lengths in organic polymers, could be applied to inorganic polymers (mostly silicates) because the underlying chemical phenomena share striking similarities, especially for glasses with covalent bonds and no ionic species. 25−27 The Flory theory provides a very simple model for the probability, P(n), of finding a chain of length n, which is equal to np n−1 (1 − p) 2 , where p is the probability to form a linkage between two monomers and the average chain length is given by 1 + p/1 − p. Moreover, Flory distributions are characterized by a single parameter p and not two as is the case for Gaussian distributions (which may not correctly reproduce the chain length distributions for arsenic-rich glasses with short chain lengths) or three for skewed Gaussian distributions. Therefore, the Flory framework was applied here as a model for the distribution of chain lengths. 77 Se is a spin 1/2 nucleus with a 7.63% natural abundance, a gyromagnetic ratio equal to 19% of γ(1 H), and a fairly large chemical shift range over 3000 ppm. 28,29 However, as many diluted spin-1/2, it usually features long longitudinal relaxation times around hundreds of seconds, which may affect the measured proportions of each selenium environment. 23 Usually, three selenium environments are distinguished depending upon the nature of the two atoms (arsenic or selenium) they are connected with. 20 However, as the 77 Se atoms are excessively sensitive to their environments, it is often observed that the chemical shifts of these broad lines vary with the composition of the sample, 20 precluding any simultaneous fitting of series of NMR spectra. Such an effect results from a dependence of th