Chemical Short-Range Order in Selenide and Telluride Glasses

Abstract

The structure of Ge20SbxSe80-x (x = 5, 15, 20) glasses was investigated by neutron diffraction, X-ray diffraction, and extended X-ray fine structure measurements at the Ge, Sb, and Se K-edges. For each composition, large-scale structural models were obtained by fitting simultaneously the experimental data sets in the framework of the reverse Monte Carlo simulation technique. It was found that the structures of these glasses can be described mostly by the chemically ordered network model. Ge-Se and Sb-Se bonds are preferred; Se-Se bonds in the Se-poor composition (x = 20) and M-M (M = Ge, Sb) bonds in strongly Se-rich glass (x = 5) are not needed. The quality of the fits was significantly improved by introducing Ge-Ge bonding in the nearly stoichiometric composition (x = 15), showing a violation of chemical ordering. The structure of Ge20SbxSe80-x was compared to that of several glasses from the three analogous systems (Ge-As-Se, Ge-As-Te, Ge-Sb-Te), and it was found that chemical short-range order becomes more pronounced upon substituting As with Sb and Se with Te. Ge-As-Se glasses behave as random covalent networks over a very broad composition range. Chemical short-range order and disorder coexist in both Te-rich and Te-poor Ge-As-Te glasses, whereas amorphous Ge14Sb29Te57 and Ge22Sb22Te56 are governed by strict chemical preferences.

Atomic level structure of Ge-Sb-S glasses: Chemical short range order and long Sb-S bonds

“…The Ge-S bond distance (2.22-2.23 Å, see Table 3) is similar to those found earlier in binary Ge-S systems (2.21-2.23 Å e.g. by ND [56], by ND and high energy X-ray diffraction measurements [49] or by pulsed ND [57] [12].…”

Chemical order in Ge-Ga-Sb-Se glasses

“…If the Mott-rule is satisfied then glassy Ge 21 respectively, which agrees well with the previous results in Ge-Ga-Se [24] and Ge-Ga-Sb-Se [5,12] glasses.…”

“…This is not the case for glassy Ge 21 Ga 5 Sb 10 Se 64 and Ge 30 Ga 5 Sb 10 Se 55 . Comparison of the Ge-Se partial pair correlation function of Ge 21 Ga 5 Sb 10 Se 64 glass with those of Ge 27.5 As 10 Se 62.5 [45], Ge 20 Sb 15 Se 65 [21] and Ge 20 Ga 10 Se 70 glasses [24] obtained with the same methodology ( Figure 5) also shows that the secondary peak between 2.7 Å and 3 Å is significant. The ratio of coordination numbers corresponding to 'normal' and long bonds is 0.2, 0.12, 0.055, 0.04 and 0.05 for Ge 30 Ga 5 Sb 10 Se 55 , Ge 21 Ga 5 Sb 10 Se 64 , Ge 27.5 As 10 Se 62.5 , Ge 20 Sb 15 Se 65 and Ge 20 Ga 10 Se 70 , respectively.…”

“…Sample preparation and density measurement are described in [21]. Densities of the glasses are shown in Table 1.…”

“…In Se-poor systems where there are not enough Se atoms to satisfy the bonding requirements of the M atoms, M-M bonds are also expected to be present.There are several studies about the structure of the ternary parent glasses (Ge-Ga-Se and Ge-Sb-Se) (see e.g. [21,24] and references therein). Diffraction and extended X-ray absorption fine structure (EXAFS) measurements proved that the structure of Ge-Sb-Se glasses can be described by the CONM…”