Structure and physical properties of glasses in the system Ag2Se–Ga2Se3–GeSe2

Abstract

Homogeneous glasses in the ternary system Ag 2 Se-Ga 2 Se 3-GeSe 2 are synthesized and their structure is characterized using Raman and one-and two-dimensional 77 Se, 71 Ga and 69 Ga nuclear magnetic resonance (NMR) spectroscopy. The structure of these glasses consists predominantly of a charge-compensated network of corner-sharing (Ga/Ge)Se 4/2 tetrahedra where the negatively charged GaSe 4/2 tetrahedra are charge balanced by the Ag cations. The deficiency in Se required to satisfy the tetrahedral coordination of Ga in glasses with Ag 2 Se:Ga 2 Se 3 < 1 is accommodated by the formation of homopolar Ge-Ge bonds. Progressive addition of Ag 2 Se to these glasses efficiently removes Ge-Ge bonding such that their concentration essentially goes to zero at the chemical threshold Ag 2 Se:Ga 2 Se 3 = 1. Further addition of Ag 2 Se beyond this threshold (i.e., Ag 2 Se:Ga 2 Se 3 > 1) results in the modification of the Ga,Ge-Se tetrahedral network via depolymerization and formation of non-bridging Se atoms. Moreover, all glasses irrespective of their Ag 2 Se:Ga 2 Se 3 ratio contain a small fraction of Se-Se bonds, implying violation of chemical order. Therefore, the compositional evolution of the structure of these glasses displays the characteristics of both the isoelectronic alkali/alkalineearth aluminosilicate glasses and the purely covalent, continuously alloyed chalcogenide networks. Such composition-dependent structural evolution is shown to be consistent with the corresponding variation in glass transition temperature and molar volume.

Synthesis and structural characterization of stoichiometric Li-Ga-Ge Sulfo-selenide glasses

“…4c) yields a CSA tensor with Δ=210 ppm (Δ=405 ppm) and η= 0.9 (η= 0.6) for the isotropic peak at 316 ppm (100 ppm). These CSA values are consistent with previous reports [16,35]. These isotropic shifts and CSA parameters are used to constrain the simulation of the 77 Se MAS NMR spectra of compositionally closely related glasses with R>1 ( Fig.…”

“…2a) This structural scenario for the LGGS/Se glasses is consistent with the observed compositional variation of T g (Table 1). Overall the same compositional trends were also observed in analogous selenide glasses in the stoichiometric Ag 2 Se/Na2Se/BaSe-Ga 2 Se 3 -GeSe 2 systems [14,16,22]. The T g of the LGGS/Se glasses with R>1 is significantly lower than that of glasses with R<1.…”

“…The 77 Se isotropic projection spectrum can be simulated with two peaks centered at 316 and 100 ppm (Fig. 4b) that can be assigned, respectively, to bridging selenium (BSe) sites in Ga/Ge-Se-Ge/Ga linkages between corner sharing Ge(Ga)S n Se n-4 tetrahedra and to NBSe sites in Ge-Se-Li environments [16,34,35]. The magnitude of the CSA  for these two sites can be gleaned from the 2D contour plot where it is clear that  for the NBSe site is larger than that of the BSe site, consistent with previous reports of such sites in selenide glasses with compensated networks [16,35].…”

“…5, Table 2). The inset of [15,16,38]. On the other hand, the 71 Ga QMAT spectrum for the R>1 glass is characterized by a δ iso of ~320 ppm and a C q of 11 MHz.…”

“…In fact, it is possible to synthesize pseudo-ternary glasses in the stoichiometric system M 2 X/M'X-Ga 2 X 3 -GeX 2 [M=alkali; M'= alkaline-earth] that are isoelectronic analogues of alkali/alkaline-earth aluminosilicate glasses with partially or fully compensated network. Recent works on Ag 2 Se/Na 2 Se/BaSe-Ga 2 Se 3 -GeSe 2 glasses have shown that the ratio of modifier to intermediate content R= M 2 X/M'X:Ga 2 X 3 is of critical importance in controlling the structural evolution of the network in these compensated chalcogenide glasses [14][15][16]. In the case of R < 1, these glasses are considered to be chalcogen deficient for satisfying the tetrahedral coordination of heteropolar bonded Ga and Ge atoms and this deficiency is accommodated in the structure primarily via the formation of Ge-Ge homopolar bonds.…”