Structure of the Intermediate Phase Glasses GeSe3 and GeSe4: The Deployment of Neutron Diffraction With Isotope Substitution

Abstract

The method of neutron diffraction with isotope substitution was used to measure the full set of partial pair-correlation functions for each of the network-forming glasses GeSe 3 and GeSe 4 , which lie at the boundaries of the so-called intermediate phase in the Ge-Se system. The results show the formation of chemically ordered networks, where selenium chains are cross-linked by Ge(Se 4) 1/2 tetrahedra, in contrast to glassy GeSe 2 where the chemical order is broken. In all of these materials, the Ge-centered structural motifs are distributed unevenly on an intermediate length scale, as indicated by the appearance of a first sharp diffraction peak in the Bhatia-Thornton concentration-concentration partial structure factor. The new experimental work provides benchmark results for guiding in the development of realistic structural models, which can be used to explore the network rigidity and other structure-related properties of the glass. In this context, there are discrepancies between experiment and the predictions of first-principles molecular dynamics simulations that are particularly marked in respect of the Ge-Ge correlation functions, which are sensitive to the connectivity of the Ge-centered structural motifs.

Structure of semiconducting versus fast-ion conducting glasses in the Ag–Ge–Se system

“…The neutron g T,N ( r ) and X-ray g T,X ( r ) total pair-distribution functions are shown in figure 5 c , d , respectively. In both cases, the first peak at 2.37(1) Åis likely to originate from a combination of Ge–Se and Se–Se correlations, as found from the measured set of g αβ ( r ) functions for the Ge 0.25 Se 0.75 base glass [ 55 ]. A second peak at ≃2.64 Åemerges with increasing silver content and, by comparison with the structures of the crystalline polymorphs of Ag 8 GeSe 6 [ 56 – 58 ], it is attributed to nearest-neighbour Ag–Se correlations.…”

“…the silver pair-distribution functions receive a larger weighting in g T,X ( r ) when compared with g T,N ( r ). For x =0, a shoulder on the low- r side of the peak at ≃3.8 Å, which is attributed to corner-sharing Ge–Ge correlations by comparison with the measured set of g αβ ( r ) functions for the base glass [ 55 ], becomes less pronounced with increasing silver content.…”

“…In the β ′ -Ag 8 GeSe 6 phase, for example, Ag has 3 or 4 Se atoms at distances in the range 2.53–2.91 Å, the nearest-neighbour Ag–Ag distances are in the range 2.99–3.18 Å and the shortest Ag–Ge distance is 3.70 Å [ 58 ]. In the Ge 0.25 Se 0.75 base glass, the measured set of g αβ ( r ) functions show both Ge–Se and Se–Se nearest-neighbours, with the bond distances and coordination numbers summarized in table 2 [ 55 ].…”

“…Results are also given from NDIS experiments on the Ge 0.25 Se 0.75 base glass [ 55 ], from previous neutron diffraction [ 24 , 29 ] and anomalous X-ray scattering [ 21 ] experiments on the silver-modified glass, from an analysis of anomalous X-ray scattering data using the reverse Monte Carlo method [ 34 , 35 ], and from a previous NDIS experiment on a related glass [ 36 ]. x r GeSe (Å) r SeSe (Å) function reference 0 2.37(1) 2.36(1) 4 0.66(5) D T,N ( r ) this work 2.37(2) — 4.00(2) — g GeSe ( r ) [ 55 ] — 2.35(2) — 0.70(2) g SeSe ( r ) [ 55 ] 4.2 2.35 2.35 4.07 0.78 — [ 35 ] 5 2.37(1) 2.35(1) 4 0.66(5) D T,N ...…”

“…Here, a starting point is provided by the ‘8-N’ rule for the base glass, where the overall coordination numbers of Ge and Se are Z Ge =4 and Z Se =2, respectively. A chemically ordered network model appears to hold for glasses such as Ge 0.25 Se 0.75 and Ge 0.20 Se 0.80 , as supported by the full set of g αβ ( r ) functions measured for these materials by using the NDIS method [ 55 ]. Hence, if the numbers of Ge and Se atoms in the base glass are denoted by N Ge and N Se , respectively, the number of Se–Se bonds can be enumerated as [ 36 ].…”