Structural properties of chalcogenide glasses and the isocoordination rule: Disentangling effects from chemistry and network topology

The structural properties of two Ge-As-Se glass compositions (${\mathrm{Ge}}₁₀{\mathrm{As}}₁₀{\mathrm{Se}}₈₀$ and ${\mathrm{Ge}}₂₁{\mathrm{As}}₂₁{\mathrm{Se}}₅₈$) are investigated from a combination of density-functional-based molecular dynamics simulations and neutron/x-ray scattering experiments. We first focus on structural characteristics, including structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions, and compare our results with the experimental data. Results leave anticipated coordinations from the octet rule (${\mathrm{Se}}^{\mathrm{II}}, {\mathrm{As}}^{\mathrm{III}}$, and ${\mathrm{Ge}}^{\mathrm{IV}}$) unchanged, and these are contrasted with respect to glasses having the same average coordination number $\overline{r}$ such as binary ${\mathrm{As}}₃₀{\mathrm{Se}}₇₀$ and ${\mathrm{Ge}}₃₃{\mathrm{Se}}₆₇$. The increase of (As,Ge) content induces a growth of ring structures that are dominated by edge-sharing motifs (four-membered rings) having mostly heteropolar bonds, while As-As and As-Ge homopolar bonds are clearly more favored than Ge-Ge. These features signal that both topological (rings) and chemical (bonds) features are different with respect to related binaries. The validity of the so-called vibrational isocoordination rule stating that properties of multicomponent chalcogenides depend solely on $\overline{r}$ is checked, and results from a vibrational analysis indicates that this rule is merely satisfied for the Se-rich composition. An inspection of correlations via the Bhatia-Thornton formalism shows that topological ordering is not only different between ${\mathrm{Ge}}₁₀{\mathrm{As}}₁₀{\mathrm{Se}}₈₀$ and ${\mathrm{Ge}}₂₁{\mathrm{As}}₂₁{\mathrm{Se}}₅₈$ but also radically contrasts with respect to the isocoordinated binary glasses and displays an obvious reduced directional bonding.