Compositional Thresholds and Anomalies in Connection with Stiffness Transitions in Network Glasses

The structural and dynamical properties of amorphous and liquid ${\mathrm{As}}_x{\mathrm{Se}}_{1-x}$ ($0.2<x<0.4$) are studied by first principles molecular dynamics. Within the above range of compositions, thresholds and anomalies are found in the behavior of reciprocal space properties that can be correlated to the experimental location of the so-called Boolchand intermediate phase in these glassy networks. These findings are associated with diffusion anomalies for the parent liquid phase, thereby linking structural and dynamical atomic-scale fingerprints for the onset of rigidity within the network, while also providing a much more complex picture than the one derived from mean-field approaches of stiffness transitions.

Figure 1

Total simulated structure factor $S_T(k)$ of amorphous ${\mathrm{As}}_{20}{\mathrm{Se}}_{80}$, ${\mathrm{As}}_{35}{\mathrm{Se}}_{65}$, and ${\mathrm{As}}_{40}{\mathrm{Se}}_{60}$ (top, black solid lines) compared to previous simulations of ${\mathrm{As}}_{40}{\mathrm{Se}}_{60}$ (bottom) from Li and Drabold (solid line, [20]) and Mauro and Varshneya (broken line, [21]). All simulated functions are compared to experimental data (blue curves : ${\mathrm{As}}_{40}{\mathrm{Se}}_{60}$, [19]; other compositions, [9]). A decomposition into partial structure factors is shown for ${\mathrm{As}}_{40}{\mathrm{Se}}_{60}$ : As-As (green), As-Se (red), Se-Se (orange). Bottom: a schematic view of the structure of a low modified ${\mathrm{As}}_x{\mathrm{Se}}_{1-x}$ glass: ${\mathrm{AsSe}}_{3/2}$ pyramids cross-link Se chains (see text for details).

Figure 2

Computed partial structure factor $S_{\mathrm{AsSe}}(k)$ for the five amorphous compositions (from to 20% to 40% As) in amorphous ${\mathrm{As}}_x{\mathrm{Se}}_{1-x}$. The red broken curves are Lorentzian fits used to determine the position $k_{\mathrm{FSDP}}$ (inset) and width $\Delta k_{\mathrm{FSDP}}$ (inset, right axis) of the As-Se FSDP as a function of composition. The grey zone indicates the rigid intermediate phase, determined experimentally [14].

Figure 3

(a) As-As (black line), As-Se (blue line), and Se-Se (red line) pair distribution function $g_{ij}(r)$ for two selected compositions in amorphous ${\mathrm{As}}_x{\mathrm{Se}}_{1-x}$ ($x=20$ and 40% As). For clarity, the As-Se pair is shown only for the 40% As system. (b) Computed fraction $n_{\mathrm{AsAs}}$ and $n_{\mathrm{SeSe}}$ of homopolar As-As and Se-Se bondings as a function of As composition. The grey zone indicates the rigid intermediate phase, determined experimentally [14]. The inset represents the total fraction of homopolar bonds as a function of composition.

Figure 4

Computed activation energy $E_A$ for diffusion of As (black symbols) and Se (red symbols) as a function of As content in amorphous ${\mathrm{As}}_x{\mathrm{Se}}_{1-x}$. The insert shows the diffusion constant at 800 K. The grey zones indicate the location of the rigid IP, determined experimentally [14]. The flexible chalcogen-rich phase is marked.