Small nanoparticle clusters limited atomic transport in self-organized topologically-disordered covalent networks

Topologically-disordered glasses possessing fully-saturated covalent
bonding in respect to known 8-N rule have been in a sphere of tight
interests because they exhibit a large variety of useful practical
applications in IR optoelectronics and photonics. They exist in many
glass-forming compositions in dependence on their connectivity defined
in mean coordination number Z taken as average number of covalent
bonds per one atom of structural unit. Within Phillips-Thorpe
mean-field rigidity theory [1,2], the covalent-bonded networks proper
to such solids reveal three distinct phases in dependence on Z –
floppy, intermediate and rigid. The networks having less than 3
Lagrangian constrains per atom are under-constrained (floppy phase)
ones, while those having more than 3 constrains per atom are
over-constrained and enthalpically-stressed ones (stressed-rigid
phase). The onset from floppy to stressed-rigid networks predicted to
be solitary [1,2], but it split into two points with accepting that
bonds are not distributed randomly revealing a tendency to
self-organization [3]. Thus the second-order transition occurs from
floppy to unstressed-rigid phase and first-order transition occurs
from unstressed-rigid to stressed-rigid phase. The stressed-free
intermediate phase having just 3 Lagrangian constrains per atom
appears so as to avoid stress, forming a so-called reversibility
window. In device application, the self-organized intermediate-phase
glasses having optimal space filling are most attractive since they
reveal unique non-aging ability. We try to examine the idea on
adaptability of covalent-linked glassy networks owing to computational
cluster-modeling approach justifying energetically the validity for
previously-reported boundaries of reversibility windows in binary
As/Ge-S/Se systems. Quantum mechanical calculations were performed
using HyperChem program, ab initio calculations with RHF/6-311G* basis
set being used to determine the total energies of clusters. All
boundary chalcogen atoms belonging to two clusters were terminated by
H atoms to be two-fold coordinated. Only half-part contributions from
these atoms were considered after subtraction both energies of H atoms
and –S(Se)-H bonds from total cluster energy. This value was taken as
a measure for cluster formation probability. The performed
calculations showed that directly linked two-cation-centered clusters
are basic glass-forming blocks in the studied glasses, they being
specifically interconnected in a space within more extended blocks to
form topologically self-organized structural motives. Despite
overconstrained nature of constituting blocks in Ge-S/Se glasses, they
are specifically distributed via optimally-constrained inter-cation
linking elements. In such a way, the pseudo-reversibility window
appears in glass compositions between Z ranging from 2.4 to 2.5 in
Ge-S/Se glasses, while the reversibility window in As-S/Se glasses is
rather shifted towards Z=2.40-2.46. 1. Phillips
J.C. J. Non-Cryst. Sol., 34, 153-181 (1979). 2. Thorpe
M.F. J. Non-Cryst. Sol., 57, 355-370 (1983). 3. Thorpe M.F., Jacobs
D.J., Chubynsky M.V., Phillips J.C. J. Non-Cryst. Sol., 266-269,
859-866 (2001). 4. Boolchand P., Georgiev D.G., Goodman
B. J. Opt. Adv. Mat., 1, 703-720