Silver transport in Ge
x
Se
1-x
: Ag materials: Ab initio simulation of a solid electrolyte
De Nyago Tafen* and D. A. Drabold
†
Department of Physics and Astronomy, Ohio University, Athens, Ohio 45701, USA
M. Mitkova
Center for Solid State Electronics Research, Arizona State University, Tempe, Arizona 85287-6206, USA
Received 14 January 2005; revised manuscript received 17 May 2005; published 16 August 2005
In this paper, we present models of Ge-Se glasses heavily doped with Ag obtained from ab initio simulation
and study the dynamics of the network with an emphasis on the motion of Ag
+
ions. The models are analyzed
with partial pair correlation functions, static structure factors, and novel wavelet techniques. The electronic
properties are characterized by the electronic density of states and analysis of specific electronic eigenstates. As
Ag content increases, the optical band gap increases. Ag diffusion is observed directly from thermal simulation.
The most diffusive Ag
+
ions move preferentially through low density regions of the network and the existence
of well-defined trapping centers is confirmed. Preliminary information about temperature dependence of trap-
ping and release is provided.
DOI: 10.1103/PhysRevB.72.054206 PACS numbers: 61.43.Fs, 66.30.Dn, 66.30.Hs, 71.23.An
I. INTRODUCTION
The chalcogenide glasses are preferred semiconducting
materials for applications. They have well-defined niches in
fiber optics,
1
optical recording,
2
phase change memory,
3
and
other technologies. Ge-Se glasses have been particularly
studied because of their ready glass formation, easy synthesis
requirements, and good chemical stability. The basic struc-
tural units are Se chains and Ge-Se tetrahedra, which may
combine in a variety of ways. Defects including homopolar
bonds exist in these systems as seen in both experiment and
theory.
The Ge-Se system was the first in which formation of an
intermediate phase was demonstrated experimentally by
Boolchand et al.
4
and further developed theoretically by
Thorpe et al.
5
In binary Ge
x
Se
1-x
glasses, the self-organized
phase exists in the 0.20 x 0.254 range, with glasses at x
0.20 regarded as floppy while those with x 0.26 stressed
rigid. Dynamic calorimetry measurements on the intermedi-
ate phase have led to the conclusion that such materials do
not age,
6
a feature that may be of importance in application
of these materials.
Silver added to chalcogenide glass hosts has attracted
widespread interest in soft condensed matter science.
7,8
The
interest emerges in part from the extensive bulk glass form-
ing tendency in the Ge-Se-Ag ternary, the spectacular en-
hancement eight orders of magnitude in electrical conduc-
tivity of glasses with Ag relative to the glassy chalcogenide
hosts, and from light-induced effects such as photodoping,
photodiffusion, and photodeposition. Although the mobile
ions in amorphous materials have been studied
10
their de-
tailed dynamics in amorphous hosts still constitutes one of
the unsolved problems of solid state ionics. The structure of
Ge-Se-Ag glass has been investigated using several experi-
mental methods, including x-ray diffraction,
11,12
neutron dif-
fraction with isotopic substitution,
13
extended x-ray-
absorption fine structure EXAFS,
14
differential anomalous
x-ray scattering
15–17
DAS and modulated differential scan-
ning calorimetry MDSC, and Raman spectroscopy.
7,8
De-
spite this impressive database, the structure of the ternary
Ge-Se-Ag glasses has not yet been completely determined.
There continues to be a debate on basic aspects of the glass
structure i.e., homogeneity and Ag coordination especially
for Se rich glasses with more than 67% Se.
Experimental evidence for macroscopic phase separation
in these materials has come from MDSC results, which indi-
cate bimodal glass transition temperatures.
8
In these experi-
ments, one T
g
is independent of glass composition and is
identified with a Ag
2
Se glass phase,
9
while the second T
g
that varies with glass composition is related to the Ge-Se
backbone. For the time scales and possibly also lengths
scales of our simulations, these effects do not emerge, but
are an interesting challenge for the future.
In a recent paper,
18
we briefly reported the motion of Ag
ions in glassy chalcogenide hosts and demonstrated the ex-
istence of ion trapping centers, which are important for re-
laxation processes in disordered systems.
19
Here, we provide
detailed information about structural and electronic proper-
ties, and also give new information about temperature depen-
dence of the trapping, and also the geometry of the traps. In
this paper, we have focused upon Ag-doped glasses contain-
ing Ge 25 at. % and Se 75 at. % we later call this GeSe
3
.
This composition is near the intermediate phase slightly into
the stressed-rigid phase. To our knowledge, this is a unique
ab initio simulation of these materials.
The rest of this paper is organized as follows. In Sec. II,
we describe the simulation procedure to fabricate the atom-
istic models, discuss the ab initio total energy functional
force and force code used and other approximations. In Sec.
III, we describe the structural properties using conventional
measures such as static structure factors and also apply a
novel wavelet method to explore intermediate range order.
Electronic properties are briefly discussed in Sec. IV, and
Section V is concerned with the dynamics of the Ag
+
ions in
the amorphous matrix.
PHYSICAL REVIEW B 72, 054206 2005
1098-0121/2005/725/0542069/$23.00 ©2005 The American Physical Society 054206-1