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Accurate bonding description in disordered
chalcogenides challenges and open issues
Carlo Massobrio Institut de Physique et Chimie
des Matériaux Strasbourg (France) (CNRS-Univ. L.
Pasteur)
CECAM, Lyon, October 18, 2005
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AX2 disordered networks (AGe, Si XSe,S,O)
How to build up a classical potential..??
Strong ionic character coulomb interaction
What else two-body or three-body parts..??
Three-body favoring edge-sharing
Drawback of available potential
models Different network-forming disordered
share similar structures No homopolar bonds No
miscoordinations same intermediate range order
behavior lack of concentration fluctuations (No
FSDP in the Scc structure factor)
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GeSe2
SiSe2
Prototypical AX2 liquids and glasses
featuring AX4 units and intermediate range order
What is their atomic structure..??

Tool first-principles molecular dynamics
Disordered SiSe2, GeSe2 , GeSe4 structure
factors, building blocks and defects
Intermediate range order (IRO) concentration
fluctuations vs charge fluctuations
chemical disorder
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What is so interesting about l- and g-GeSe2 ??
I.T. Penfold and P.S. Salmon, PRL 67, 97 (1991)
FSDP in both the total...
(first sharp diffraction peak)
FSDP
and in the concentration- concentration structure
factor
Scc/cge cse 1cgecse(Sgege-Sgese)
cgecse(Ssese-Sgese)
For point charges (classical MD) no FSDP in SCC
charge-charge structure factor
Scc/cgecse SZZ ?
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First-principle molecular dynamics results
? total neutron structure factor
Theory FSDP absent
Note (dashed lines) LDA is inaccurate..!!!
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Liquid GeSe2
Which kind of network..??

GeSe4 tetrahedra and miscoordinations
n cge(ngege ngese) cse (nsesensege)
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Facts about g-SiSe2
In the crystal ES (edge-sharing) tetrahedra only
In the glass, both ES and corner-sharing (CS)
present
L.F. Gladden and S.R. Elliott, PRL 59, 908 (1987)
M.Tenhover et al Solid State Comm. 65, 1517
(1988)
D. Selvanathan et al PRB 61, 15061 (2000)
Open issues which is the predominant structural
sequence of tetrahedra..?? How many Si(0), Si(1),
Si(2)...??
Si(1)
Si(0)
Si(2)
0,1,2 number of fourfold rings for a given Si
By classical MD, see G.A. Antonio et al PRB 45,
7455 (1992)
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Amorphous SiSe2
M. Celino PhD thesis
Enhanced chemical order
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Understanding Scc and Szz structure factors
Scc cacx1cacx((SAA-SAX)(SXX-SAX))
zi ionic charges (Ge 4, Se 6)
POINT-LIKE CHARGE (PLC) MODEL
zvi point-like charges Ge 4, Se -2 for GeSe2,
Ge 4, Se-1 for GeSe4
How close are Scc and Szz..??
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FPMD calculations no FSDP in Szz
General postulate charge-charge correlations
absent on IRO scales
Charge neutrality on IRO scales is a primary
constraint
How to rationalize the appearance of FSDP in
Scc..??
Scc
l-GeSe2
Which atomic structure when FSDP is absent (or
present) in Scc..??
Szz
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Correlation between FSDP in Scc and chemical
disorder (II)
Constraint charge neutrality on IRO scales
Case II small deviations from chemical order
g-SiSe2 (theory)
Distinct FSDP in Scc
The same situation is encountered in liquid and
amorphous GeSe4 (theory) and in the neutron
scattering results for liquid GeSe2 (P.S. Salmon)
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Correlation between FSDP in Scc and chemical
disorder (III)
Constraint charge neutrality on IRO scales
Case III large deviations from chemical
order l-GeSe2 (theory)
case (II) (l-GeSe2, exp) compared to case (III)
(l-GeSe2, theory)
Vanishing FSDP in Scc
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Disordered networks 3 classes of systems
identified
Perfect network charge neutrality does not
require any local variation of the
concentration no FSDP in Scc Ex disordered
SiO2 (case I)
Moderate chemical disorder Occurrence of
different valence states Variations of
concentration induce FSDP in Scc Ex l-GeSe4,
g-SiSe2, l-GeSe2 (exp) (case II)
High chemical disorder FSDP in the total S(k)
but no FSDP in Scc Ex l-GeSe2 (theory) (case
III)
Intensity of FSDP in Scc
Case II
Case III
Case I
Chemical disorder
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Simulation of disordered structures two
additional points of methodology
System size and periodicity are they compatible
with IRO..?
Validity of phenomenological models analysis via
first-principles calculations
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Two ways are available to calculate the structure
factor
Real space
Reciprocal space
Total S(k)
FSDP appears in the Ge-Ge S(K) when
correlations beyond 6Å are accounted for
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Elliott theory remarkable but...
...A residual peak still noticeable
in the S
(voids-cations) at the FSDP location!
CC
No evidence for plane formation
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