Peierls distortion in High Pressure liquid arsenic

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Peierls distortion in High Pressure liquid arsenic

• Laura Crapanzano
• European Synchrotron Radiation Facility
• France

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Introduction

• Liquid structural polymorphism
• Peierls distortions in solid state
• Arsenic the state of the art

Description of the experiment
Conclusions and Acknowledgements
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Liquid structural polymorphism
The existence of two distinct structural forms of
liquid (physically distinguishable but not
chemically!!!) is common experience in
multi-component systems
Popular examples are the liquid crystals and the
solutions
In principle, liquid polymorphism exist and then
also transition between the allotropes
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Peierls distortions
The case of the V, VI and VII groups
Considering the overlap of the p-orbitals, the
most stable structure is obviously the simple
cubic structure. With this structure all the
elements of these groups should be metallic
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What happens?
The coordination number follows the Octet
Rule NC 8 N (where N is the number of the
valence electrons)
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The semiconductor character of these elements can
be described as the result of an electronic
instability
The formation of a gap near the Fermi level
stabilizes the distorted structure by an
electronic energy gain.
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Arsenic at room conditions
At 1 atm et room temperature the structure is A7
type
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Liquid arsenic

• Above the triple point arsenic does not undergo a
  semiconductor-metal transition.
• The coordination number of liquid Arsenic at
  low pressure (37 atm) is 3 exactly as in the A7
  crystal .

Liquid arsenic conserves the distorted structure
of the solid phase!
(Bellissent et al., PRL, 59 (6),661 (1987))
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From theoretical studies we expect that solid A7
as becomes simple cubic at 25 GPa
In order to check the lost of the Peierls
distortion in the liquid we performed different
isobaric measurements at high pressure
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The experiment.
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Sample environment
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The Soller slits system
In monochromatic mode the high background coming
from high pressure cells (boron gasket, graphite
heater, h-BN) strongly deteriorates the data
quality
Soller slits system adapted to the P-E press
geometry
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Analysis difficulties due to the Sollers system
The angular dependence due to the presence of the
Sollers system have to be considered in the
datas correction by the introduction of two
normalizing functions
Cell volume angular dependence g(q) Sample
volume angular dependence j(q)
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Data correction
External factors corrections
Intensity normalization
g(r) extraction
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Classic S(q) extraction
S(q) extraction after consideration of all the
external factors
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Results
The coordination number increase from 3 to 5.5
going from 37 atm to 6 GPa
There is no relevant temperature effect on the
liquid structure The position of the peaks does
not change
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RMC why?
Because good quality data are available from XRD
with the Paris-Edinburgh press at High pressures
Because it is very difficult to have density data
for liquids at extreme conditions
RMC why not?
Because the q-range of the P-E data is limited
and than only particular systems can be studied
Systems for which the molecular structure is
well-known
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Conclusions
XRD with the Paris-Edinburgh press is a powerful
tool in order to obtain good quality data for
liquids in extreme conditions The data analysis
of liquid As is a good example of application of
a large volume cell
As results are in good agreement with the
theoretical predictions at high pressure The
Peierls distortions lost can be observed in
liquid systems by increasing pressure
RMC could be used in data analysis of high
pressure data It can be an ideal data analysis
tool when no density data are available for the
studied system
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Acknowledgements
ESRF, France Wilson Crichton ,Mohamed Mezouar and
Giulio Monaco
Universite de Liege, Belgium Jean-Pierre Gaspard
Centre de Recherches sur les Mécanismes de la
Croissance Cristalline, France Christophe Bichara

CEA, France Robert Bellissent
Thanks for your attention