Snmek 1

1
Relaxation of Structural Units in MD Simulated
Silicate Glasses
Jan Machácek, Ondrej Gedeon Institute of
Chemical Technology, Technická 5, Prague, CZ-166
28, Czech Republic, Jan.Machacek_at_vscht.cz Marek
Lika Vitrum Laugaricio - Joint Glass Center of
Institute of Inorganic Chemistry SAS, Alexander
Dubcek University of Trencín and RONA Lednické
Rovne, tudentská 2, Trencín, SK-911 50,Slovak
Republic
2
Introduction

• Structure of silicate glass - described by
  Q-species - Si4n
• NMR, Raman spectroscopy
• Unusual coordination of silicon, high-pressure
  systems
• CRD(Si) 5, 6
• Si55 - evidence from MAS-NMR /Stebbins 1988/
• high pressure
• high cooling rates
• binary potassium silicate
• 20 mol alkali oxide
• Role of Si55 in diffusion of alkalis? /Farnan
  1998/
• intermediate in diffusion mechanism of oxygen ?
• Presence of Si55 in MD glasses
• pair-wise potentials
• caused by rapid cooling rates
• maximum of Si5n 20 mol Na2O

3
Introduction
Na
Na
Na
/Farnan 1998/
4
Aim of this all

• Exploration of behaviour of Simn units (m is
  O n is BO)
• at various temperatures (1000K - 2500K)
• Description of reactions or mutual
• transformations Simn ? Sikl
• Tracing of decay of initial number
• of various Simn in time
• Discussion of such decay in term of ionic
  migration

5
Some computational details

• MD simulation, DL_POLY program /Smith 1995/
• Na2O2SiO2 glass, 1494 atoms
• Pair-wise potential
• LR Coulombic SR Buckingham /van Beest 1990,
  Yuan 2001/
• (A) Preparation of MD glass
• cooling from random configuration at 5000 K down
  to 300 K,
• total time 960 ps
• configurations from 1000 K to 2500 K collected
  every 100 K
• (B) Tracing of Simn units for one temperature
  value
• NVE ensemble, system relaxed
• 50 ps run
• atom positions collected every 0.01 ps into
  HISTORY file
• cut-off for NBO(BO) identification 2.46 Å
• determination of Simn units
• summation of coordination changes Simn ?
  Sikl

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Results and Discussion

• Most frequent Simn?Sikl reactions
• Chem. equilibrium, temperature 2500 K
• Red numbers absolute number of
• transformations per 1 ps
• Vertical lines BO ? NBO conversions
• Horizontal lines changes in Si
• coordination
• Diagonal line addition/subtraction of
• oxygen (NBO is formed) less
• probable, non-elementary
• Most frequent are 55 ? 44, 54 ? 43 transitions
• At lower temperatures Si6n and Si53 units
  vanish

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Results and Discussion
Decay of Si43 units at various temperatures.
Newly generated units during a simulation run are
not included in the curves.
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Results and Discussion

• Decay curves cannot be described by a simple
  exponential decay
• We assume two decay processes with different
  reaction rates

• A is norm. number of Simn, k1 and k2 are
  kinetic constants of fast and slow processes ?
• Half time of decay of Simn unit is defined by
  eq. ti ln(2) / ki
• Temperature dependence of ti reveals two
  processes ?
• Fast - can be attributed to fast moves or
  thermal vibrations of oxygen atoms crossing
  cut-off distance 2.46 Å
• Slow - dominates at lower temperatures,
  Arrhenian fit of t2(T) dependence gives energy
  barrier of 0.69 eV (close to Eact for diffusion
  in Na-disilicate glass)

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Results and Discussion
Temperature dependence of half times of decay of
Si43 units. The inset demonstrates the
superposition of two processes fast - dominating
the beginning of the decay, and slow -
determining decay after the fast process is
exhausted.
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Conclusions

• Analysis of glass structure in terms of Simn
  units was performed
• Elemental transformations comprise only the
  addition/subtraction of BO and the interchanges
  between NBO and BO
• Decay of Si43 units can be decomposed into
  two processes
• fast was attributed to the vibration of atoms
  (mainly oxygen)
• slow revealed the Arrhenius temperature
  dependence with an energetic barrier of 0.69 eV
  and was attributed to the ionic migration.
• Acknowledgements
• This work was supported by the Slovak Grant
  Agency for Science through the grant No.1/0218/03
  and by the Grant Agency of the Czech Republic
  through the grant No 104/03/0976. It was alsoa
  part of the research project CEZ MSM 223100002
  Preparation and properties of advanced materials
  - modelling, characterization, and technology.