Molecular dynamics study of Li2SiS3 glass

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SOLID STATE IONICS

Solid State Ionics 91 (1996) 285-288

EISEVIER

Molecular

dynamics study of Li,SiS,

M. Seshasayee”,

glass

K. Muruganandam

Department of Physics, Indian Institute of Technology, Madras 600 036, India

Received 3 December 1995; accepted 5 July 1996

Abstract Molecular dynamics simulation has been carried out for highly conducting Li,SiS, glass at temperatures ranging from 300-1800 K in order to investigate the glass structure and dynamical behaviour of the mobile ions Li’. The glass transition temperature obtained from the change in energy and diffusion constant D of Li’ as temperature is lowered occurs around 750 K. Silicon is predominantly pentacoordinated to sulphur atoms at an average distance 2.19 A, the geometry around Si being approximately square pyramidal. The glass matrix shows little change with temperature. Room temperature value of diffusion constant (0) of Li’ is 3.53 X lo-” mz SC’. Lithium ions are surrounded by four sulphur atoms, Li-S distance being 2.61 A. Keywords: Glass; Lithium; Silicon; Sulphide; Molecular dynamics; Diffusion

1. Introduction Among the glassy solid electrolytes, Li based oxide glasses are materials of choice for use in high energy batteries, since Li is light and most electropositive. Considerable enhancement in conductivity takes place if oxygen is replaced by the more polarizable sulphur [l-4]. The recently prepared binary and ternary glasses in Li-S system has different glass formers, namely sulphides of silicon [ 11, germanium [ 131, boron [4] and phosphorus [3]. Glasses in these systems have ionic conductivity (a) comparable to those of the best crystalline ionic conductors, in the range 1O-3 R-’ cm-’ at room temperature. Conductivity studies on binary glass

*Corresponding author.

- lithium

L&S-SiS, [l] show (T to be 5 X 10e4 K’ cm-’ at 25°C increasing several fold when doped with lithium halides [2]. Similar results are obtained for L&S-P,S, [3] and Li,S-B,S, [4]. ‘Li NMR study of Li,S-SiS, glass [5] shows enhancement in conductivity and in correlated movement of Li ions with increasing Li content in the glass. “Si MAS-NMR work [6] on L&S-SiS, glasses find evidence for the presence of edge sharing SiS, tetrahedra in these systems. A similar observation is made by Angel1 [7] from molecular dynamics study on glasses in L&SSiS, system. Neutron [8] and X-ray RDF [9] studies on Li,SiS, glass show that Si is tetrahedrally coordinated to S with less tendency for edge sharing. Neutron RDF gives tetrahedral coordination of S around Li at 2.70 A whereas in X-ray RDF Li-S occurs at 2.67 A with coordination number 5.3. This work is taken up to probe the short range order

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286

M. Seshasayee,

K. Muruganandam

I Solid State Ionics 91 (1996) 285-288

Table 1 Potential uarameters

a,

6)

b,, (A, c,, (kJ A”

mol-‘)

Li-Li

Si-Si

s-s

Li-Si

Li-S

Si-S

1.7 0.08

1.45 0.072

3.72 0.284

1.6 0.076

3.08 0.182

2.95 0.178

0

0

2889

0

0

0

f,=5.428 kJ/A/mol. Z(Li)=O.l,

Z(Si)=2.6

and Z(S)=

-1.4.

existing in these glasses applying molecular dynamics (MD).

the technique

of

2. Experimental We performed constant volume MD simulation in an ensemble containing 360 atoms to which periodic boundary conditions were applied. The potential used was similar to the one used by Habasaki and Okada [ 101, namely @,j = ZiZje2/r +f,b,,

exp([aij - r]lp) - cij/r6

The potential parameters summarized in Table 1 were adjusted to obtain peaks in correlation function of Si-S and Li-S, which agreed with peaks in X-ray

RDF [9] of Li,SiS, glass. The melt was cooled from 1800 to 300 K in steps of 50 K, running for 9 ps at each temperature. A time step of lo-l5 s was used throughout. At each temperature, the system was allowed to equilibrate for 3 ps. The configurations in the subsequent 6000 steps at constant energy were used for calculating pair correlation function and running coordination number. Plot of pair correlation function at 300 K is given in Fig. 1. Mean square displacement (MSD) for temperatures 300-1500 K in steps of 100 K were calculated by storing position coordinates of all atoms for every five steps (5 X lo-l5 s) for 6 ps. Similar procedure was followed for calculating the velocity auto correlation function (VACF). Diffusion constant for Li was calculated from a plot of MSD vs. temperature and the glass transition temperature T, from plots of total energy vs. temperature which is shown in Fig. 2 and In D vs.1000/T which is shown in Fig. 3.

33OK

3. Results and discussion 20

15

10

5

0 1.9

2.4

23

Fig. 1. Pair correlation functions Li-S and Si-S pairs at 300 K.

3.4

of Li-Li,

3.9

Si-Si,

4.4

S-S,

Li-Si,

Si-S pair correlation function at 300 K shows a well defined Si-S peak at 2.19 A. The running coordination number (RCN) for Si-S has a long plateau at 5. These parameters have the same value at all temperatures indicating the preservation of the basic SiS, polyhedra. SiS, crystal structure results [l l] show chains of edge shared SiS, tetrabedra, with Si-S = 2.14 A, the Si-Si distance along the chain being 2.13 A. This leads to the conclusion of edge shared tetrahedra in the structure. In the high pressure modification of SiS, crystal [12], the SiS, tetrahedra are comer sharing as in SiO, structure with Si-S=2.13 A, Si-Si=3.47 A. Our room temperature MD data shows that 77% of Si atoms are surrounded by five sulphurs, 17% with six and 6% with four sulphur atoms. The angular distribution

M. Seshasayee, K. Muruganandam I Solid State Ionics 91 (1996) 285-288

287

- 4.36

1

Temperature

(‘K)

Fig. 2. Plot of total energy vs. temperature.

-23

f 0.0

I OS

I LO

I I.5

I 2.0

I

I

I

7.5

3.0

3.5

lOOO/ T Fig. 3. Plot of logarithmic 1000/T for lithium ions.

diffusion

constant

D (cm’/s)

vs.

of S-Si-S around five coordinated Si atoms show peaks at 90 and 170”. This distribution favours approximate square pyramidal geometry. Si generally exhibits only four coordination though there are

cases of pentacoordinated Si. Two Si-Si peaks occur at 3.49 and 4.05 for all temperatures studied. RCN plot for Si-Si at room temperature exhibits a plateau at 1 for distances 3.6-3.9 A and another plateau at 4 starting at 4.5 A. The plateau at short distance disappears above T,, though the plateau at large distance is present. In addition, at low temperatures (below T,) the second peak shows a shoulder. These facts indicate that the motion causing relative orientation of the neighbouring SiS, polyhedra gradually decreases as temperature decreases. MD results at RT also show that 45% of SiS, polyhedra are edge sharing, though the first Si-Si peak occurs at 3.49 A. This distance is longer than found in edge sharing SiS, structure and is due to the lithium atoms present in the glass network. MD studies on 3Li,S. 2SiS, and Li,S*3SiS, glass reported by Angel1 [7], show a similar Si-Si distance of 3.5 A. The number of SiS, tetrahedra above glass transition temperature increases while SiS, polyhedra decreases with increasing temperature, these changes being considerable. In the same temperature range, the number of SiS, polyhedra changes only slightly showing that the geometry around Si is not greatly influenced by the onset of the glass transition, at least in the time

288

M. Seshasayee, K. Muruganandam I Solid State Ionics 91 (1996) 285-288

scale studied here. For counting the coordination number, the cut-off was taken at the distance where the pair correlation function crosses 1 after the peak to take into account the broadening of the peak with increase in temperature. The percentage of nonbriding sulphur atoms (NBS) observed above and below T, shows an expected increase from 48% at 3006< to 55% at 1000 K. The Li-S peak occurs at 2.61 A for all temperatures studied. The presence of a mild plateau in RCN of Li-S at 4 at low temperatures and its disappearance at high temperatures shows some local ordering of four sulphur atoms around Li at low temperatures. A much shorter Li-S distance of 2.15 A with for sulphur around Li is reported by Angel1 in Li,Si,S, with four S*- surrounding Li+ at 1000 K. The reason for such a short Li-S distance is not clear, since even in Li,S crystal structure, the Li-S distance is 2.47 A, which is the shortest one can expect. The diffusion constant D of Li+ ions at 1000 K is 3.8 X 10m9 m2 s-r which is higher than the value of 1.25 X 10m9 m2 s-’ reported [8] for Li,Si,S, glass. The reason for this may be the non-inclusion of polarization interactions in Li,Si,S,. The glass transition temperature T, was obtained from the plots of total energy vs. T [Fig. 21 and In D vs. 1000/T [Fig. 31. The change in slope in these plots occurs at Tg. T, measured at change in slope in these graphs occur at 800 and 750 K respectively, which are higher than the experimentally measured value 611 K [l]. The high fictive temperature Tf (at which the glass would be in metastable equilibrium if it could be brought to T, instantaneously from the melt) of the glass is probably the reason for this difference in T,. Velocity auto correlation function of Li is less oscillatory than that of Si and S as it should be for a mobile species.

Conclusions At all the observed temperatures, Si is predominantly pentacoordinated to S at a distance of 2.19 A with square pyramidal geometry. Temperature seems to have very little effect on these parameters. The plateau at 4 in the RCN plot of Li-S below Tg shows some ordering of sulphur atoms around Li at a distance of 3 A. The diffusion constant D of Li’ at room temperature is 3.53X lo-” m2 s-r as calculated from the MSD plot. The glass transition occurs between 750 to 800 K.

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