Structural study of Li2S-GeS2 glasses: GeS network and local environment of Li
Introduction
The structure of germanium disulphide (GeS2) glass has been studied by using various experimental techniques such as Raman spectroscopy [1], extended X-ray absorption fine structure (EXAFS) [2], X-ray diffraction [3] and neutron diffraction [4,5]. The results of these studies show that GeS4 tetrahedra are dominant fundamental units and they are connected by themselves to form the network structure. Of particular interest in the structure of GeS2 glass may be the connectivity of GeS4 tetrahedra. The network structure of this glass is formed by the connection of a considerable number of edge-sharing GeS4 tetrahedra in addition to corner-sharing ones. Both types of the configurations are different in GeGe distances from each other. Namely, a GeGe correlation peak between the edge-sharing GeS4 tetrahedra is separately observed around 2.9 Å in the real-space data [[2], [3], [4], [5]] while the corner-sharing GeS4 tetrahedral configurations have a longer GeGe distances. The peak identification of the GeGe correlations between edge- and corner-sharing GeS4 tetrahedral configurations has been made by reference to the high temperature crystal structure of α-GeS2 [6].
Reverse Monte Carlo (RMC) method has been used for building a three-dimensional structure model with fitting to the measurement data [7]. Recently the present author achieved the building structure models of GeS2 glass by using RMC simulation in which a constraint for edge- and corner-sharing GeS4 tetrahedral configurations was introduced [5]. In the RMC simulation, the two-dimensional layer network model based on the crystal structure of α-GeS2 was examined as well as the three-dimensional random network model. Resultantly, the models reproduced the measurement data well, showing the validity of the constraint for GeS4 tetrahedral configurations.
From a technological point of view, on the other hand, there has been considerable interest in lithium sulphide glasses due to their potential applications for solid state electrolytes [[8], [9], [10], [11]]. Li2S-GeS2 glasses samples can be prepared over a wide range of compositions and they exhibit high ionic conductivity at room temperature (~ 1 × 10−5 Scm−1) for the composition of (Li2S)50(GeS2)50 [[12], [13], [14], [15]]. In order to understand the ionic conduction mechanism, detailed information about the atomic structure, particularly with respect to the host network and the environment of mobile Li ions. Neutron diffraction is a very powerful technique for obtaining information about the location of Li because the coherent neutron scattering length of Li is negative and its absolute value is comparable to those of Ge and S. The Neutron diffraction technique has been applied to Li-containing chalcogenide glasses, sometimes using isotopic substitution method [16]. The present author and co-workers reported neutron diffraction results of (Li2S)x(GeS2)100-x glass samples (x = 20, 30, 40 and 50) [17]. Later RMC simulation was used to construct a three-dimensional structure model of (Li2S)50(GeS2)50 glass using the neutron diffraction data as well as newly measured X-ray diffraction data [18]. The result of the RMC study shows that the GeS network is mainly formed by corner-sharing GeS4 tetrahedra as well as by a few edge-sharing ones, and Li ions are mainly coordinated by four sulphur atoms. However, GeGe correlations between GeS4 tetrahedra in the RMC model does not always coincide with the proper distance for each configuration. Hence a constraint for the GeGe distances of edge- and corner-sharing GeS4 tetrahedral configurations is needed for constructing a more precise model of the host GeS network. In addition, the Li ion environment obtained from the previous RMC model are also capable of improvement. Unlike in the case of a covalent bond network in which constraints for coordination numbers or distinctive configurations based on experimental evidences are valid for improving the reliability of the model, it is difficult to determine the precise location of ions having Coulomb interaction just by using RMC simulation. In order to eliminate an energetically unreasonable structure, molecular dynamics (MD) simulation is useful. Actually, MD simulation has been used for Na containing chalcogenide glass systems such as Na2S-GeS2 [19], Na2S-SiS2 [20], Na2S-P2S5 [21] and Na2S-As2S3 [22] and significant progress has been made in understanding the structures of these glasses.
In this work, accurate three-dimensional atomic structure models for (Li2S)x(GeS2)100-x glasses were built by a combination of RMC and MD methods through the following steps. The model of host GeS network for each glass sample was constructed by using RMC simulation with fitting to the measured X-ray diffraction data, in which a constraint for the GeGe distances corresponding to edge- and corner-sharing GeS4 tetrahedral configurations was introduced. The locations of Li ions were determined by using MD simulation using the RMC model of host GeS network.
Section snippets
Theory of X-ray and neutron diffraction
The X-ray and neutron total structure factors, SX(Q) and SN(Q), where Q is the magnitude of momentum transfer, are respectively derived from the scattering intensities, IX(Q) and IN(Q), by using the Faber-Ziman definition [23]:wherewhere fl(Q), bl and cl are respectively the X-ray atomic form factor, the neutron coherent scattering length and the concentration of species l.
SX(Q) and SN(Q) are respectively a weighted
Experimental procedure
The (Li2S)x(GeS2)100-x glass samples (x = 0, 20, 30 and 40) used for X-ray diffraction measurements were prepared by reacting Li2S and GeS2 using the same procedure as reported previously [17].
The densities of the samples were measured at room temperature using a gas pycnometer (Quantachrome, Micro-Ultrapyc 1200e) with high-purity helium (99.9999%) as the displacing fluid and the measurement values are presented in Table 1.
The X-ray diffraction measurements were carried out using a horizontal
RMC simulation
The three-dimensional atomic configurations of GeS network for (Li2S)x(GeS2)100-x glasses (x = 20, 30, 40 and 50) as well as GeS2 glass were generated by using the RMC method [7] with fitting to the corresponding X-ray structure factor. Because of the large X-ray atomic form factors of Ge and S compared to that of Li, the X-ray diffraction data mainly provides information about the GeS network. Cube simulation boxes with periodic boundary conditions containing 4800 atoms (4992 atoms for GeS2
X-ray diffraction results
The measured SX(Q) for (Li2S)x(GeS2)100-x glasses (x = 0, 20, 30 and 40) are shown in Fig. 1, along with the previous result for (Li2S)50(GeS2)50 glass [18]. The general features of SX(Q) for these glasses are similar but clear differences are observed in the first and the second peaks at about 1.1 Å−1 and 2.1 Å−1, respectively. The first peak, which is the so-called first sharp diffraction peak (FSDP), is associated with medium-range ordering with the periodicity of 2π/Q. The heights of the
Conclusions
The static structures of (Li2S)x(GeS2)100-x glasses (x = 0, 20, 30, 40 and 50) were investigated by using a combination of diffraction measurements and simulation techniques. The RMC models show that a certain number of the edge-sharing GeS4 tetrahedra are present even when the number of connections between GeS4 tetrahedra decreases with increasing Li2S content. In addition, the MD results show that Li ions are located in a deformed octahedral site consisting of BS and NBS atoms.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
The synchrotron X-ray diffraction experiment was performed at the BL04B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI).
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