Photon Factory Activity Report 2014 #32 (2015) B

BL-7C/2013G187

Local structure change around germanium due to crystallization of LiGe2(PO4)3 withNASICON-type structure
Shun Fujieda1, Hiroshi Arima2, Toru Kawamata3, Yuki Koshika1, Kozo Shinoda1,Kazumasa Sugiyama2, Shigeru Suzuki1
1Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira,Aoba-ku, Sendai 980-8577, Japan
2
Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577,
Japan
3
Research Center for Electron Photon Science, Tohoku University, 1-2-1 Mikamine, Taihaku-ku,Sendai 982-0826, Japan

2 Experiment
The mixture of chemicals such as Li2CO3, GeO2 andNH4H2(PO4)3 was melted in a platinum crucible at 1673
K for 10 min in air. In order to obtain glass specimen, themelt was quenched onto a copper plate and then
immediately pressed by a copper block. The crystal
specimen with the NASICON structure was prepared byannealing the glass specimen at 1123 K for 6 h.
The XAFS measurements were carried out at the Ge K
absorption edge (11.1036 keV) using the synchrotron
radiation facility at the beam line station BL-7C of thePhoton Factory, Tsukuba, Japan. To obtain the adequate
intensity, appropriate amounts of the specimens werefinely ground and mixed with BN powders, and then their
X-ray absorption spectra were measured in thetransmission mode. The X-ray absorption spectra wereanalyzed by the program REX2000.

3 Results and Discussion
Radial structure functions (RSFs) around germanium
atoms obtained by Fourier transform of extended X-ray
absorption fine structure spectra are presented in Fig. 1.
The RSF of the glass specimen exhibits a correlation peakwhich is attributable to the coordination of oxygen. It is
well known that the Ge-O atomic distance in thecrystalline GeO2 is closely related with the coordination
number [2]. The Ge-O atomic distance of the quarts likeGeO2 with a four-hold coordination has been reported to
be located at the shorter atomic distance side incomparison with that of the rutile like GeO2 with a sixhold coordination. It should be noted that the Ge-O
correlation peak in the RSF of the glass specimen islocated at a longer distance side in comparison with thatof the crystal specimen. Accordingly, the change ofcoordination number of germanium is probably caused by
the crystallization of the NASICON-type LiGe2(PO4)3
compound from the glass phase.

LiGe2(PO4)3glass

Ge K0FT

1 Introduction
Rechargeable lithium ion batteries using non-aqueous
liquid electrolytes are applied for many electronic devices
because of large energy density. However, these liquid
electrolytes generally consist of flammable organic
solvents. To improve the safety of rechargeable lithium
ion batteries, superior solid lithium ion conductors withhigh lithium ionic conductivity and high stability should
be developed.
The sodium (Na) super ionic conductor (NASICON)type LiGe2(PO4)3-based compounds have been reported toexhibit the high stability in air and non-reactivity withlithium metal. Many efforts have been dedicated toincrease the lithium ionic conductivity of LiGe2(PO4)3based compounds. It has been reported that the lithium
ionic conductivity of the crystal phase of LiGe2(PO4)3
systems is much larger than that of the glass phase which
can be obtained by the quenching of molten glass [1].
Thus, it is meaningful to compare the structure of thecrystal phase with that of the glass phase in LiGe2(PO4)3based systems. However, structure of the glass phase ofLiGe2(PO4)3-based systems is not completely understoodyet. In this study, the local structure around germanium
atoms of the glass and crystal phases of LiGe2(PO4)3 wasinvestigated by X-ray absorption fine structure (XAFS)
measurements.

crystal

00.05

0.10

0.15
R (nm)

0.20

0.25

Fig. 1: Radial structure functions (RSFs) aroundgermanium atoms of the glass and crystal
specimens of LiGe2(PO4)3.
References[1] J. Fu, Sol. State Ion. 104 (1997) 191.
[2] M. Alami et al., J. Sol. State Chem. 90 (1991) 185.
* fujieda@tagen.tohoku.ac.jp