The structure of Ge2Sb2Te5 and the mechanism ofits rapid phase change
S. Koharaa, K. Oharaa, L. Temleitnera, K. Kobayashib, J. Akolac,d,e, R. O. Jonese,f,T. Matsunagag, R. Kojimag, N. Yamadag, and M. Takatah,iaSPring-8/JASRI, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan
Beamline Station at SPring-8, National Institute for Materials Science, 1-1-1 Kouto, Sayo-gun,Hyogo 679-5198, Japanc
Department of Physics, Tampere University of Technology, P.O. Box 692, FI-33101 Tampere,FinlanddNanoscience Center, Department of Physics, University of Jyväskylä, P.O. Box 35, FI-40014
Jyväskylä, FinlandeInstitut für Festkörperforschung, Forschungszentrum Jülich, D-52425 Jülich, Germanyf
German Research School for Simulation Sciences, FZ Jülich and RWTH Aachen University,
D-52425 Jülich, Germanyg
Panasonic Corporation, 3-1-1 Yagumo-Nakamachi, Moriguchi, Osaka 570-8501 Japan
hSPring-8/RIKEN, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japani
Department of Advanced Materials Science, School of Frontier Science, The University of Tokyo,5-1-5, Kashiwanoha, Chiba 277-8561, JapanbABSTRACT
We have determined the crystalline and amorphous phases of GST using a combination of advanced
synchrotron radiation experiments and massively parallel molecular dynamics (MD) / density
functional (DF) simulations and reverse Monte Carlo (RMC) methods. We focus on the structuralsimilarity of the two phases, paying particular attention to the bond angles (defined by three atoms)
and the ring statistics (connectivity of atoms) beyond the nearest neighbors. Furthermore, we address
the relationship between bond recombination and phase change speed.

INTRODUCTION
Since its discovery in 1987,1) much work has been carried out on the structure of Ge2Sb2Te5 (GST)
and the reasons for its rapid phase change. The “umbrella flip model” proposed by Kolobov et al.,2)
in particular, led to some controversy and to the recognition that knowledge of the structure ofamorphous GST was crucial. Many experiments and computer simulations have addressed thisproblem.3)
To understand the structure of Ge2Sb2Te5 and the mechanism of its rapid phase change, we haveused a combination of theory and experiment. We have performed synchrotron x-ray diffraction(XRD) measurements4,5) and x-ray photoelectron spectroscopy (XPS) measurements6) on bothcrystal and amorphous Ge2Sb2Te5 and have performed a very extensive density functional theory(DF) / molecular dynamics (MD) simulation on 460 particles7) to obtain a model of the amorphousphase. This model was recently refined on the basis of reverse Monte Carlo simulation (RMC) toreproduce both XRD and XPS data.8) Here we apply RMC simulation techniques to the crystal phaseof Ge2Sb2Te5 (rock salt structure) and compare the three-dimensional configurations of theamorphous and crystal phases of Ge2Sb2Te5 at intermediate range by deriving bond angle
distributions and ring statistics, i.e. the atomic connectivity. We discuss the relationship between
bond recombination and the speed of phase change.

EXPERIMENTS
Sample preparation
The specimen for high-energy x-ray diffraction experiments was made by laminating an organic film
sheet on a glass disc with a diameter of 120 mm and sputtering to form the recording film with athickness of 200 – 500 nm. The organic film was scraped from the glass disc, and the specimen wasremoved from the glass substrate using a spatula. The composition of the sample was examined byinductively coupled plasma atomic emission spectrometry. The 300-nm thick samples fortime-resolved x-ray diffraction experiments were prepared by depositing Ge2Sb2Te5 on a SiO2 glass
substrate (12 cm diameter and 0.6 mm thick) with a 2-nm thick 80 mol% ZnS-20 mol% SiO2 cap layer.

High-energy x-ray diffraction experiment
The high-energy x-ray diffraction experiment was carried out at the high-energy x-ray diffraction
beamline BL04B2.9) The diffraction patterns of powder sample in a thin walled (10 m) tube of 0.5
mm diameter (supplier: GLAS Müller, D-13503 Berlin) and an empty tube were measured in atransmission geometry. The collected data were corrected using standard programs,9) and thecorrected data were normalized to give the coherent scattering intensity I(Q) and the Faber-Ziman
structure factor S(Q).10)

RMC simulation
The RMC method11) is a useful tool for constructing three-dimensional structural models ofdisordered materials using mainly experimental diffraction data. In this technique, the atoms of an
initial configuration are moved to minimize the deviation from experimental diffraction and/orEXAFS data. This technique has been applied extensively to disordered materials that do not exhibit
structural periodicity12) and to crystalline materials to understand local structure and disorder.13,14) Inthis study, we have applied the RMCPOW code13) to the metastable cubic phase to derive a
3-dimensional atomic configuration. A 10 x 10 x 10 supercell configuration of 7200 particles wasused to fit the coherent scattering intensity I(Q). The Sb and Ge atomic positions were interchanged
during the simulation.

RESULTS
It is well known that crystalline Ge2Sb2Te5 has a rock salt (NaCl-type) structure with 20% vacancies
and random Ge/Sb occupation of one sublattice.4) Figure 1 shows the coherent scattering intensity
I(Q) obtained by RMC simulations. The agreement between experimental data and the RMC modelis excellent.
The partial pair correlation functions, gij(r), of crystal Ge2Sb2Te5 (c-GST) obtained from the RMCmodel are shown in Fig. 2 together with the data of amorphous Ge2Sb2Te5 (a-GST).8) It isremarkable that c-GST exhibits significant oscillations indicating long-range periodicity in thecrystal phase. The most striking feature in a-GST is that both Ge-Te and Sb-Te exhibit a well defined
first peak whose amplitude is comparable to those of c-GST. This feature shows that AB (A=Ge, Sb,B=Te) alternation is significant in a-GST, which is an important feature of this material. However,the positions of Ge-Te and Sb -Te correlation peaks in a-GST are smaller than those in c-GST,
suggesting that both Ge-Te and Sb-Te form covalent bonds with smaller coordination numbers in theamorphous phase.
Figure 3 shows bond angle distributions of Te-Ge-Te and Te-Sb-Te in c-GST and a-GST. Crystalline
GST shows a well-defined peak at 90 degrees arising from the rock salt structure. The bond angle
distributions of Te-Ge-Te and Te-Sb-Te in a-GST are similar to those of c-GST, although the peakwidth in a-GST is slightly larger than in c-GST.
The ring statistics, which show the connectivity of Ge-Te and Sb-Te, is an important structuralparameter for understanding the similarity between c-GST and a-GST. The calculation used theshortest-path analysis, where we count the number of atoms in the shortest closed loop from a given
atom. The calculated ring statistics for c-GST and a-GST are shown in Fig. 4, where A-B-A-B rings
are fourfold rings found in the NaCl lattice. As can be seen in the figure, c-GST consists of four- andsixfold rings due to the 20% vacancies on 4(a) sites. On the other hand, a-GST has a significant
number of fourfold rings, which can be recognized as nuclei of crystal. This feature can be seenclearly in Fig. 5, where structural units found in the NaCl lattice are evident. These results suggestthat the similarity of bond angle distribution and ring statistics in c-GST is a-GST is a unique featureof GST. Furthermore we propose that the numerous 4-fold rings with ABAB alternation (A: Ge, Sb;

B: Te) is the key to the rapid crystallization of a-GST, since such fourfold rings can act as nuclei, andthe presence of many four- and sixfold rings (see. Fig. 4) allows the phase change to occur with lessbond breaking and formation.

CONCLUSIONS
We have applied RMC simulation to synchrotron x-ray diffraction data on crystalline Ge2Sb2Te5 andhave compared the results with those of amorphous phase. The structures of c-GST and a-GST have
similar bond angle distributions and ring statistics. Furthermore, a-GST has numerous fourfold rings
with ABAB alternation (A: Ge, Sb; B: Te). We propose that they act as nuclei in the rapid
phase-change to the NaCl crystalline phase and that this feature is the basis of rapid nucleation
driven crystallization with little bond breaking and formation.

ACKNOWLEDGEMENTS
We thank Prof. László Pusztai for fruitful discussions. This work was supported by CREST “X-ray
pinpoint structural measurement project - Development of the spatial- and time-resolved structural
study for nano-materials and devices” and the Academy of Finland and the Japan Science andTechnology Agency via the Strategic Japanese-Finland Cooperative Program on “FunctionalMaterials”. The MD/DF simulations were performed on the Jugene (IBM BlueGene/P)
supercomputer in the Forschungszentrum Jülich with grants from the FZJ and the John von
Neumann Institute for Computing (NIC).

REFERENCES
1. N. Yamada et al., Jpn. J. Appl. Phys. 26, Suppl. 26–4, 61 (1987).
2. A. Kolobov et al., Nature Mater. 3, 703 (2004).
3. M. Wuttig and N. Yamada, Nature Mater. 6, 824 (2007).
4. N. Yamada and T. Matsunaga, J. Appl. Phys. 88, 7020 (2000).
5. S. Kohara et al., Appl. Phys. Lett. 89, 201910 (2006).
6. J. J. Kim et al., Phys. Rev. B 76, 115214 (2007).
7. J. Akola and R. O. Jones, Phys. Rev. B 76, 235201 (2007).
8. J. Akola et al., Phys. Rev. B 80, 020201R (2009).
9. S. Kohara et al., J. Phys.: Condens. Matter 19, 506101 (2007).
10. T. E. Faber and J. M. Ziman, Philos. Mag. 11, 153 (1965).
11. R. L. McGreevy and L. Pusztai, Mol. Simul. 1, 359 (1988).

12. R. L. McGreevy, J. Phys.: Condens. Matter 13, R877 (2001).
13. A. Mellergård, R. L. McGreevy, Acta Cryst. A55, 783 (1999).
14. M. Tucker et al.: J. Phys.: Condens. Matter 19, 335218 (2007).

Fig. 1

Fig. 2

Normalized x-ray scattering intensity I(Q) for c-GST.

Partial pair correlation function gij(r) for c-GST(dotted lines) and a-GST (solid lines).

Bond angle distributions for c-GST(dotted lines) and a-GST (solid lines).

60

Crystal40
Fraction (arb. unit)

Fig. 320025

Amorphous

20
15
10
50

3

4

5

6

7

8

910 11 12 13

n-fold ring

Fig. 4

Ring statistics for c-GST and a-GST.

Fig. 5

Atomic configuration of c-GST and a-GST obtained from RMC simulation.