[実験課題名]
ホモロガス構造をもつカルコゲナイド化合物の精密結晶構造解析
[Title of Experiment]
Structural investigations of chalcogenide compounds crystallized in the homologous phase
[実験責任者 / Project Leader]
松永 利之 / Matsunaga Toshiyuki (0005185)
[ビームライン / Beamline]
BL02B2
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For about ten years, we have been investigating the structural features for several pseudo-binary chalcogenide compounds, GeTe-Sb2Te3 or GeTe-Bi2Te3, since these compounds are used for rewritable optical recording materials as well as for future non-volatile electronic memories and thermoelectric energy conversion devices. These pseudo-binary systems are known to form various intermetallic compounds represented by chemical formula, (GeTe)n(Sb2Te3)m or (GeTe)n(Bi2Te3)m [1], which are called homologous series compounds. Although these materials are very important to exploit new devices with better performance, almost all of these compounds have very complicated structures with long-period layer stackings, which makes precise structural analyses difficult. For this reason, the detailed structural analysis for GeBi4Te7, which is one of these homologous compounds as well, has not been made yet. However, this time, we successfully analyzed the structure of this compound using a large Debye-Scherrer camera installed in BL02B2 at the Japan Synchrotron Radiation Research Institute (SPring-8) [2].
We prepared the GeBi4Te7 by melting stoichiometric mixtures of 99.999%-pure Ge, Bi, and Te in an argon atmosphere and then quenched it into ice water. The resulting alloy ingot was annealed at 723 K for 15 days in an argon atmosphere. The powder specimen produced by crushing the completed ingot was packed in a quartz capillary tube with an internal diameter of 0.2 mm. This structure had, as shown in Fig. 1, a complicated long stacking trigonal structure ( a= 4.3451(1) and c= 23.8205(8)Å in the space group of P-3m1 at 90 K ). Ge and Bi atoms randomly occupy 1(b) and two different 2(d) sites, and Te atoms are located at other four sites (see Table I). This structure is an isostructure of GeSb4Te7.
For the GeTe-Sb2Te3 and GeTe-Bi2Te3 homologous series, the crystal structures of Ge3Sb2Te6 [3], Ge2Sb2Te5 [4], and GeSb2Te4 [5, 6], Ge2Bi2Te5[7], have already been analyzed. In these materials, partial disordering between the Ge and Sb(/Bi) atoms occurred across their atomic sites. We then expected that a similar partial disordering would have taken place in GeBi4Te7, and conducted Rietveld analysis regarding and as independent variables and assuming the following constraints among the three Ge/Bi sites:
(see Fig. 2)
where X and Y in gXY represent atom species and atomic site, respectively, and g means occupancy factor. The final structural analysis results are shown in Table I and Fig. 1. As shown in this table, it was revealed that, in the GeBi4Te7 crystal, Ge and Bi atoms coexist at each of their three sites. However, Ge/Bi(1) and (2) sites prefer Bi atoms, whereas Ge/Bi(3) site is occupied by more Ge atoms than the other two sites. This suggests that the structure of this material can be approximated by the perfectly ordered one shown in Fig. 1. We have also taken the powder diffraction data for PbBi4Te7 and SnBi4Te7 together with GeBi4Te7. We are now examining these materials to clarify their structural features. It is expected that these materials have isostructures of each other. We will show the analysis results for those materials near future.
Table and figure captions
Table I. Refined structural parameters for GeBi4Te7 at 90 K. Standard deviations are shown in parentheses. Of the six kinds of g parameters at the Ge/Bi sites, and were set as the independent variables in this analysis.
Fig. 1. Perfectly ordered crystal structure of GeBi4Te7 shown in perspective, in which red and blue spheres represent Ge or Bi atoms, and yellows indicate Te atoms.
References
[1] L. E. Shelimova, O. G. Karpinskii, V. S. Zemskov, and P. P. Konstantinov, Inorg. mater. 36, 3, 302 (2000).
[2] E. Nishibori, M. Takata, K. Kato, M. Sakata, Y. Kubota, S. Aoyagi, Y. Kuroiwa, M. Yamakata, and N. Ikeda, Nucl. Instrum. Methods A 467−468, 1045 (2001).
[3] T. Matsunaga, R. Kojima, N. Yamada, K. Kifune, Y. Kubota, and M. Takata, Appl. Phys. Lett. 90, 161919 (2007)
[4] T. Matsunaga, N. Yamada, and Y. Kubota, Acta Crystallogr. B, 60, 685 (2004).
[5] O. G. Karpinsky, L. E. Shelimova, M. A. Kretova, and J-P. Fleurial, J. Alloys Compd. 268, 112 (1998).
[6] T. Matsunaga and N. Yamada: Phys. Rev. B, 69, 104111 (2004).
[7] T. Matsunaga, R. Kojima, N. Yamada, K. Kifune, Y. Kubota, and M. Takata, Acta Crystallogr. B, 63, 346 (2007) |