Structural Features of Ge1Sb4Te7, an Intermetallic Compound in the

5750

Chem. Mater. 2008, 20, 5750–5755

Structural Features of Ge1Sb4Te7, an Intermetallic Compound in the GeTe-Sb2Te3 Homologous Series Toshiyuki Matsunaga,* Rie Kojima,† Noboru Yamada,† Kouichi Kifune,‡ Yoshiki Kubota,§ and Masaki Takata| Materials Science and Analysis Technology Center and AV Core Technology DeVelopment Center, Matsushita Electric Industrial Co., Ltd., Osaka, 570-8501 Japan, CREST-JST, Saitama, 332-0012 Japan, Faculty of Liberal Arts and Sciences and Graduate School of Science, Osaka Prefecture UniVersity, Osaka, 599-8531 Japan, and SPring-8/RIKEN, Hyogo, 679-5148 Japan ReceiVed December 7, 2007. ReVised Manuscript ReceiVed July 9, 2008

The structural and bonding nature of Ge1Sb4Te7 is investigated using an X-ray diffraction method and calculations based on density-functional theory (DFT). This material’s crystal is confirmed to have a 12-layered, close-packed cubic stacking structure (P3jm1). In this crystal, Te atoms occupy their own specific layers; however, it has been revealed that Ge and Sb atoms are not located in their respective layers but cause partial atomic disordering across other layers. The structure consists of two kinds of NaCl slabs stacked alternately, and the calculations demonstrate that adjacent slabs are connected by a van der Waals-type weak force and that Ge1Sb4Te7 forms a compound semiconductor with a very narrow band gap.

Introduction Today, the most widely used memory materials for rewritable phase change optical disks, such as DVD-RAM (digital versatile disk-random access memory) and Blu-ray discs, are pseudobinary compounds located at the tie line between GeTe and Sb2Te3. These chalcogenide materials are of considerable importance, because alloys in this system are used for rewritable optical recording materials as well as for future nonvolatile electronic memories and thermoelectric energy conversion devices. Consequently, many investigations have been conducted on the compounds in this system.1,2 To effectively use these materials’ properties in developing superior devices, however, it is important to first elucidate the crystallographic characteristics. It is known that this compound has two types of crystalline phase: one is a metastable phase with NaCl-type simple structures appearing over a wide composition range of GeTe, from 100 mol % to at least 33.3 mol %,3,4 and the other includes a wide variety of stable phases with complicated structures represented by the chemical formula n(GeTe) · m(Sb2Te3).5 The structures in the latter stable phases are closely related to each other; they can be described as structures with cubic close-packed * To whom correspondence should be addressed. Fax: 81-6-6906-3407. E-mail: [email protected]. † Matsushita Electric Industrial Co., Ltd., and CREST-JST. ‡ Faculty of Liberal Arts and Sciences, Osaka Prefecture University. § Graduate School of Science, Osaka Prefecture University. | SPring-8/RIKEN and CREST-JST.

(1) Wuttig, M.; Yamada, N. Nat. Mater. 2007, 6, 824. (2) Shelimova, L. E.; Karpinskii, O. G.; Konstantinov, P. P.; Kretova, M. A.; Avilov, E. S.; Zemskov, V. S. Inorg. Mater. 2001, 37 (4), 342. (3) Matsunaga, T.; Kojima, R.; Yamada, N.; Kifune, K.; Kubota, Y.; Tabata, Y.; Takata, M. Inorg. Chem. 2006, 45, 2235. (4) Matsunaga, T.; Morita, H.; Kojima, R.; Yamada, N.; Kifune, K.; Kubota, Y.; Tabata, Y.; Kim, J.-J.; Kobata, M.; Ikenaga, E.; Kobayashi, K. J. Appl. Phys. 2008, 103, 093511.

periodicity ( · · · abcabc · · · ), in which the stacking rules of the Ge, Sb, and Te layers are different from each other. The atoms in each layer distribute to form a triangular net in the a-b plane.4 These stable compounds can also be expressed as trigonal structures with cubic periodicity formed by closepacked stacks of NaCl slabs along the c-axis in hexagonal notation. Each NaCl slab consists of alternating stacks of Te and Ge/Sb layers, and both ends of the slab are covered with Te layers without exception. The adjacent Te layers in these compounds are assumed to be connected by a weak interaction, such as van der Waals bonding; on the other hand, the atoms within the slabs are considered to be strongly connected with adjacent atoms by covalent bonding.5 Ge1Sb4Te7, one of the compounds in the GeTe-Sb2Te3 pseudobinary system, was discovered about 40 years ago by electron diffraction studies to have a 12-layered homologous crystal structure with the space group P3jm1.6 Since then, however, no precise structural analysis has been carried out. The aim of this study is to investigate the crystal structure and the characteristics of its stable phase; we scrutinized the structure using a synchrotron powder diffraction method and examined its bonding nature through molecular orbital and band calculations. These examinations revealed that Ge and Sb atoms are not located at their specific sites but are randomly distributed across other sites, and they demonstrated that two adjacent NaCl slabs are connected by a van der Waals-like weak force, clearly illustrated in Figure 3 below.

(5) Karpinsky, O. G.; Shelimova, L. E.; Kretova, M. A.; Fleurial, J.-P. J. Alloys Compd. 1998, 268, 112. (6) Petrov, I. I.; Imamov, R. M.; Pinsker, Z. G. SoV. Phys. Crystallogr. 1968, 13 (3), 339.

10.1021/cm703484k CCC: $40.75  2008 American Chemical Society Published on Web 08/27/2008

Structural Features of Ge1Sb4Te7

Figure 1. Observed (+) and calculated (gray line) X-ray diffraction profiles of Ge1Sb4Te7 at 90 K. The profiles are shown on a logarithmic scale, and under them, reflection markers are indicated by vertical spikes. A difference curve (observed-calculated) appears at the bottom of the figure on a linear scale.

Experimental Section We prepared Ge1Sb4Te7 by melting a stoichiometric mixture of 99.999%-pure Ge, Sb, and Te in a silica tube filled with argon gas at 1073 K and then quenching it in ice water. The resulting alloy ingot was annealed at 723 K for 32 days in an argon atmosphere. The powder specimen produced by crushing the finished ingot was packed into a quartz capillary tube with an internal diameter of 0.3 mm to ensure a sufficiently large number of crystallites in the sampling volume for the X-ray measurements. The opening of the capillary was sealed using an oxyacetylene flame to insulate it against the atmosphere. We performed diffraction experiments using the BL02B2 beamline at the Japan Synchrotron Radiation Research Institute.7 The wavelength of the incident beam was λ ) 0.42986 Å. Intensity data were collected using a Debye-Scherrer camera with a 287 mm radius. An imaging plate with a pixel area of 100 µm2 was used as the detector. Angular resolution was 0.02°. We performed a low-temperature experiment while nitrogen gas at 90 K was blown onto the capillary tube. The crystal structure was examined and refined using the Rietveld method.8 The program used for this purpose was RIETAN.9 As mentioned above, a very short wavelength was used for the diffraction measurements to raise the transmission efficiency of the incident X-ray beam, which instead decreased the data points in terms of Bragg peak. To improve the accuracy of the Rietveld analyses, we then obtained intensity data in increments of 0.01° by reading the imaging plate for a pixel area of 50 µm2. The energy of the synchrotron’s radiation was confirmed by recording the diffraction intensity of CeO2 (a ) 5.4111 Å) powder as a reference specimen at room temperature under the same conditions. Neutral atomic scattering factors were used for the structural analysis, and isotropic thermal vibrations were assumed for all of the atomic sites.

Chem. Mater., Vol. 20, No. 18, 2008 5751

With this stacking, which contains two kinds of NaCl blocks (slabs), S5 and S7, a complete unit cell can be formed. We conducted a Rietveld analysis assuming this perfectly ordered structural model. This analysis gave a small reliability factor (RI ) 0.78%), which would seem to indicate a satisfactory result. However, some other intermetallic compounds in this pseudobinary system, such as Ge3Sb2Te6,10 Ge2Sb2Te5,11 and Ge1Sb2Te4,5,12 have been precisionexamined using X-ray diffraction methods. The results revealed that in these structures, Te atoms occupied their specific layers, whereas the Ge and Sb atoms were located in other layers with partial atomic disordering. We expected that a similar partial disordering would take place in this Ge1Sb4Te7, and thus we conducted Rietveld analysis with 2:2d 5:2d gSb and gGe as independent variables, assuming the following constraints among the three sites of 2:2d, 5:2d, and 7:1b: 2:2d 5:2d 5:2d 7:1b 7:1b g2:2d Ge ) 1 - gSb , gGe ) 1 - gSb , gGe ) 1 - gSb ,

and

7:1b 2:2d 5:2d ) 4 - 2 × gSb - 2 × gSb (2) gSb

(A) Structure Determination. The structure of the Ge1Sb4Te7 (P3jm1) compound is assumed to have the following stacking.6

where X and Y in gXY represent atom species and atomic site (see Table 1), respectively, and g is the occupancy factor. The final results of the structural analysis are shown in Table 1 and Figure 1, where it can be seen that this assumption produced satisfactory results. RI went from 0.78% down to 0.52% through this analysis, which ensures that Ge and Sb atoms are actually scattered across other sites; however, the ratio of the two atoms differs among sites. (Note that it is very difficult for us to distinguish whether the Te sites contain a small number of Sb atoms by the X-ray diffraction method, since their atomic numbers are very close to each other; we thus assume that the Te sites are occupied only by themselves, as well as in Ge3Sb2Te6, Ge2Sb2Te5, and Ge1Sb2Te4.) The Ge1Sb4Te7 crystal, as shown in Figure 2a, has a 12layer structure in which the two kinds of NaCl blocks are alternately stacked along the c-axis. Blocks S5 and S7 are seen in the structures of Sb2Te3 and Ge1Sb2Te4, respectively. In other words, the structure of Sb2Te3 consists of closepacked cubic stacks of only block S5, while that of Ge1Sb2Te4 is formed only by S7. Interatomic distances obtained for the Ge1Sb4Te7 structure are shown in Table 2. Each atom at the seven sites is coordinated by six other atoms. However, except for the Te(1) and Ge/Sb(3) atoms, the other five atoms have 3 + 3 neighbors at shorter and longer distances, as in the cases of Ge3Sb2Te6, Ge2Sb2Te5, and Ge1Sb2Te4, with interatomic distances scattered from 2.942 to 3.691 Å (90 K). In particular, the Te-Te distance of 3.691 Å between the two neighboring NaCl slabs is markedly longer than the others, which makes the interplanar distance between Te-Te layers longer than any of the others, as in the other homologous phases (the Te-Te distances for

(7) Nishibori, E.; Takata, M.; Kato, K.; Sakata, M.; Kubota, Y.; Aoyagi, S.; Kuroiwa, Y.; Yamakata, M.; Ikeda, N. Nucl. Instrum. Methods, A 2001, 467-468, 1045. (8) Rietveld, H. M. J. Appl. Crystallogr. 1969, 2, 65. (9) Izumi, F.; Ikeda, T. Mater. Sci. Forum 2000, 321-324, 198.

(10) Matsunaga, T.; Kojima, R.; Yamada, N.; Kifune, K.; Kubota, Y.; Takata, M. Appl. Phys. Lett. 2007, 90, 161919. (11) Matsunaga, T.; Yamada, N.; Kubota, Y. Acta Crystallogr., Sect. B 2004, 60, 685. (12) Matsunaga, T.; Yamada, N. Phys. ReV. B 2004, 69, 104111.

Results and Discussion

5752

Chem. Mater., Vol. 20, No. 18, 2008

Matsunaga et al.

Figure 2. (a): Atomic arrangement of Ge1Sb4Te7 in the perfectly ordered structure model (see stacking sequence (1)) shown in a stereographic view, in which red, yellow, and blue spheres represent Ge, Sb, and Te atoms, respectively. Each layer in the stacking forms a triangular net in the a-b plane. (b): Electron density distribution in the unit cell was obtained using MEM, and (c): that using band calculation is depicted in perspective as the equidensity contour surface of 0.25 e/Å3. The contour maps inserted in these cells show the electron density distributions on the lattice planes containing one Te(2) and two Te(3) atoms. The contours are drawn on a logarithmic scale from 0.025 through 0.25 e/Å3. Table 1. Refined Structural Parameters for Ge1Sb4Te7 at 90 Ka atom

site

g

x

y

z

B (Å2)

Te(1) Ge/Sb(1) Te(2) Te(3) Ge/Sb(2) Te(4) Ge/Sb(3)

1:1a 2:2d 3:2d 4:2c 5:2d 6:2d 7:1b

1.0 0.153/0.847(17) 1.0 1.0 0.155/0.845(15) 1.0 0.331/0.617

0 1/3 2/3 0 1/3 2/3 0

0 2/3 1/3 0 2/3 1/3 0

0 0.08365(21) 0.15490(14) 0.27122(12) 0.34002(18) 0.42508(14) 1/2

0.21 (2) 1.05 (4) 0.21 0.21 1.05 0.21 1.05

a The space group is P3jm1. Standard deviations are shown in 2:2d parentheses. Of the six kinds of g parameters at the Ge/Sb sites, gSb 5:2d and gSb were set as independent variables in this analysis. Overall temperature factors were assumed for the three Ge/Sb sites and for the four Te sites. Final R-factors and lattice parameters are Rwp ) 5.10%, Rp ) 3.60%, RI ) 0.52%, Rwp expected ) 1.05%, a ) 4.2360 (3) Å, and c ) 23.761 (2) Å.

Figure 3. Total energy dependences for (a): Ge1Sb4Te7 and (b): Sb5Te7 on the Te(2)-Te(3) interatomic distance between the two adjacent NaCl slabs (see Figure 2(a)) obtained by band calculation. In these structures, each Te(3) (/Te(2)) atom is located right above (/below) the center of a triangle formed by three neighboring Te(2) (/Te(3)) atoms. (The Te-Te interplanar distance corresponds to the gap width between these two slabs.) The solid lines in the Figures were fitted to the calculation results (0) using the leastsquares method assuming the Lennard-Jones-type potential.

Ge3Sb2Te6, Ge2Sb2Te5, and Ge1Sb2Te4 are 3.729, 3.742, and 3.663 Å at 90 K, respectively). These Te-Te layers, as mentioned above, are assumed to be connected by a van der

Waals-type weak force; we examined the bonding nature of this crystal by performing two kinds of DFT (densityfunctional theory) calculations as shown in the following section. (B) Examination of the Bonding Nature. It was revealed that in the Ge1Sb4Te7 crystal, Ge and Sb atoms coexist at each of their three sites as shown in Table 1. However, the Ge/Sb(1) and (2) sites prefer Sb atoms, whereas the Ge/Sb(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 stacking sequence (1). In performing molecular orbital calculations (DV-XR/discrete variational-Hartree-Fock-Slater method), we employed this perfectly ordered structural model to simplify the calculations, because it is very difficult to construct structure models with the actual disordered atomic arrangement. The cluster model for each site is shown in Table 2. The analytical program used for these calculations was SCAT.13 The results showed that the bond overlap population (∼0.013 e) between the Te(2) and Te(3) atoms, (13) Adachi, H.; Tsukada, M.; Satoko, C. J. Phys. Soc. Jpn. 1987, 45, 875.

Structural Features of Ge1Sb4Te7

Chem. Mater., Vol. 20, No. 18, 2008 5753

Table 2. Interatomic Distances for Ge1Sb4Te7 at 90 Ka Te(1)

Ge/Sb(1) ×6 3.1515 (32) Å

Ge/Sb(1) Te(1) Te(2) Te(2) Te(3)

Ge/Sb(1) ×3 2.9745 (34) Å Te(3) ×3 3.6906 (33) Å Te(2) ×3 3.6906 (33) Å Ge/Sb(2) ×3 2.9417 (29) Å

Ge/Sb(2) Te(3) Te(4) Te(4)

×3 3.1515 (32) Å ×3 2.9745 (34) Å

×3 2.9417 (29) Å ×3 3.1727 (35) Å

Ge/Sb(2) ×3 3.1727 (35) Å Ge/Sb(3) ×3 3.0249 (20) Å

Ge/Sb(3) Te(4)

×6 3.0249 (20) Å

-0.218 e

Te57Sb42

Table 3. Least Squares Fitting Results for the Curve Represented by Eq 3a ε

R

n

b

-2305.709(2) eV/mol

3.732(5) Å

10.4(4)

0.078(7) eV/mol

0.110 e 0.215 e -0.121 e -0.072 e

Te61Sb38 0.117 e 0.199 e

a

Te56Sb37Ge3 0.194 e 0.013 e

The estimated standard deviations for the last digits are in parentheses. The total energy is assumed to be in proportion to the potential energy, and the difference between them is represented as constant b.

Te56Sb33Ge7 0.013 e 0.186 e

0.154 e -0.190 e

Te52Sb29Ge12 0.217 e 0.088 e Te56Sb22Ge18 0.099 e 0.145 e

0.233 e

Te56Sb24Ge19 0.146 e

a

Standard deviations are shown in parentheses. The molecular orbital calculation results performed by assuming a perfectly ordered structural model are also tabulated. The fifth and sixth columns indicate the net charges and the bond overlap populations obtained, respectively. The clusters used for the calculations, which are configured with the atoms present within r ≈ 9 Å, are shown in the last column.

a measure of the number of electrons that contribute to the covalent bond between them, was very small. This implies that the adjacent NaCl slabs in the crystal are connected by very weak covalent bonding. On the other hand, the bond overlap populations for the other atomic pairs are fairly large, which means that the atomic pairs within an NaCl slab are connected by much stronger covalent forces than is the Te(2)-Te(3) pair. For instance, the overlap population for the (Ge/)Sb(1)-Te(2) pair is 0.19-0.20 e and that for Te(3)-(Ge/)Sb(2) pair is 0.19-0.22 e. These anisotropic bondings for Te(2) and Te(3) will cause a repulsive dipolar interaction at their interplanar gap. To examine the bond nature between adjacent NaCl slabs, we carried out a series of band calculations, varying the distance between the two slabs. The calculations were performed using pseudopotentials with electronic configurations of Ge 4s24p2, Sb 5s25p3, and Te 5s25p4 in the generalized gradient approximation of the plane-wave method. K points in a reciprocal unit cell were divided into 6 × 6 × 2; the tetrahedron method was used for the total energy calculations. The program used was CHASE-3PT.14 The two kinds of NaCl slabs, S5 and S7, were fixed to maintain the atomic configurations shown in Table 1 and to hold a perfectly ordered atomic occupation; only the interplanar distance between the two slabs was set as a variable parameter. Figure 3(a) shows the total energies obtained by these calculations. It is known that the interaction energy (V) between two atoms bonded by a van der Waals-like weak force can be described by the Lennard-Jones-type potential15 V(r) ) 4ε[(R/r)n - (R/r)6] (3) where ε is the depth of the potential well and R is the distance at which the potential is zero. The first term on the right(14) CHASE-3PT: The “PHASE” computer program was created by the members of the national project “Frontier Simulation Software for Industrial Science (FSIS)”, and Advancesoft Co., Ltd., has developed and released this software as “Advance/PHASE” (http://www. advancesoft.jp/) (15) Rohrer, G. S., Structure and Bonding in Crystalline Materials; Cambridge University Press: Cambridge, U.K., 2001.

Figure 4. DOS diagram for (a) Ge1Sb4Te7 obtained using band calculation. Diagrams for the two hypothetical structures, (b) Sb5Te7 and (c) Ge5Te7, are also shown for comparison. The Fermi level corresponds to 0 eV. These calculations were performed using the respective perfectly ordered structure models, in which the atoms were placed at the lattice points of the structure with a 100% Te-Te interlayer gap (see Figure 2a and Table 2). These three DOS patterns are similar in figure to each other: narrow band gaps between the valence and conduction bands can be seen at or near the Fermi level. The Fermi level lies in the forbidden, conduction, or valence band corresponding to the number of valence electrons in the respective compound.

hand side of this equation expresses repulsive force, and the second represents attractive force. The power index n tends to prefer values around 12 in most cases. We fitted this equation to the above band calculation results using the leastsquares method (Figure 3a). The parameters obtained are tabulated in Table 3. As seen in this Figure, the total energy varies with the interatomic distance (r) between the two adjacent Te atoms following eq 3, which demonstrates that the neighboring NaCl slabs are connected by weak interaction according to this equation. It is significant that the interatomic distance giving the minimum total energy is somewhat longer than that obtained by the Rietveld method. As in this case, the parameters determined by DFT calculations and experiments generally show a slight difference from each other; however, these discrepancies will be taken up in future study. The band calculations also show that this material is a compound semiconductor. Figure 4a shows the density of states (DOS) obtained at the point where the Te(2)-Te(3) distance is 3.6906 Å (see Figure 2a and Table 2), revealing that a very narrow gap exists between the valence and conduction bands. The remaining calculations all showed the

5754

Chem. Mater., Vol. 20, No. 18, 2008

presence of very narrow forbidden bands (which, however, gradually widen with increasing Te-Te distance). Shelimova et al.2 experimentally observed that a series of (GeTe)n(Sb2Te3)m homologous compounds involving Ge1Sb4Te7 are degenerate p-type semiconductors. If there are no defects in the crystal and structure, Ge1Sb4Te7 can be regarded as a compound semiconductor with a very narrow band gap. We confirmed the above results, that is, the van der Waalstype bonding nature between the adjacent Te-Te layers and the semiconducting characteristics, by performing the following examinations. This bonding nature has been obtained, as mentioned above, by calculation using a perfectly ordered structure model, expected to closely approximate the actual disordered structure. As shown in Table 1, however, the major atoms in the Ge/Sb sites are Sb atoms. We then conducted the band calculations using CHASE-3PT code and another perfectly ordered structure model in which all of the Ge/Sb sites are occupied only by Sb atoms. This second model (chemical formula: Sb5Te7) can also be assumed to closely approximate the actual structure. These calculations provided us, as shown in Figure 3b, almost the same results as those in Figure 3a; the total energy varies with the interatomic distance between the two adjacent Te atoms following the Lennard-Jones-type potential. The DOS obtained at the same Te(2)-Te(3) distance of 3.6906 Å, shown in Figure 4b, exhibited almost the same pattern as that seen in Figure 4a (which can be also seen in Figure 4c below); this strongly suggests that the atomic site’s configuration itself forms a band structure, namely molecular orbitals, specific to this crystal, almost regardless of the arrangement of the Ge and Sb atoms in their atomic sites. (This band structure feature is commonly seen not only in this Ge1Sb4Te7 stable phase but also in other stable phases or the metastable phase in the GeTe-Sb2Te3 pseudobinary system.3,10,16-18 ) To confirm this inference, we further conducted a band calculation assuming another hypothetical structure in which all of the Ge/Sb sites are occupied only by Ge atoms (Ge5Te7). Even the calculations under this third model gave us almost the same total energy dependence as those seen in a and b in Figure 3, even though its composition is considerably different from the original. The DOS pattern obtained, which is shown in Figure 4c, also exhibits a clear resemblance to the other two. The patterns seen in b and c in Figures 4 also show very narrow gaps between the valence and conduction bands, just as seen in Figure 4a; however, it can be observed that the Fermi level lies not in the forbidden band but in the conduction and in the valence band, respectively. Because the Sb atom has one more p electron than in the Ge atom, it is presumed that in Sb5Te7 these excess electrons are distributed in the conduction band. On the other hand, in Ge5Te7, holes occupy the valence band instead, because Ge5Te7 has fewer electrons than Ge1Sb4Te7. (16) Wuttig, M.; Lu¨sebrink, D.; Wamwangi, D.; Wełnic, W.; Gilleβen, M.; Dronskowski, R. Nat. Mater. 2007, 6, 122. (17) Edwards, A. H.; Pineda, A. C.; Schultz, P. A.; Martin, M. G.; Thompson, A. P.; Hjalmarson, H. P.; Umrigar, C. J. Phys. ReV. B. 2006, 73, 045210. (18) Matsunaga, T.; Kojima, R.; Yamada, N.; Kifune, K.; Kubota, Y.; Takata, M. Structural investigation of Ge1Sb6Te10, Sb8Te9, and Bi8Te9, intermetallic compounds in the chalcogenide homologous series. Acta Crystallogr., Sect. B 2008, Submitted.

Matsunaga et al.

All of the above findings substantiate the conclusion that the actual Ge1Sb4Te7 crystal with a disordered atomic arrangement is a semiconducting substance and that the two adjacent NaCl slabs in the layer stacking are connected by a van der Waals-type force. (C) Electron Distribution in the Crystal. We performed maximum entropy method (MEM)19 analysis using a computer program called ENIGMA20 to observe the electron distribution in the Ge1Sb4Te7 crystal. This analysis revealed an electron distribution specific to the anisotropic bondings among the atoms of this crystal. It is known that MEM analysis obtains more precise distributions in a crystal than the conventional Fourier analysis. Figure 2b shows the electron distribution in a whole unit cell obtained by this analysis together with that of the valence electrons by the band calculation (Figure 2c). These two distributions show a good accordance with each other and, moreover, that higher electron densities are located along the bonds with shorter interatomic distances than along those with longer distances. However, few bonding electrons are observed between the two adjacent Te layers. The contour maps inserted in b and c in Figure 2 indicate the electron density distributions for a lattice plane including Te(2)-Te(3). These maps, which are also in good accordance with each other, show that the electron densities between the two neighboring Te atoms are very low. In particular, the density at the trough along these two atoms is 90-100 me/Å3, which proves that they are only very weakly connected with each other. These results demonstrate that the atoms in an NaCl slab strongly connect with their coordination atoms belonging to the same slab; on the other hand, they are weakly bonded with the Te atoms at the ends of the two neighboring slabs. (D) Origin of the van der Waals-Type Bonding. It is presumed that the van der Waals-type weak connection between the adjacent slabs is formed by the two opposite forces, electric repulsion and attractive covalent bonding. For the three atoms Ge (1.8), Sb (1.9), and Te (2.1), the electronegativity gets higher in that order, as shown in the parentheses.15 If the compound Ge1Sb4Te7 realized an ideal ionic crystal, one may expect that this crystal would ionize to such a compound as Ge2+4Sb3+7Te2- when taking the valence electron structures of these atoms into account. (It is known that, in fact, the GeTe low-temperature rhombohedral phase4 is a ferroelectric substance;21 the Ge and Te atoms in this phase are shifted away from each other along the 3-fold rotation axis 22). The above molecular orbital calculation also provided us with the ionic characteristics of the atoms. The net (effective) charges, which mean the number of excess electrons that an atom in a cluster has received from its surrounding atoms, can be estimated by Muliken’s method. The estimation results are tabulated in Table 2. The DV-XR code supplies the net charge for every (19) Takata, M.; Nishibori, E.; Sakata, M. Z. Kristallogr. 2001, 216, 71. (20) Tanaka, H.; Takata, M.; Nishibori, E.; Kato, K.; Iishi, T.; Sakata, M. J. Appl. Crystallogr. 2002, 35, 282. (21) Polatoglou, H. M.; Theodorou, G.; Economou, N. A. J. Phys. C: Solid State Phys. 1993, 16, 817. (22) Chattopadhyay, T.; Boucherle, J. X.; von Schnering, H. G. J. Phys. C: Solid State Phys. 1987, 20, 1431.

Structural Features of Ge1Sb4Te7

atom in each cluster. Those shown in Table 2 are mean values for the atoms located at the same atomic sites in all clusters employed for the calculations. As expected from the electronegativities, the atoms in this crystal are weakly ionized, although not to the extent seen in the ideal ionic crystal, because the Te atoms tend to accept electrons, whereas Ge and Sb are donors. Both ends of the slabs are, as mentioned above, covered with Te layers. These positively and negatively charged Sb and Te layers, by means of the repulsive dipolar interaction mentioned above, act to separate the opposing slabs from each other ( · · · -Sbδ+-Teδ- r repulsion f Teδ--Sbδ+- · · · ); on the other hand, they are covalently bonded, which works to withstand this repulsion. It is thus concluded that these two opposite forces produce by far the longest interplanar gap in the crystal and hence the van der Waals-type bonding between two adjacent NaCl slabs. Conclusions The Ge1Sb4Te7 intermetallic compound has a 12-layered, close-packed cubic stacking structure (P3jm1) in which the Ge/Sb layer and Te layer are piled alternately. Te atoms occupy their own specific layers, whereas Ge and Sb atoms are randomly located at the other layers, causing a disordered atomic occupation in these layers. The atoms in this crystal are weakly ionized; the Te atoms tend to accept electrons, whereas Ge and Sb are donors. The crystal structure is

Chem. Mater., Vol. 20, No. 18, 2008 5755

composed of two kinds of NaCl slabs stacked alternately. The atoms within the slabs are strongly connected to adjacent atoms by a covalent bond; on the other hand, adjacent pairs of NaCl slabs are held together by a van der Waals-like weak force. Assuming that no defect exists in the crystal or structure, Ge1Sb4Te7 can be considered a compound semiconductor with a very narrow band gap. Acknowledgment. The synchrotron radiation experiments were performed on the BL02B2 beamline at SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI). We express our sincere gratitude to Drs. K. Kato and K. Osaka at JASRI and to graduate students N. Yasukawa, A. Yoshimura, and T. Murata at the Graduate School of Science at Osaka Prefecture University for their assistance with the experiment. The structural model in Figure 2a was displayed using Java Structure Viewer (JSV 1.08 lite), created by Dr. Steffen Weber. The electron density map in Figure 2b was drawn using a program called Limner, created by Y. Shimizu and Dr. H. Tanaka of Shimane University. We are indebted to them for allowing us to use their programs. We thank Dr. H. Tanaka for the computer program ENIGMA, which was used for the MEM analysis. Supporting Information Available: Crystallographic data in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org. CM703484K