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NaxMg5xGa9 a New Intermetallic Phase as a Link Between Zintl Phases and Metals.

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mol-'; pcald=1.981 g cm-'; measurement: Syntex P3 four-circle
diffractometer, graphite monochromator, scintillation counter,
3.5"<2$<55.0', w-scan, variable scan speed dependent on I, 296 K ;
MaK,. radiation; v-scan; seven reflections hkl, program SHELX76, direct methods, least squares refinement, full matrix, 27 variables;
N(hkl)=577, 418 with 1 > 3 o ( l ) ; R,,,,=0.042, R,,,,,,,>=0.032. Further details of the crystal structure determination can be obtained from the
Fachinformationszentrum Energie, Physik, Mathematik GmbH, D-75 14
Eggenstein-Leopoldshafen2, on quoting the depository number CSD53310, the names of the authors and the journal citation.
161 H. Jedlicka, F. Benesovsky, H. Nowotny, Mh. Chemie I00 (1969) 844.
[7] H. G. van Schnering, W. Wichelhaus, M. Schulze Nahrup, Z. Anorg.
Allg. Chem. 412 (1975) 193.
[X] H.-P. Abicht, W. Honle, H. G. von Schnering, Z. Anorg. Allg. Chem. 519
(1984) 7.
[9] Compare: H. Schafer, B. Eisenmann, W. Miiller, Angew. Chem. 85
(1973) 742; Angew. Chem. Inf. Ed. Engl. 12 (1973) 694; H. G. von
Schnering, ibid. 93 (1981) 44 and 20 (1981) 33; H. Schafer, Annu. Reu.
Marer. Sci. I5 (1985) 1.
[lo] W. Schnick, M. Jansen, Angew. Chem. 97 (1985) 48; Angew. Chem. Inl.
Ed. Engl. 24 (1985) 54; M. Lueken, M. Deussen, M. Jansen, U. Hesse,
W. Schnick, Z . Anorg. Allg. Chem. 553 (1987) 179.
[I I ] U. Wedig, Stuttgart, unpublished results.
[12] F. Seel, H. J. Guettler, Z. Narurforsch. 830 (1975) 88.
-b
Friauf polyhedra
Fig. 1. Projection of the hexagonal structure of Na,Mg,-,Cia, along the caxis [141: Ga, open circles: Na, Mg, full circles; Friauf polyhedra, black;
metal atoms outside of the unit cell are not shown.
Table 1. Na,Mg5_,Ga9; structural parameters: space group P6/rnmm-Ddh
(No. 191). Z=6, a= 15.43(1), c=8.998(6)
single-crystal structure solution:
four-circle diffractometer, 461 reflections hkl, R,(uniso)=0.056 [13]. The relatively large standard deviations of the occupation factors of the split positions are attributed to strong correlation of the closely-neighboring positions
in the structure refinement. This does not, however, alter the fact that split
positions are present.
A;
NaxMg5-,Ga9, a New Intermetallic Phase as a
Link Between Zintl Phases and Metals
By Reinhard Nesper*
Dedicated to Professor Hans Bock on the occasion of his
60th birthday
Binary compounds of the alkali metals with the elements
of group 13 such as NaTl belong to the classical Zintl
phases,"] i.e., they are compounds with defined compositions.
In contrast, the ternary compounds of the alkali o r alkaline earth metals with group 13 elements often exhibit a
phase width, with which the dependence of bonding relationships upon the valence-electron number can be studied. The binary phases NaGe,lz1 Na7Ga,3,[2'31Na z&a39,[41
Mg,Gaz,'51 MgzGa,'61 MgGa,"' MgGaZ['l and MgGa,['] are
known border members of the ternary system Na/Mg/Ga.
In the course of systematic studies of this system, we have
discovered the new ternary compound Na,Mg, -,Gas,
which is not a derivative of the binary phases. The compound exhibits, typical for intermetallic compounds, a
phase width 2 5 x 5 3.
The synthesis was carried out by heating a mixture of
the pure elements in welded niobium ampoules to 1100 K
and then cooling over a period of 24 h. The product has a
metallic luster and is both air- and moisture-sensitive. The
compound is not ductile but hard and brittle, resembling in
this respect the classical valence compounds and Zintl
phases. The compound exhibits, however, a specific resistance of p(300 K) IZ. 10 R cm, which lies within the range of
resistances found for metals and increases with rising temperature. Na,Mg5 -,Gag gives a n X-ray powder pattern
which is distinctly different from those of the binary
phases NaGa,, Na,Ga,,, Na,,Ga,,,
MgGa and MgGa,.
Single-crystal photographs show predominantly the Laue
symmetry 6/rnrnrn (Table I), which is modulated by composition-dependent superstructures.
Na,Mg,-,Ga,
crystallizes in a new structure type, in
which the Ga-atoms form a three-dimensional framework
of linked icosahedra and Friauf polyhedra (truncated tet[*] Dr. R. Nesper
Max-Planck-lnstitut fur Festkorperforschung
Heisenbergstrasse I , D-7000 Stuttgart 80 (FRG)
58
0 VCH Verlagsgeselisebaft m6H. 0-6940 Weinheim. 1989
Ga1
Gal'
G a 1"
Ga2
Ga2'
Ga3
Ga4
Ga5
M1
MI'
M2
M3
M4
M5
0.2062
0.2000
0.168(5)
0.353(1)
0.35 l(2)
0.491 l(4)
0.4501(2)
0.3958(3)
0
0
0.130( 1)
0.2088(7)
0.387(2)
113
0
0
0
0
0
0.1596(4)
0.9002(4)
0.7915(6)
0
0
0.261(2)
0.418( I )
0
213
0.2000
0.1600
0.2300
0.3600
0.3300
1 /2
0.2578(8)
0
0.260(3)
0.340(3)
1/2
0.19 l(2)
0
0.306(4)
315(26)
315(26)
315(26)
2 19(18)
2 19(18)
lOl(26)
196(21)
98132)
722(230)
722(230)
2 18( 1 19)
299(89)
343(134)
212( 1 17)
0.72(14)
0.18( 14)
O.lO( 14)
0.58(2)
0.42(2)
1
I
I
0.12( 10)
0.88(10)
I
1
1
1
rahedra, cf. Fig. 1). These two types of polyhedron are typical for intermetallic phases. The Ga framework is not significantly influenccd by the superstructure and the cation
positions within the framework are all localized unambiguously. The mean distances between the Ga atoms of the
icosahedra are similar (2.69-2.73 A), while that of Ga5 in
the Friauf polyhedra is ca. 0.1 A longer (2.81 A). Although
these distances are somewhat longer than the sum of the
covalent radii (2rmv= 2.52 A), quantum mechanical calculations have shown that this effect does not necessarily
lead to the loss of countable local bonds. In related aluminides, whose electronic structures follow, to a large extent,
the valence rules according to quantum mechanical calculations,"O1 bonds which are about 0.2 A longer than 2rc,,
are also found.
The superstructure results essentially from changes in
the region of the six-fold tube. A further Ga atom is located here, which is terminally bound to the icosahedron
and shows a significant elongation of its electron density
(Fig. 2). This nonuniformity can be well described by three
different Ga positions (Gal, Gal' and Gal"), each of which
is bound in a different manner to the neighboring Ga atoms.
Thus, by combination of the positions Gal, G a l ' and
Gal", limiting cases for the local chemical bonding can be
0570-0833/89/0101-0058$ 02.50/0
Angew. Chem. Int. Ed. Engl. 28 ('1989) No. I
formulated, in which one, two or three bonds per Ga atom
are attained (Gal, one bond: Gal-Ga2; Gal', two bonds:
Gal'-Gal', -Gal'; Gal", three bonds: Gal"-Gal",
-Gal", -Ga2'; cf. Fig. 2). Mean distances of 2.60-2.83 A
are also obtained for these limiting cases.
cies according to (lb)Ga4Q,(2b)Ga3Q and (3b)GazQ.The
observed displacements of the electron density with formation of the superstructure(s) indicate those positions which
represent different bonding situations.
The phase width of Na,Mg,- ,.Gag includes the formulas
I 0
0
2
IWll
H
0
,
0
0
I
5
*
2
lPl
,
-
O
i
l
j
.
5
[OlO]
Fig. 2. Electron density of the plane (Oyz).The distribution of the electron density of Gal can be described by three positions, which have bond
orders of one, two and three to the neighboring Ga atoms (cf. a, b, c; Ga: open circles; Na, Mg: full circles).
The differentiation between Na and Ga exclusively on
the basis of the crystallographic data is, in this case, no:
possible. The distances M-Ga lie in the range 3.2 to 3.5 A
and give coordination numbers of 10 to 12 for all M atoms
except for M1 and Ml', which have only six neighbors. A
decision between Na-Ga and Mg-Ga contacts is not possible on this basis. If one, however, sets the requirement that
the balance of charge between the Ga polyanions and the
cations has to be satisfied locally as is found, for example,
in Zintl phases, then all the positions cannot be occupied
by Mg atoms. In order to analyze the electron balance using the Zintl-Klemm concept)"] the icosahedra were
treated as closo-polyhedra as described by Wade."z1The
icosahedron is coordinated in the first coordination sphere
by four metal atoms, only half of which are at its disposal
(M,,,Ga:P). The X I 2framework of the Friauf polyhedron
can be understood in terms of classical, two-electron twocenter bonds."'] Since the polyhedron is linked to three
icosahedra, only three Ga-atoms, each of them fourbonded, remain per formula unit. They are not part of the
icosahedra. Accordingly, the following electron balance
can be written for the basic framework:
The charge required for the basic framework is already satisfied by six of the metal positions even if these are occupied only by Na atoms. A variable charge for the remaining atoms (Na or Mg) can only be compensated on the local level by the bond order of Gal (Gal', Gal").
A further metal position (Ml) lies in the center of the
large tube and is a direct neighbor of Gal. Depending on
the occupancy of this position (Na or Mg), Gal, Gal' and
Gal " receive differing amounts of transferred charge, to
which the different bonding situations are correlated. Consequently, Gal can be formulated as a 7e-, 6e- or Se-speAngew. Chem. Int. Ed. Engl. 28 (1989) No. I
with 2 1 x 1 3 , as revealed in this structural chemical study.
NaxMg5-&a, possesses the typical properties (silver metallic luster, electrical conductivity) and the typical structural building blocks of intermetallic phases. Nevertheless,
its bonding can be understood in terms of classical valence
rules and the Zintl-Klemm concept." 'I These findings are
supported by experimental and quantum mechanical studies of related ternary aluminides."']
Submitted: September 13, 1988 [Z 2964 IE]
German version: Angew. Chem. 101 (1989) 99
[ I ] E. Zintl, J. Goubeau, W. Dullenkopf, Z. Phys. Chem. A 154 (1931) 1; E.
Zintl, A. Harder, ibid. A 154 (1931) 47: E. Zintl, W. Dullenkopf, ibid. 8 1 6
(1932) 183; E. Zintl, H. Kaiser, Z. Anorg. Allg. Chem. 211 (1933) 113.
[2] U. Frank-Cordier, G. Cordier, H. Schafer, Z. Naturforsch. 8 3 7 (1982)
119: G. Bmuone, Acta Crystallogr. 8 2 5 (1969) 1206.
(31 Sh. R. Zhakupov, Russ. J. Phys. Chem. 54 (1980) 584.
[41 U. Frank-Cordier, G. Cordier, H. Schafer, Z . Naturforsch. 8 3 7 (1982)
127; R. G. Ling, C. Belin, Acta Crysralfogr. 8 3 8 (1982) 1101.
[5] K. Schubert, K. Frank, R. Gohle, A. Maldonado, H. G. Meissner, A.
Raman, W. Rossteutscher, Naturwissenschaften 50 (1963) 41.
161 K. Frank, K. Schubert, J. Less-Common Metals 20 (1970) 215.
[7] K. Schubert, F. Gauzzi, K. Frank, Z. Metallkd. 54 (1963) 422.
181 G. S. Smith, K. F. Mucker, Q. Johnson, D. H. Wood, Acta CrystaNogr.
8 2 5 (1969) 549.
[91 8. Predel, D. W. Stein, 1. Less-Common Metals 18 (1969) 203: G. S.
Smith,. Q. Johnson, D. H. Wood, Acta Crystallogr. 8 2 5 (1969) 554.
[lo] R. Nesper, Habilitationsschrifr, Universitat Stuttgart 1988.
[ I l l W. Klemm, Proc. Chem. Sac. (London) 1959. 329; W. Klemm: Festkorperprobleme. Vieweg, Braunschweig 1963; E. Mooser, W. B. Pearson,
Phys. Rev. 101 (1956) 1608.
I121 K. Wade, Adu. Inorg. Chem. Radiochem. 18 (1976) 1. 67.
1131 G. M. Sheldrick, E. Egert: SHELXS Programsfor Crystul Structure Solulion, Universitat Gottingen 1984; G . M. Sheldrick, SHELX-76 Programs
for Crystal Structure Determination, University of Cambridge 1976.
[I41 C. K. Johnson, ORTEP 11, niermal Ellipsoid Plot Program, Oak Ridge,
TN, USA 1976.
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