Intermetallics: Synthesis, Structure, Function by Pöttgen Rainer Johrendt Dirk

Author:Pöttgen, Rainer,Johrendt, Dirk
Language: eng
Format: epub
Publisher: De Gruyter
Published: 2014-07-10T16:00:00+00:00

Fig. 3.73 The crystal structures of AuSn, AuNiSn2, Nb3Sn, and Ni3Sn4. Relevant coordination polyhedra, interatomic distances, and atom designations are indicated.

Besides the binary transition metal stannides a variety of ternary ordered stannides is known. As an example we present the AuNiSn2 [9] structure in Fig. 3.73. The gold and nickel atoms show a 1:1 ordering on the gold sites of the AuSn structure discussed above. The Au/Ni ordering drastically reduces the space group symmetry from P63/mmc to P3m1. The difference in size between gold and nickel leads to a stacking of larger AuSn6 and smaller NiSn6 octahedra along the c axis with Au–Sn and Ni–Sn distances of 281 and 264 pm, respectively. The knowledge on noble metal stannides is important, since in high-value electronic devices more and more contacts and joints are based on noble metals in order to reduce corrosion phenomena.

A second example concerns the ternary zirconium stannides ZrNiSn and ZrNi2Sn [10]. In both structures, the zirconium and tin atoms build up a rock salt-type substructure. Half of the tetrahedral voids left by this substructure are filled in ZrNiSn and all of them in ZrNi2Sn (Fig. 3.74). Alternatively one can describe ZrNi2Sn as a filled version of ZrNiSn. However, these two stannides do not form a continuous solid solution ZrNi1+xSn. The different occupancy of the tetrahedral voids has a drastic effect on the space group symmetry. The half-Heusler phase crystallizes with a non-centrosymmetric space group. ZrNiSn and other half-Heusler type stannides (MgAgAs type), have intensively been investigated with respect to their thermoelectric properties [11]. Improvement of the properties is possible by different dopings. Such Heusler phases have been prepared with many transition metals. Even if the properties are promising, in those cases where neighboring transition metals are used, the site assignments on the basis of X-ray data often remains an open question.

The family of binary and ternary rare earth stannides is much larger than the one of the transition metal stannides. The crystal chemical details of these stannides have been reviewed by Skolozdra [12]. The research concerned the determination of the phase diagrams (isothermal sections at different temperatures) as well as the structure determinations of new phases. Depending on the RExTySnz composition, the stannides show different crystal chemical peculiarities. Most structures contain two- or three-dimensional [TySnz] polyanionic networks which leave cavities or channels for the rare earth elements. Within the networks one can observe also T–T bonding and especially in the tin-rich compounds extended tin substructures. Some of the T-rich stannides show segregation of the transition metal, leading to interesting structures. So far, the rare earth-rich parts of the RE–T–Sn systems have only scarcely been studied.

The cerium, europium, and ytterbium based stannides have intensively been investigated with respect to their physical properties, when searching for intermetallics with valence instabilities. To give some examples, CeNiSn [13] and CeRhSn [14] show intermediate cerium valence. Both structures contain only one crystallographic cerium site. CeRuSn [15] is a static intermediate-valent cerium compound with one purely trivalent cerium site and one Ce(4 –δ)+ site.

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