Alkaline earth-gold-aluminides: synthesis and structure of SrAu₃Al₂, SrAu_2.83Al_2.17, BaAu_2.89Al_2.11 and BaAu_7.09Al_5.91

1 Introduction

Gold-aluminum intermetallics have widely been studied in microelectronics, since the aurides Al₂Au₅ (called white plague) and Al₂Au (called purple plague) frequently occur at solder joints through inter-diffusion processes at higher temperatures. The main problem with auride formation at the interfaces concerns drastic differences in the electrical conductivity as compared to the pure metals as well as the brittleness of the intermetallic compound which might lead to cracks at the joints.

Apart from these technical applications, gold intermetallics display a rich crystal chemistry and fascinating bonding peculiarities, a consequence of relativistic effects [1–4]. If a highly electropositive element (alkali (A), alkaline earth (AE) or rare earth (RE) metal) reacts with gold and a p element of the 3^rd, 4^th, or 5^th main group (E), structures with one-, two-, or three-dimensional [Au_xE_y] polyanions form. Typical examples are the linear phosphido-aurate ions [AuP₂]^5– in K₅[AuP₂] [5], the planar 2D [AuP]^2– and [Au₂Sn]^6– polyanions in SrAuP [6] and Er₂Au₂Sn [7], or the wurtzite-related three-dimensional [AuSn]^3– network in CeAuSn [8].

So far, the systems AE-Au-Al have only scarcely been studied. Few compounds with calcium and strontium have been reported: TiNiSi-type CaAuAl [9], CaAu_0.8Ga_3.2 and CaAu_0.9Ga_3.1 with BaAl₄-type structure [10, 11], CaAu₃Al₇ [12], and the large series of aluminum-rich Ca₃Au_6+xAl₂₆T (T = Ti, Zr, Hf, V, Nb, Cr, Mo, W, Mn, Re) and Sr₃Au_6+xAl₂₆T (T = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Re) [12, 13] compounds with a stuffed variant of the cubic BaCd₁₁ type. Further strontium compounds were reported recently. Sr₂Au₆Al₃ [14] is the first representative of the Sr₂Au₆Zn₃ type with Al₃ triangles. This structural motif also occurs in SrAu_4.06Al_2.94, SrAu_5.05Al_1.95, Sr₂Au_7.32Al_1.67, and Sr₂Au_6.18Al_2.82 [15]. The triangles show small degrees of Au/Al mixing.

When searching for further representatives of this structural family we obtained a series of by-products with different structure types, namely SrAu₃Al₂, SrAu_2.83Al_2.17, BaAu_2.89Al_2.11 and BaAu_7.09Al_5.91. The synthesis and crystal chemistry of these aluminides are reported herein.

2 Experimental

3 Results and discussion

Phase analytical studies in the systems AE-Au-Al led to four new ternary compounds with pronounced three-dimensional polyanionic networks formed by the gold and aluminum atoms. The new compounds crystallize with three different structure types which will be discussed separately.

We start our discussion with the structure of SrAu₃Al₂. The gold and aluminum atoms show complete ordering and one can describe the SrAu₃Al₂ structure as a ternary ordered version of the low-temperature (LT) modification of SrZn₅ [21]. So far only few ternary examples are known for that structure type: RbAu₃Ga₂ and CsAu₃Ga₂ with complete gold-gallium ordering [24] as well as BaPd_1.57Zn_3.43 and BaAu_1.41Zn_3.59 with pronounced Pd/Zn and Au/Zn mixing, respectively [25].

A view of the SrAu₃Al₂ structure approximately along the b axis is presented in Fig. 1. The shortest interatomic distances occur between the gold and aluminum atoms (252–262 pm). They compare well with the sum of the covalent radii of 259 pm [26]. Additionally we observe Au–Au (277–306 pm) and Al–Al (274–275 pm) contacts, which are close to the respective distances in the elements (286 pm in fcc aluminum and 288 pm in fcc gold) [27]. Together, the gold and aluminum atoms built up a three-dimensional network of condensed tetrahedra. The larger strontium atoms fill the cavities within this network. They have 18 nearest Au/Al neighbors. As expected from the course of the electronegativities, the nine nearest neighbors are gold atoms (329–349 pm Sr–Au). A similar bonding situation has been observed for Sr₂Au₃In₄ (308–355 pm Sr–Au) [28], SrAuSn (327–341 pm Sr–Au) [29] and Sr₂Au₆Al₃ (325–335 pm Sr–Au) [14]. All of these distances compare well with the sum of the covalent radii of 326 pm for Sr + Au [26].

Fig. 1:

The structures of SrAu₃Al₂, SrAu_2.83Al_2.17 and BaAu_2.89Al_2.11. Alkaline earth, gold and aluminum atoms are drawn as medium grey, blue and magenta circles, respectively. Mixed occupied sites are marked by segments. The tetrahedral gold-aluminum substructures are emphasized.

With a lower gold content we obtained SrAu_2.83Al_2.17 which crystallizes with the BaZn₅ structure [21], space group Cmcm, similar to the barium compound BaAu_2.89Al_2.11. This structural arrangement has previously been reported for BaAg_2.4Al_2.6 [30] and KAu_3.08Ga_1.92 [31]. It is remarkable, that all four ternary representatives show one mixed occupied site; the 4c site for the three aluminum compounds but the 8e site for the gallium one.

Exemplarily we discuss the SrAu_2.83Al_2.17 structure for the series of BaZn₅-related phases. A view of the structure approximately along the c axis is presented in Fig. 2. The gross structural features and the course of the interatomic distances (see Tables 4 and 5) are very similar to the fully ordered variant SrAu₃Al₂ discussed above. The mixed occupied sites correspond to the common atom of the corner-sharing tetrahedra. The only peculiarity that should be mentioned are the enhanced U₃₃ parameters of the Au2 positions. This might be a hint for some short-range order.

Fig. 2:

The crystal structure of BaAu_7.09Al_5.91 (NaZn₁₃ type, Fm3̅c). Barium sites are drawn in medium grey color, the mixed occupied Au/Al sites in black and white segments. The left-hand drawing emphasizes packing of Au2/Al2 centered icosahedra and barium centered 24-atom snub cubes. The polyhedral subunits and their connectivity through tetrahedra are emphasized at the right-hand part.

The fourth compound crystallizes with the cubic NaZn₁₃ type structure. Both zinc sites (96i and 8b) show Au/Al mixing, leading to the composition BaAu_7.09Al_5.91 for the investigated crystal. Besides the solid solution BaAu_xZn_13–x [32], BaAu_7.09Al_5.91 is one of the few gold-containing members with NaZn₁₃-type structure. The crystal chemistry and chemical bonding of binary and ternary NaZn₁₃ type phases are well documented [33, 34]. Herein we only briefly discuss the structural peculiarities of BaAu_7.09Al_5.91. The unit cell of BaAu_7.09Al_5.91 is shown in Fig. 2. The basic polyhedral building units are Au2/Al2 centered icosahedra and barium centered 24-atom snub cubes which are packed in a CsCl manner. Due to different orientations of the icosahedra, a 2 × 2 × 2 superstructure occurs. Another geometrical description would be an fcc packing of icosahedra with icosahedra of the other orientation in all octahedral voids and the 24-atom snub cubes in all tetrahedral voids. Although these explanations are of purely geometrical nature, they offer an easy way to illustrate the large unit cell which comprises 112 atoms. BaAu_7.09Al_5.91 crystallizes with the undistorted cubic NaZn₁₃ type. Several ordering variants forcing symmetry reductions are summarized in ref. [35].

Since both network sites (96i with almost equal occupancy and 8b with predominant aluminum occupancy) show Au/Al mixing, we can only discuss average distances (Table 6). They compare well with the sum of the covalent radii of Au + Al of 259 pm as well as with fcc gold (288 pm), similar to the three phases discussed above. The differently oriented icosahedra are connected via short Au/Al–Au/Al distances forming tetrahedra between the larger polyhedra.

Finally we draw back to the formulæ of SrAu₃Al₂, SrAu_2.83Al_2.17, BaAu_2.89Al_2.11, and BaAu_7.09Al_5.91 which are written in the classical style for ternary aluminides/intermetallic aluminum compounds [4]. However, keeping the course of the Pauling electronegativities [25] of gold (2.54) and aluminum (1.61) in mind, we can safely ascribe auridic character to the gold atoms, similar to many other series of related intermetallic gold compounds, e.g. RE₄Mg₃Au₁₀ [36].