RE₃Au₅Zn (RE = Y, Sm, Gd–Ho) – A new structure type with five- and six-membered rings as building units in a gold network

2.1 Synthesis

Starting materials for the intermetallic compounds RE₃Au₅Zn (RE = Y, Sm, Gd–Ho) were sublimed ingots of the rare earth metals (Smart Elements, >99.9 %), pieces of a gold bar (Allgussa AG, >99.9 %), and zinc granules (Merck, >99.9 %). For the syntheses, the elements were weighed in the ideal 3:5:1 atomic ratio and arc-welded [14] into tantalum containers under an argon pressure of ca. 800 mbar. Argon was purified with titanium sponge (900 K), silica gel, and molecular sieves. The ampoules were sealed in quartz tubes (for oxidation protection) under vacuum and placed in a muffle furnace. They were rapidly heated to 1473 K and kept at that temperature for a few minutes with subsequent lowering of the temperature at a rate of 40 K h^–1. Finally the samples were kept at 1073 K and 723 K for 10 h each, followed by switching off the furnace and cooling to ambient temperature. The polycrystalline, bronze-colored samples showed metallic luster. No reactions with the container material were observed. The samples are stable in air over weeks. After selecting single crystals for the X-ray diffraction experiments the samples were ground, cold-pressed to pellets and annealed in sealed evacuated silica ampoules in a resistance furnace at 1173 K (Y, Tb, Dy) or 973 K (Sm, Gd, Ho) for 14 days in order to enhance phase purity and overall crystallinity.

2.2 X-ray diffraction

All intermetallic RE₃Au₅Zn samples were ground to fine powders and characterized through Guinier powder patterns (Enraf-Nonius camera, type FR 552): imaging plate detector, Fujifilm BAS-1800, CuK_α1 radiation and α-quartz (a = 491.30, c = 540.46 pm) as an internal standard. The lattice parameters (Table 1) were deduced from least-squares fits and the unit cell volumes showed the expected course of the lanthanide contraction (Fig. 1). Correct indexing was ensured by intensity calculations [15]. The powder (Table 1) and single crystal data (Table 2) are in good agreement.

Fig. 1:

Course of the cell volume in the RE₃Au₅Zn series.

Irregularly shaped single crystals were selected from the crushed yttrium and terbium samples prior to the annealing step and fixed to quartz fibers using bees wax. The crystals were first characterized on a Buerger camera (using white Mo radiation) to check their quality. Intensity data of both single crystals were collected at room temperature on a Stoe IPDS-II image plate system (graphite-monochromatized MoK_α radiation; λ = 71.073 pm) in oscillation mode. Numerical absorption corrections were applied to the data sets. Details of the data collections and the crystallographic parameters are summarized in Table 2.

Further details of the crystal structure investigation may be obtained from Fachinformationszentrum Karlsruhe, 76344 Eggenstein-Leopoldshafen, Germany (fax: +49-7247-808-666; e-mail: crysdata@fiz-karlsruhe.de, http://www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting the deposition number CSD-430572 (Y₃Au_4.92Zn_1.08) and CSD-430573 (Tb₃Au₅Zn).

2.3 EDX data

The crystals measured on the diffractometer were analyzed using a Zeiss EVO MA10 scanning electron microscope using Y, TbF₃, Au, and Zn as standards for the semiquantitative EDX analysis. No impurity elements (especially from the container material) were observed. The experimentally observed compositions (33 ± 2 at% Y:55 ± 2 at% Au:12 ± 2 at% Zn and 31 ± 2 at% Tb:59 ± 2 at% Au:10 ± 2 at% Zn) were close to the ones obtained from the single crystal structure refinements (33.3 at% Y:54.7 at% Au:12.0 at% Zn and 33.3 at% Tb:55.5 at% Au:11.1 at% Zn). The standard deviations result from the irregular surface of the crystals (conchoidal fracture).

3.1 Structure refinements

Both diffractometer data sets showed C-centered orthorhombic lattices and the systematic extinctions were compatible with space group Cmcm. The starting atomic parameters were deduced from Direct Methods with shelxs-97 [16] and both structures were refined using shelxl-97 [17] (full-matrix least-squares on F²) with anisotropic displacement factors for all sites. The following refinement of the occupancy parameters led to a fully ordered structure in case of Tb₃Au₅Zn. The corresponding yttrium analog showed a mixed occupancy at the 8 g site (91.7(5) % Au/8.3(5) % Zn). This mixed occupancy was refined with least-squares variables in the final cycles, leading to the composition Y₃Au_4.92(1)Zn_1.08(1) for the investigated crystal. Difference Fourier syntheses revealed no residual peaks. The refined atomic positions, displacement parameters, and interatomic distances (exemplarily for Tb₃Au₅Zn) are given in Tables 3–5.

3.2 Crystal Chemistry

The compounds of the series RE₃Au₅Zn (RE = Y, Sm, Gd–Ho) belong to a new orthorhombic, C-centered structure type with space group Cmcm, Pearson code oC72 and Wyckoff sequence h²gf²c⁴. The unit cell of the terbium compound with its different building units is presented in Fig. 2. A view along the crystallographic a axis shows the network consisting of six-membered puckered gold rings running along the b direction. Condensation of the hexagons through Au–Au bonds leads to five-membered rings and consequently to the formation of a three-dimensional network with boat, envelope, and chair conformations. Atoms of the rare earth element are placed within the cavities of this gold network; RE1 and RE2 are exclusively surrounded by six- and RE3 as well as RE4 by five-membered rings. However, every third cavity offered by the gold substructure is not filled by a rare earth atom but by a Zn₂ dumbbell. Each atom of the latter one is fixed by a 4 + 2 coordination in form of a trigonal prism (Fig. 3).

Fig. 2:

Crystal structure of Tb₃Au₅Zn. Terbium, gold, and zinc atoms are represented as gray, blue and magenta circles, respectively. The gold substructure as well as the zinc dumbbells embedded in the network are emphasized. For clarity the whole arrangement is shown approximately along the a axis (upper part); detailed views on the five- and six-membered rings are given in the lower part. Numbers label crystallographically independent atomic sites.

Fig. 3:

Description of the Tb₃Au₅Zn structure underlining the trigonal prismatic coordination of the zinc atoms by the gold network approximately along a (left) and c (right).

Two different bonding situations occur for the gold atoms within the network: Au2 and Au3 are tetrahedrally surrounded, whereas the Au1 atoms have a square pyramidal coordination. Both coordination modes are well known in the crystal chemistry of gold-rich, intermetallic compounds. A framework of gold tetrahedra is part of almost every intermetallic gold compound with an AlB₂ superstructure [18, 19]. The square pyramidal coordination is similar to the BaAl₄ type and related structures [20]; however, in contrast to the gold substructure in the 3-5-1 phases, no binary BaAl₄ phases containing gold have been reported [21]. The shortest Au–Au contacts in the gold substructure of Tb₃Au₅Zn amount to 283 pm. These distances can be related to a bonding Au–Au character when compared with elemental gold (fcc, 288 pm [22]).

The shortest bond lengths within the Tb₃Au₅Zn structure occur between gold and zinc (267 pm). They are slightly longer than the sum of the covalent radii (259 pm [23]) and suggest attractive interactions between these two elements. Similar ranges of Au–Zn distances occur in the structures of EuAuZn (271–283 pm) [24], CeAu₄Zn₂ (286 pm Au–Zn) [13], or Ca₄Au₁₀Zn₃ (260–287 pm Au–Zn) [25].

Zn–Zn contacts within the dumbbells are significantly longer (281 pm Zn1–Zn2) and in line with those observed for the Zn₃ building units in A₂Au₆Zn₃ (A = Sr, Eu, Ba; 281 pm Zn–Zn) [25–27] and the zinc chains in CeAu₄Zn₂ (266 pm) [13]. All of these Zn–Zn distances are comparable with that of elemental zinc (hcp with 6 × 266 and 6 × 291 pm [22]). A charge assignment to the zinc dumbbells is certainly difficult. In view of the high electronegativity differences between terbium and zinc as compared to gold we expect charge transfer from both, electropositive terbium and zinc to the gold atoms, classifying Tb₃Au₅Zn as an auride.

The rare earth atoms are either located between two five- or two six-membered rings and are therefore surrounded by ten or twelve nearest gold neighbors. The shortest Tb–Au contact shows a length of 292 pm (Tb3–Au2), slightly below the sum of the covalent radii (293 pm [23]). Comparable distances can be found in binary Tb-Au compounds, i.e. Tb₃Au₄ (293 pm) [28]. Keeping the low zinc content in the new 3-5-1 compounds in mind it is not surprising that the bonding characteristics are in close agreement with the binaries. Intermetallics of a higher zinc or in general of an X element content (X = d¹⁰ or main group element) show significantly longer Tb–Au contacts, i.e. 310 pm in TbAuZn [29], 321 pm in Tb₃Au₂Al₉ [30] and 323 pm in TbAu₃Al₇ [31].

The gold substructure of the RE₃Au₅Zn compounds is distinctly different when compared to the many known gold rich phases. The simultaneous appearance of five- and six-membered rings is a very rare structural motif and to the best of our knowledge not known in the context of any other structure type. An obvious relation occurs with the Gd₇Au₁₀ (I4₁/acd) [32] and Hf₇Au₁₀ (Zr₇Ni₁₀ type [33], Cmce) [34] structures. A comparison with Hf₇Au₁₀ is given in Fig. 4. A view of the Hf₇Au₁₀ structure approximately along the a direction reveals – with the help of hafnium atoms for constructing the network – similar strands of five- and six-membered rings as described for the Tb₃Au₅Zn structure. Stacking of these strands becomes obvious from a side view. Similar to the Tb₃Au₅Zn structure, the puckered five-membered rings are arranged around the hafnium atoms that are not involved in the network. Such double layers are further condensed in a direction via Au–Au bonds. The different stacking sequences of the gold layers in both structures are marked in the left hand drawings of Fig. 4; ABAB for Tb₃Au₅Zn and AA′B′B for Hf₇Au₁₀.

Fig. 4:

Comparison of the Tb₃Au₅Zn structure (top) with the binary compound Hf₇Au₁₀ (Zr₇Ni₁₀ type [33], bottom) [34]. Numbers label crystallographically independent atomic sites. The stacking sequences of the gold sublayers are indicated.