Birgit Gerke and Rainer Pöttgen

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

De Gruyter | Published online: February 29, 2016

Abstract

RE3Au5Zn (RE = Y, Sm, Gd–Ho) intermetallic compounds were synthesized by melting the elements in sealed tantalum tubes. They exhibit a new structure type which was studied by X-ray diffraction on powders and refined from single crystal diffraction data: Cmcm, a = 736.9(2), b = 1489.4(2), c = 1330.4(3) pm, wR2 = 0.0371, 1184 F2 values and 55 variables for Y3Au4.92Zn1.08 and a = 739.0(1), b = 1495.8(2), c = 1339.2(2) pm, wR2 = 0.0325, 1410 F2 values and 54 variables for Tb3Au5Zn. The network consists of five- and six-membered gold rings in puckered conformations. Atoms of the rare earth elements are placed within the cavities of this network where every third cavity is filled by a Zn2 dumbbell. The structure is discussed in detail and compared with the gold substructure of Hf7Au10.

1 Introduction

The structural diversity of gold in intermetallic compounds is much broader than that of other noble metals. The key parameter explaining the peculiar crystal chemical behavior relies in relativistic effects, leading to pronounced 6s-5d mixing [1, 2], a smaller effective radius and an enhanced electronegativity, close to the values of selenium and tellurium. Already a survey of the binary alkaline earth (AE) and rare earth (RE) gold phases reveals quite different gold substructures with a direct dependence on the gold content. AE- and RE-rich compounds like AuBe5 [3] contain isolated gold atoms (i.e. no Au–Au bonding), while gold dumbbells occur in Ca5Au4 [4], Yb5Au3 [5] and Eu3Au2 [6]. An increasing gold content leads to a higher degree of Au–Au condensation, leading to zig-zag chains (CeAu [7]), condensed tetrahedra (CaAu5 [8]), honeycomb (BaAu2 [9]) or Kagomé (EuAu5 [10]) networks. An overview on the structural chemistry of the AEAux phases is given in [11].

Ternary systems AE-Au-X and RE-Au-X (X = d10 transition metal or a p element of the 3rd, 4th or 5th group) offer further possibilities for Au–Au bonding besides Au–X bonding with new gold substructures [12]. A recent example concerns the series of REAu4Zn2 (RE = Ce, Pr, Nd) compounds [13] which contain Au4 squares that are not known from binary phases. Herein we report on the new phases RE3Au5Zn (RE = Y, Sm, Gd–Ho) which consist of a unique three-dimensional gold network which is composed of condensed pentagons and hexagons.

2 Experimental

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, CuKα1 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.

Table 1

Lattice parameters (Guinier powder data) of the orthorhombic compounds RE3Au5Zn (RE = Y, Sm, Gd–Ho). Standard deviations are given in parentheses.

Compound a (pm) b (pm) c (pm) V (nm3)
Y3Au5Zn 736.9(2) 1489.4(2) 1330.4(3) 1.4602
Sm3Au5Zn 746.7(2) 1514.5(3) 1354.3(5) 1.5315
Gd3Au5Zn 741.9(2) 1500.7(3) 1342.4(3) 1.4946
Tb3Au5Zn 739.0(1) 1495.8(2) 1339.2(2) 1.4803
Dy3Au5Zn 734.6(3) 1487.7(3) 1330.9(4) 1.4545
Ho3Au5Zn 732.4(2) 1482.5(2) 1326.2(2) 1.4400
Fig. 1: Course of the cell volume in the RE3Au5Zn series.

Fig. 1:

Course of the cell volume in the RE3Au5Zn series.

Table 2

Crystal data and structure refinement for Y3Au4.92(1)Zn1.08(1) and Tb3Au5Zn, space group Cmcm, Z = 8; Pearson code oC72.

Empirical formula Y3Au4.92(1)Zn1.08(1) Tb3Au5Zn
Molar mass 1306.01 1526.96
Unit cell dimensions a = 737.39(2) a = 737.92(2) pm
(single crystal data) b = 1490.49(4) b = 1494.04(5) pm
c = 1332.09(3) c = 1339.06(6) pm
V = 1.4641 V = 1.4763 nm3
(powder data) a = 736.9(2) a = 739.0(1) pm
b = 1489.4(2) b = 1495.8(2) pm
c = 1330.4(3) c = 1339.2(2) pm
V = 1.4602 V = 1.4803 nm3
Calculated density, g cm–3 11.88 13.70
Crystal size, μm3 40 × 30 × 25 45 × 30 × 30
Diffractometer IPDS-II IPDS-II
Radiation; wave length λ, pm MoKα; 71.073 MoKα; 71.073
Transm. ratio (max/min) 0.141 / 0.027 0.122 / 0.041
Absorption coefficient, mm–1 125.3 130.0
F(000), e 4303 4960
θ range, deg 3–30 3–32
Range in hkl ±10, ±20, +18 ±10, ±22, +19
Total no. reflections 3966 4944
Independent reflections/Rint 1184/0.0435 1410/0.0276
Reflections with I ≥ 2 σ(I)/Rσ 860/0.0332 981/0.0250
Data/ref. parameters 1184/55 1410/54
Goodness-of-fit on F2 0.928 0.891
Final R1/wR2 indices [I ≥ 2 σ(I)] 0.0189/0.0344 0.0166/0.0305
R1/wR2 indices (all data) 0.0359/0.0371 0.0352/0.0325
Extinction coefficient 0.000296(7) 0.000128(4)
Largest diff. peak/hole, e Å–3 2.79/–1.82 1.99/–1.76

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 MoKα 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: , http://www.fiz-karlsruhe.de/request_for_deposited_data.html) on quoting the deposition number CSD-430572 (Y3Au4.92Zn1.08) 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 Results and discussion

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 Y3Au4.92(1)Zn1.08(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 35.

Table 3

Atomic coordinates of Y3Au4.92(1)Zn1.08(1) and Tb3Au5Zn, space group Cmcm.

Atom Wyck. x y z
Y3Au4.92(1)Zn1.08(1)
 Y1 4c 0 0.0736(1) 1/4
 Y2 4c 0 0.6777(1) 1/4
 Y3 8f 0 0.36960(8) 0.03742(7)
 Y4 8f 0 0.11882(8) 0.54887(7)
 Au1a 8g 0.30964(5) 0.37333(3) 1/4
 Au2 16h 0.21556(6) 0.21725(2) 0.10310(3)
 Au3 16h 0.28434(6) 0.02787(2) 0.10526(3)
 Zn1 4c 0 0.2801(1) 1/4
 Zn2 4c 0 0.4665(1) 1/4
Tb3Au5Zn
 Tb1 4c 0 0.07191(5) 1/4
 Tb2 4c 0 0.67887(5) 1/4
 Tb3 8f 0 0.36954(4) 0.03841(3)
 Tb4 8f 0 0.11848(4) 0.54984(3)
 Au1 8g 0.30837(5) 0.37323(3) 1/4
 Au2 16h 0.21460(6) 0.21660(2) 0.10293(3)
 Au3 16h 0.28536(6) 0.02821(2) 0.10494(3)
 Zn1 4c 0 0.2794(1) 1/4
 Zn2 4c 0 0.4671(1) 1/4

aMixed site occupancy: 91.7(5) % Au/8.3(5) % Zn.

Table 4

Anisotropic displacement parameters (pm2) of Y3Au4.92(1)Zn1.08(1) and Tb3Au5Zn, space group Cmcm.a

Atom Wyck. U11 U22 U33 U12 U13 U23 Ueq
Y3Au4.92(1)Zn1.08(1)
 Y1 4c 79(6) 119(6) 66(6) 0 0 0 88(3)
 Y2 4c 79(6) 105(6) 67(7) 0 0 0 83(3)
 Y3 8f 55(4) 39(4) 76(4) 0 0 11(4) 57(2)
 Y4 8f 65(4) 72(4) 74(4) 0 0 2(4) 70(2)
 Au1 8g 74(2) 97(2) 97(2) –1(2) 0 0 89(2)
 Au2 16h 98(1) 73(1) 87(2) 32(1) –14(1) –6(1) 86(1)
 Au3 16h 97(1) 67(1) 83(2) 13(1) 19(1) 8(1) 82(1)
 Zn1 4c 113(8) 97(8) 68(8) 0 0 0 93(3)
 Zn2 4c 104(8) 105(8) 71(8) 0 0 0 93(3)
Tb3Au5Zn
 Tb1 4c 134(3) 153(3) 128(3) 0 0 0 138(1)
 Tb2 4c 137(3) 154(3) 134(3) 0 0 0 142(1)
 Tb3 8f 116(2) 92(2) 146(2) 0 0 4(2) 118(1)
 Tb4 8f 123(2) 127(2) 135(2) 0 0 9(2) 128(1)
 Au1 8g 124(2) 152(1) 166(1) 3(2) 0 0 147(1)
 Au2 16h 156(1) 124(1) 152(1) 28(1) –15(2) –9(1) 144(1)
 Au3 16h 152(1) 117(1) 152(1) 16(1) 23(2) 8(1) 140(1)
 Zn1 4c 163(8) 133(7) 124(8) 0 0 0 140(3)
 Zn2 4c 155(8) 131(7) 137(8) 0 0 0 141(3)

aThe anisotropic displacement factor exponent takes the form: −2π2[(ha*)2U11 + ··· + 2hka*b*U12]; Ueq is defined as one third of the trace of the orthogonalized Uij tensor.

Table 5

Interatomic distances (pm) in the structure of Tb3Au5Zn. All distances within the first coordination spheres are listed. Standard deviations are given in parentheses.

Tb1: 4 Au3 294.1(1) Au2: 1 Zn1 269.7(1)
1 Zn1 310.3(2) 1 Au3 286.6(1)
2 Au1 329.2(1) 1 Tb3 291.5(1)
4 Au2 332.8(1) 1 Tb2 294.0(1)
2 Tb4 391.1(1) 1 Tb4 297.6(1)
Tb2: 4 Au2 294.0(1) 1 Au2 297.9(1)
1 Zn2 316.8(2) 1 Tb3 311.3(1)
2 Au1 323.4(1) 1 Au1 313.8(1)
4 Au3 337.2(1) 1 Au2 317.2(1)
2 Tb3 393.0(1) 1 Tb4 332.3(1)
Tb3: 2 Au2 291.5(1) 1 Tb1 332.8(1)
2 Au3 292.2(1) Au3: 1 Zn2 266.9(1)
2 Au3 299.0(1) 1 Au2 286.6(1)
2 Au2 311.3(1) 1 Tb3 292.2(1)
1 Zn1 313.8(1) 1 Au3 293.5(1)
1 Zn2 318.7(1) 1 Tb1 294.1(1)
2 Au1 363.7(1) 1 Tb3 299.0(1)
2 Tb4 370.2(1) 1 Au1 310.3(1)
1 Tb2 393.0(1) 1 Tb4 313.1(1)
1 Tb4 393.7(1) 1 Au3 317.2(1)
Tb4: 2 Au2 297.6(1) 1 Tb4 325.1(1)
2 Au1 303.4(1) 1 Tb2 337.2(1)
2 Au3 313.1(1) Zn1: 2 Au1 267.7(1)
2 Au3 325.1(1) 4 Au2 269.7(1)
2 Au2 332.3(1) 1 Zn2 280.8(2)
2 Tb3 370.2(1) 1 Tb1 310.3(2)
1 Tb4 378.8(1) 2 Tb3 313.8(1)
1 Tb1 391.1(1) Zn2: 4 Au3 266.9(1)
1 Tb3 393.7(1) 2 Au1 267.7(1)
Au1: 1 Zn1 267.7(1) 1 Zn1 280.8(2)
1 Zn2 267.7(1) 1 Tb2 316.8(2)
1 Au1 283.2(1) 2 Tb3 318.7(1)
2 Tb4 303.4(1)
2 Au3 310.3(1)
2 Au2 313.8(1)
1 Tb2 323.4(1)
1 Tb1 329.2(1)
2 Tb3 363.7(1)

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. 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).

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) [2527] 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.

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.

Acknowledgments

We thank Dipl.-Ing. U. Ch. Rodewald for collecting the single crystal intensity data. This work was financially supported by the Deutsche Forschungsgemeinschaft. B.G. is indebted to the Fonds der Chemischen Industrie and the NRW Forschungsschule Molecules and Materials – A Common Design Principle for PhD fellowships.

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Received: 2015-12-12
Accepted: 2016-1-10
Published Online: 2016-2-29
Published in Print: 2016-5-1

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