Synthesis, structure and physical properties of the new Zintl phases Eu11Zn6Sb12 and Eu11Cd6Sb12

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Abstract

Reported are the syntheses, crystal structure determinations from single-crystal X-ray diffraction, and magnetic properties of two new ternary compounds, Eu11Cd6Sb12 and Eu11Zn6Sb12. Both crystallize with the complex Sr11Cd6Sb12 structure type—monoclinic, space group C2/m (no. 12), Z=2, with unit cell parameters a=31.979(4) Å, b=4.5981(5) Å, c=12.3499(14) Å, β=109.675(1)° for Eu11Zn6Sb12, and a=32.507(2) Å, b=4.7294(3) Å, c=12.4158(8) Å, β=109.972(1)° for Eu11Cd6Sb12. Their crystal structures are best described as made up of polyanionic 1[Zn6Sb12]22- and 1[Cd6Sb12]22- ribbons of corner-shared ZnSb4 and CdSb4 tetrahedra and Eu2+ cations. A notable characteristic of these structures is the presence of Sb–Sb interactions, which exist between two tetrahedra from adjacent layers, giving rise to unique channels. Detailed structure analyses shows that similar bonding arrangements are seen in much simpler structure types, such as Ca3AlAs3 and Ca5Ga2As6 and the structure can be rationalized as their intergrowth. Temperature-dependent magnetization measurements indicate that Eu11Cd6Sb12 orders anti-ferromagnetically below 7.5 K, while Eu11Zn6Sb12 does not order down to 5 K. Resistivity measurements confirm that Eu11Cd6Sb12 is poorly metallic, as expected for a Zintl phase.

Graphical abstract

The synthesis, structure determination from single-crystal X-ray diffraction and magnetic properties of Eu11Zn6Sb12 and Eu11Cd6Sb12 are reported. Both compounds crystallize with the monoclinic space group C2/m, and their structure can be viewed as built of ZnSb4 or CdSb4 tetrahedra, which are connected through common corners and exo-bonds.

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Introduction

Zintl phases attract much attention from a fundamental standpoint because of the appeal of their diverse crystal and electronic structures [1], [2], [3]. Recently, reports on unusual magnetic, electronic and thermoelectric properties within the realm of some ternary Zintl phases [4], [5], [6], [7], [8], [9] have generated renewed interest in these materials from a practical position, and have opened new research opportunities for solid-state chemists.

In the past 2–3 decades, the traditional Zintl boundaries have been extended to include some of the lanthanides and the transition metals [3]. Many such compounds have already been synthesized and structurally characterized [3], [4], [5], [6], [7], [8], [9], [10], [11]; however, quite surprisingly, much phase space still remains unexplored. A brief survey of the literature from the last 5–8 years reveals a wealth of new bonding patterns and properties within the ternary manganese pnictides alone—Sr21Mn4Sb18 [12], EuMn2P2 [13], Sr2MnSb2 [14], CaMn2Sb2 [15], Eu10Mn6Sb13 [16], Ca21Mn4Bi18 [17], among others. These recent findings, together with the fact that there are great similarities between the crystal chemistry of Mn- and many of the Zn- and Cd-compounds, the structures of which are often based on isolated or condensed tetrahedra, inspired us to begin systematic studies on the chemistry and properties of other members of this potentially very large family.

We have already reported on several new compounds in the A–Zn–Pn and A–Cd–Pn systems (A=divalent alkaline- or rare-earth metals; Pn=pnicogen). Some specific examples include: Ba11Cd8Bi14 [18], Yb2CdSb2 and Ca2CdSb2 [19], Eu10Cd6Bi12 [20], Sr21Cd4Sb18 and Ba21Cd4Sb18 [21], Ba11Cd6Sb12 [22]. Herein, we provide new results from these ongoing efforts by reporting the syntheses and the structural characterization of the first Eu-compounds with the C-centered monoclinic Sr11Cd6Sb12 type (Pearson's symbol mC58) [23], Eu11Zn6Sb12 and Eu11Cd6Sb12. This structure is based on antimony tetrahedra centered by Zn or Cd atoms, which share corners to form a unique polyanionic ribbons. Since the subtleties of this bonding arrangement and its electronic requirements have been discussed at length elsewhere [22], [23], the focus of this paper is on the “structural genealogy” of the structure and its similarities with the structures of other well-known ternary Zintl phases. Aside from this comparative analysis, we also discuss the complicated phase relationships in the corresponding phase diagrams, as well as the temperature dependence of the magnetization of both Eu11Zn6Sb12 and Eu11Cd6Sb12. Resistivity data taken on a single-crystal of Eu11Cd6Sb12 are also presented.

Section snippets

Synthesis

All manipulations were carried out in an argon-filled glove box or under vacuum. The metals were purchased from Alfa or Aldrich and were used as received: Eu (ingots, 99.9%), Sb (shot, 99.99%), Zn (shot, 99.999%), Cd (shot, 99.999%), Ca (granules, 99.9%), Pb (granules, 99.999%). Pb-flux reactions in alumina crucibles [24] and on-stoichiometry reactions in welded niobium tubes were employed for the syntheses. To prevent oxidation, heating of the reaction mixtures was done in evacuated fused

Crystal structure

Eu11Cd6Sb12 and Eu11Zn6Sb12 are rare-earth metal analogs of Sr11Cd6Sb12 and Ba11Cd6Sb12 [22], [23]. Their structures are best viewed as being made of isolated cations and a covalently bonded polyanionic sub-network of CdSb4 or ZnSb4 tetrahedra. They share common corners to form ribbons, which run parallel to the crystallographic b-direction (Fig. 1). The cations, i.e., Sr2+, Ba2+ and Eu2+ provide the electrons needed for the bonding and fill the space within the polyanionic sub-lattice. An

Conclusions

The synthesis, crystal structure from single-crystal X-ray diffraction and magnetic properties of Eu11M6Sb12, where M=Cd, Zn have been reported. Both compounds crystallize with the monoclinic Sr11Cd6Sb12-type structure, which is built of MSb4 tetrahedra. Detailed structural relationships, including analysis of various bonds, comparisons, coordination environments and derivation from simple and well-known structure types, are provided. The magnetic susceptibility of both compounds suggests that

Acknowledgments

Svilen Bobev acknowledges the University of Delaware (start-up grant) and the Donors of the American Chemical Society Petroleum Research Fund for the financial support of this research.

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