Stability and electronic properties of isoelectronic heteroatomic analogs of Sn52-

https://doi.org/10.1016/j.cplett.2011.02.035Get rights and content

Abstract

The electronic structure and stability of three gas phase heteroatomic clusters, Ga2Bi3-, In2Bi3-, and In2Sb3-, which are isoelectronic with the Zintl ion Sn52-, are examined using photoelectron spectroscopy and first-principles theoretical investigations. While all the isoelectronic clusters are stable with high adiabatic detachment energies, the HOMO–LUMO gap, absolute stability and the relative stability of the isomers depend on the atomic size and point of substitution. Theoretical analysis reveals the variations are attributable to atomic size differences, which affect covalent bonding within the cluster. The studies offer a strategy for controlling properties of clusters that might be incorporated into cluster-assembled materials.

Research highlights

► Ga2Bi3-, In2Bi3-, and In2Sb3- are isoelectronic to the Zintl ion Sn52-. ► Photoelectron spectra and theoretical studies reveal similar electronic structures. ► HOMO–LUMO gaps and stability of isomers depend on size and location of heteroatoms.

Introduction

Materials in which atomic or molecular clusters serve as the building blocks motivate our desire to understand and manipulate cluster properties [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Since the characteristics of clusters change with size, composition, and charge state, cluster assembled materials offer the prospect of nanoscale materials with atomic control. This is different from top-down approaches that reside primarily in the ‘scaling regime’ of nanoscience. Here, properties vary less dramatically than in the cluster regime where every atom and every electron matters [12]. One important class of cluster assembled materials, the Zintl phases, were discovered almost 70 years ago [13]. The building blocks of Zintl phases are the multiply anionic, post-transition metal clusters that are stabilized by alkali or alkali-based countercations [14], [15], [16], [17], [18], [19], [20]. A viable strategy to synthesize such Zintl phases from new clusters is to conduct experiments on Zintl analogs (clusters that are isoelectronic with species already known to form Zintl assemblies). One way to accomplish this is to study gas phase clusters, since numerous gas phase studies have established that the Zintl analog clusters appear as the most abundant species in mass spectra [4], [21], [22], [23]. As the gas phase synthesis of such a series of clusters is straightforward it offers a viable pathway to compare a large range of candidates for Zintl phases [21], [22], [23], [24], [25], [26], [27], [28].

In the current study, we have examined the properties of a series of isoelectronic heteroatomic clusters. Heteroatomic clusters offer an additional degree of freedom by which the properties of nanoscale materials may be manipulated [29], [30], [31], [32], [33], [34], [35], [36]. This is possible because heteroatomic substitutions allow the size and charge state of the components to be easily varied. We choose the five-atom analogs of Sn52- for this study because the parent ion forms cluster assemblies [37], and because five-atom clusters provide more options for substitution than the traditional tetrahedral or distorted tetrahedral four-atom clusters. The Sn52- cluster is a well known Zintl anion, whose stability is attributed to Wade–Mingos rules, as it has five atoms and 12 valence p-electrons with a trigonal bipyramidal geometry [38], [39]. Previous studies in our group have pointed out that Ga2Bi3-, which is isoelectronic with Sn52-, is a very stable species that displays a high abundance in the mass spectrum, high adiabatic electron detachment energy (AEDE), and a large gap between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO–LUMO gap) [29]. In another communication, similar results were found [30] for In2Bi3-, an analog of Sn52-; another species isoelectronic with Sn52- is In2Sb3-. Here, we perform a comparative study of these three clusters and show how the size of the atoms and point of substitution affects the electronic structure and stability of the clusters. We employ photoelectron spectroscopy experiments to give a fingerprint of the electronic structure and allow for a comparison of stability. Our theoretical investigations allow control over the substitution, and reveal the evolution of electronic structure with the choice of heteroatomic dopants and point of substitution.

Section snippets

Methods

The experimental details for the current study are provided elsewhere [40]. In brief, InxBiy-, GaxBiy-, and InxSby- clusters were formed by using ¼′′ 50:50 molar ratio In-Bi, Ga-Bi, and In-Sb molded rods in a laser vaporization source. Helium was used as a carrier gas and the clusters were mass analyzed using Wiley McLaren time-of-flight mass spectrometry [41]. The photoelectron spectra for the clusters were obtained using a magnetic bottle time-of-flight photoelectron spectrometer [42],

Results

The photoelectron spectrum of In2Sb3- is shown in Figure 1 and is compared with the spectra of Ga2Bi3- and In2Bi3-. The adiabatic electron detachment energy (AEDE) is assigned by extrapolating the leading edge of the first peak. This method may provide a valid estimation of the true AEDE when the vibrational progression is not well-resolved. The clusters studied here have rigid trigonal bipyramid structures that do not substantially change upon electron addition, yielding a good Frank–Condon

Conclusions

The present studies have examined the effects of substitution in a series of IIIsingle bondV gas phase clusters that are isoelectronic with Sn52-. These heteroatomic clusters are singly charged and demonstrate enhanced stability as seen through the AEDE, R.E., A.E., and HOMO–LUMO gap. The ground state geometry in all three cases is that in which the Group V atoms form the equatorial triangle, while the Group III atoms cap the trigonal bipyramid. This enhances the bonding within the equatorial triangle,

Acknowledgement

We gratefully acknowledge financial support from the US Department of the Army through a MURI Grant No. W911NF-06-1-0280.

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