This project aims to understand the relationships among the composition, structure, and electronic structure in complex intermetallic compounds and their properties. Using rationally designed syntheses, we will seek to develop new classes of intermetallic compounds that are based on the lanthanide elements and the heavier carbon analogues, Silicon and Germanium. This research will involve thorough and systematic crystallographic studies (single- crystal and powder X-ray and/or neutron diffraction) of the complicated crystal chemistry and the possible stoichiometry breadths of many of these phases. The derived principles will be then applied for rationalization of the structures of more complex ternary phases and the corresponding phase equilibria. The fundamental goal of these investigations will be gaining better understanding of the structural relations together with the effect they have on the magnetic and electronic properties. Ultimately, the comprehensive knowledge gained from these studies will be used as a foundation for the reasoned tuning of properties of interest of the newly synthesized materials. It is anticipated that this project will advance the P.I.'s group to the forefront of this research and will allow the principle investigator to make significant contributions to the fast growing field of solid-state and materials chemistry.
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The efforts outlined in this proposal are broadly aimed at establishing the fundamentals and the infrastructure for solid-state and materials chemistry education and research within the Department, the College and the University of Delaware as a whole. The proposed studies are highly interdisciplinary, encompassing chemistry, physics, and materials science, and will give excellent training in solid-state research to graduate and undergraduate students. Special attention will be paid to recruiting students from traditionally under- represented backgrounds and encouraging their active participation in the project development and advancement. This will allow them to be more competitive on the job market by going on to various careers and actively contributing to the nation's scientific progress. Also, the successful outcome of the research program discussed in the proposal is anticipated to boost the scientific presence of the University of Delaware in the solid-state chemistry arena, and to have a direct positive impact on intra- and inter-departmental collaborations across campus.
Over the course of the last five and a half years, under the project DMR-0743916 we have published 25 papers directly resulting from this award. 3 graduate students and 3 post-docs contributed to the work and were paid stipends from the grant. Assisted by about a dozen undergraduates, the named individuals carried out a substantial body of work. Some of the recently discovered classes of compounds have indicated an unexpected wealth of new crystal chemistry and properties and are described next. The published, as well as some fresh results and work in-progress have been cited as proof-of-principle studies in this proposal for renewed funding. The first examples from our previous studies shown here are based on the idea to use the similar atomic sizes of some of the alkaline-earth and the rare-earth metals to study the effects of the "cations". Since the systems of interest are typically metallic, one can expect that the packing of the metal atoms (i.e., the geometric considerations) will dominate over the valence electron concentration (i.e., the electronic structure). The interplay between these two factors has received much attention in the solid-state literature, and we have shown that we can use this approach to make new solids and/or to fine-tune properties in a given structure. For instance, we synthesized and characterized a series of A2[n+m]In2n+mGe2[n+m] phases (A stands for Eu or Yb, mixed with Ca, Sr, or Ba). Their crystal structures are best viewed as intergrowths of Mo2FeB2-like (2-1-2) and TiNiSi-like (2-2-2) fragments in different ratios and stacking order. The basic building blocks are InGe4 tetrahedra, InGe4 squares and Ge2 dimers (Figure 1). Notably, all members of this series were found to exist only with mixed cations. Then, based on the established structural trends, we attempted to extend the series and to synthesize higher-order homologues by employing different combinations of rare-earth/alkaline-earth metals. These efforts led to the discovery of other new phases A5In3Ge6 and A3In2Ge4. Their structures are more complex—A5In3Ge6 features InGe4 tetrahedra and Ge4 tetramers, while the A3In2Ge4 structure is based on InGe4 tetrahedra and Ge chains with intricate topology of cis- and trans-bonds (Figure 2). Interestingly, similar Ge chains have been observed in the Li-containing compounds a-Sr2LiGe3 and Eu2LiGe3. For the latter, π-delocalization of the Ge 4p-orbitals has been considered, and the special characteristics of the Li metal—its small radius and high ionic potential in particular—have been attributed as a key to the formation of such chains. We also identified a region of poorly-localized electron density at one of the Ge sites (with trigonal planar environment), as well as some peculiarities in the electronic structure calculations (A3In2Ge4 is one electron-rich)—both facts are suggesting that a small disorder at the Ge site could be concealed by the "average" structure, but might be critical to the optimization of the bonding interactions. Following this hypothesis, we set out to investigate other possible structures, where similar bonding patterns exist. Drawing on the structural analogies between the [Ge2] chains in A3In2Ge4 and Sr2LiGe3, it was thought that small amounts of Li can be used to fine-tune the valence electron concentrations in the compounds under investigation. These studies, however, did not yield the sought-after phases. Instead, as described next, the phases ALi1–xInxGe2 (x ≈ 0.1) and A2(Li1–xInx)2Ge3 (x ≈ 0.3) were obtained (A = Sr, Ba, Eu). The crystal structures of these compounds can be described as one-dimensional [Ge2] chains with a basic repeating unit of (ct)n, where c refers to the cis- and t refers to the trans-conformation, with A2+ cations filling the space between these polyanionic fragments (Figure 3). The [Ge2] chains in the "1-1-2" phase have exactly the same topology as those in LT-LaGe (LT=low-temperature). After recognizing this analogy, we proposed that the crystal structure of the "1-1-2" phase can be derived from LT-LaGe, which in turn, is a derivative of the AlB2 type. To illustrate this idea, let us consider hypothetical AGe2 phase with honeycomb [Ge2] layers, as shown in Figure 4(a). The A-cations form trigonal prisms around each Ge, forming slabs in a plane perpendicular to the hexagonal axis. Upon a patterned removal of a half of the Ge atoms, in a way that [Ge2] chains of alternating cis- and trans-bonds are left behind, a half of the A6-trigonal prisms become empty. The array of empty and filled prisms can be described as puckered "layers", running parallel to the direction of the plot, as shown in Figure 4(b). One can readily see that the latter represents the structure of LT-LaGe. Parenthetically, a different way of removing a half of the Ge atoms from the same [Ge2] layers will yield [Ge2] zig-zag chains, which are the same as in the high-temperature LaGe (FeB structure type) or in SrGe (CrB structure type).
