Synthesis of a New Class of Hybrid Solids via Salt Inclusion
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Synthesis of a New Class of Hybrid Solids via Salt Inclusion Qun Huang, Mutlu Kartin, Xunhua Mo, and Shiou-Jyh Hwu Department of Chemistry, Clemson University, Clemson, SC 29634-0973, U.S.A. ABSTRACT Via salt inclusion methods, we have recently isolated a new class of transitionmetalcontaining hybrid solids that consist of a composite structure of covalent and ionic lattices. These new solids can be synthesized by conventional high-temperature, solid-state methods employing reactive molten alkali and alkaline-earth metal halide salts. Single crystal structure studies have revealed fascinating extended salt lattices that exhibit structural directing effects that give rise to a variety of nano-structured covalent oxide frameworks. Depending upon the composition of incorporated salt and the coordination environment of halide anions, resulting covalent lattices range from sheets, clusters, to porous structures. Due to the weak interaction between the two chemically dissimilar lattices, the salt lattice, in some cases, is removable showing reversible salt-intercalation at room temperature. In this report, we will demonstrate the utilities of salt-inclusion reactions in the formation of metal-oxide nanostructures in the selected compound families. We will also give some highlights on the recent discoveries of noncentrosymmetric solids via newly exploited salt-inclusion methods. INTRODUCTION Traditional synthesis of organic-inorganic hybrid materials employing organic molecules as a structure directing agent has demonstrated a wide variety of host-guest chemistry in the forms of bulk crystalline solids, thin films and nanocomposites [1-3]. The organic moieties offer an intimate control at the interface to facilitate templating in open-frameworks, to act as linkers between inorganic chains, and to arrange around zero-dimensional clusters. Our recent investigations in salt-inclusion reactions have revealed a fascinating class of new hybrid materials of mixed covalent and ionic lattices [4-9]. Similar to organic-inorganic hybrid frameworks, the two chemically dissimilar lattices do exhibit, by broad definition, a host-guest relationship. In light of our first discovery of non-centrosymmetric compound CuPO4·BaCl [4], we have expanded our investigation in order to further our understanding of salt inclusion and its role in special framework formation. The chemical systems under investigation are transition metal phosphates and arsenates where the extended covalent frameworks consist of mixed MOn (n = 4-6; M = Cu, Mn, Nb, Mo) and M'O4 (M' = P, As) polyhedral linkages. The polyhedra in most cases share common vertex oxygen atoms, as shown in Fig. 1a-e, in order to minimize the repulsion between highly charged cation centers. Occasionally edge sharing does occur, as shown in Fig. 1f, and the resulting polyhedra consequently adopt off-centered distortions due to the cation displacement [10]. In any event, due to the already demonstrated versatility of mixed polyhedral frameworks, we have been able to isolate a significant
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