The Versatile Chemistry and Noncentrosymmetric Crystal Structures of Salt‐Inclusion Vanadate Hybrids

Abstract

Hybrid solids containing chemically dissimilar components have attracted much interest in advanced materials synthesis due to their structural versatility and multifunctional properties. Metal-organic frameworks (MOFs), for instance, have been extensively explored for their potential applications in technologically important fields such as heterogeneous catalysis, gas storage, and sensors. [1][2][3] These solids are moleculebased composite materials that contain metal ions or metal ion clusters as nodes and organic ligands as linkers. Specialframework MOFs, including noncentrosymmetric (NCS) solids, can be constructed by design due to the modular nature of their structures. Salt-inclusion solids (SISs), which are a newly emerging class of hybrid compounds, are reported to exhibit covalent metal oxide frameworks integrated with ionic lattices including alkali/alkaline-earth metal halides and others such as ammonium halides. [4][5][6][7] These all-inorganic SISs possess a fascinating structural chemistry complementary to MOFs where bonding at the interface of the dissimilar components appears to be directional. This property has been revealed through the formation of salt-templated porous frameworks [6] and NCS lattices. [7] In general, SISs are synthesized in reactive molten-salt media at 100-150 K above the melting point of the salt employed. Although no reaction mechanism has been formulated, one can imagine that the metal oxides are first "dissolved" in the corrosive molten salt and then, upon cooling, the covalent lattice aggregates around the inherent structure of the molten ionic salt [8] to form the resulting special frameworks. Employing this concept, we have recently expanded our studies in saltinclusion chemistry into the vanadate and non-oxide systems, [9] and this is the first of several reports in the area of SISs of vanadium(V)-based mixed-metal oxides to depict the rich structural chemistry inherent in this system.Prior to this study, most efforts in the field of salt-inclusion synthesis had been focused on creating mixed-framework solids based on transition metal (TM) phosphates, arsenates, and silicates. [4][5][6][7] Substituting the (XO 4 ) 3À oxoanions (X = P, As) in the previously studied systems with the fully oxidized (VO 4 ) 3À anion has allowed new structural chemistry to be unveiled. Mixing vanadium with a second TM is particularly attractive as the resulting compounds could have potential applications in areas linked to catalysis, batteries, and magnetism. This is in part due to the variable crystal chemistry of the added transition metals as well as the utility of the unique frameworks created by the salt. Structurally, like their silicate counterparts, fully oxidized vanadium(V) compounds could achieve more diversity due to their ability to form vanadate units of varying size, shape, and geometry, such as dimers, trimers, chains, and rings. Unlike Si, P, and As, however, V can adopt several different coordination modes and can therefore achieve even greater structural versatility. Furt...

Structural and spectroscopic study of Mg13.4(OH)6(HVO4)2(H0.2VO4)6

“…Since vanadium can exist in several oxidation states and different coordination geometries a great number of structure types among metal vandates were discovered. Furthermore, metal vanadates can form non-centrosymmetric structures that may show some important properties, such as non-linear optical effects, piezoelectricity, ferroelectricity, and pyroeletricity [1]. A variety of polyoxovanadates have been synthesized under hydrothermal conditions.…”

Cs7Sm11[TeO3]12Cl16 and Rb7Nd11[TeO3]12Br16, the new tellurite halides of the tetragonal Rb6LiNd11[SeO3]12Cl16 structure type

“…The four halogen atoms from the Ln(4)O 4 X 4 antiprisms also belong to the MX 8 cubes. The interplay between the {MLn 11 (Fig. 2, d) [17][18][19][20], the LnO 4 X 4 antiprisms never stack in "eclipsed", only in "staggered" motif.…”

“…An alternative approach has been developed for the pure inorganics, which is based on the so-called "saltinclusion solids". These marry two chemically dissimilar constituents, which are usually the covalent and ionic sub-frameworks, in one structure [8][9][10][11][12]. In fact, these "salt inclusions" often act as templates for pores, channels, or interlayer spaces of the covalent framework.…”

Noncentrosymmetric Inorganic Oxide Materials: Synthetic Strategies and Characterisation Techniques

“…Another novel strategy for synthesising and designing NCS materials has been described by Hwu et al [80][81][82][83] He has focused on salt-inclusion solids whose framework consists of mixed ionic and covalent sub-lattices. Hwu et al have utilised a combination of acentric polyanions, such as those found in silicates, along with first-order Jahn-Teller cations and chlorinecentred, acentric secondary building units (SBUs).…”