| Two families of microporous solids were investigated to observe the effects of variation of the guests and framework. Sodalites are open framework aluminosilicates with cages of 6Å diameter containing a combination of cations, anions, and water molecules with a net charge of +3 to balance the negative charge of the framework. By varying the identities of the guest cations, a segregation was seen in the properties of the dehydrated sodalites M6[AlSiO 4]6. For M = Na and K strong framework-distorting electrostatic forces were evidenced by the presence of superstructure reflections in their X-ray diffraction patterns and multiple 27Al sites in their MQMAS NMR spectra. For M = Li, Tl, and Ag, covalent bonding between the guest and the sodalite framework was indicated by short M-O bonds, a large 27Al NMR quadrupolar effect, and splittings in the framework modes in the infrared spectra of these compounds. Upon doping K6[AlSiO 4]6 with rubidium metal, a number of interesting properties were discovered. Rb4.7K3.3(e−) 2[AlSiO4]6 was investigated by NMR, ESR, SQUID, and rudimentary conductivity measurements. At high temperatures conducting behavior is evidenced by a broad ESR signal, a Knight shifted 87 Rb NMR signal, and measurable conductivity of approximately 1 S/cm. The unusual properties of this “expanded metal” include a temperature dependent Knight shift, and a metal to antiferromagnetic insulator transition at 100K.; Group IV clathrates are also cage structures, with frameworks consisting predominantly of tetrelide elements (Si, Ge) encapsulating individual alkali or alkaline earth atoms in each cage. The clathrate type I structure is amenable to substitution of framework elements. Many such modified structures can be grown as large single crystals in a gallium flux. A series of Ba8Ga 16Ge30 compounds doped with varying amounts of antimony was prepared this way and the transport properties investigated. The antimony appears to form a dopant band in the gap of the clathrate; this results in an increase in the Seebeck coefficient but poor conductivity. A clathrate type II compound, Rb8Na16Si136, was also investigated. Temperature dependent NMR Knight shifts and paramagnetic susceptibility indicates this material is a narrow band metal. |
