Hydrothermal synthesis, crystal structure, solid-state NMR spectroscopy, and ionic conductivity of Na5InSi4O12, a silicate containing a single 12-membered ring

Na5InSi4O12 has been synthesized by a high-temperature, high-pressure hydrothermal method and characterized by single-crystal X-ray diffraction and solid-state NMR spectroscopy. It is isotypic to Na5ScSi4O12. The structure consists of 12-membered single rings of corner-sharing SiO4 tetrahedra linked together via corner sharing by single InO6 octahedra to form a 3-D framework that delimits two types of channels along the c-axis. There are six independent sodium sites. The Na atoms within the 12-ring channels are immobile and the Na atoms within 7-ring channels are highly mobile. The Si-29 MAS NMR exhibits one Q(3) and one Q(4) resonance, each of which is split into two peaks because of inequivalent chemical environments caused by partially occupied Na sites in the 7-ring channels. Na ion conductivity measurements on a pressed pellet of the title compound were performed. The Arrhenius plot of dc conductivity shows a change in slope near 220 degreesC. We attribute this to the activation of the Na site that is situated between 7-ring channels and may serve as a connecting link between the channels. Crystal data: trigonal, space group R-3c (No. 167), a = 21.7158(9) Angstrom, c = 12.4479(7) Angstrom, and Z = 18.