Ammonium Vanadium Bronze, (NH4)2V7O16, as a New Lithium Intercalation Host Material.

A new type of ammonium vanadium bronze, (NH4)2V7O16, was synthesized by the hydrothermal method. The triclinic crystal structure (P1̅) is successfully identified by the single-crystal X-ray diffraction method. The layered structure is similar to that of other vanadium bronzes but with an unprecedented stoichiometry and crystal structure. The structure is composed of a stack of V7O16 layers along the c axis, and two NH4+ ions occupy the interlayer space per formula unit. Each ammonium ion is hydrogen-bonded to four lattice oxygen atoms, resulting in a stable structure with a large interlayer space, thus enabling the intercalation of various guest ions. Lithium ions are electrochemically intercalated into (NH4)2V7O16, with an initial discharge capacity of 232 mAh g-1 and an average discharge voltage of 2 V (vs Li/Li+). Upon the first discharge, lithium ions are inserted, whereas ammonium ions are extracted. Upon charging, a reverse reaction takes place. However, only a fraction of the extracted ammonium ions are reaccommodated. Despite the small quantity, the reinsertion of ammonium ions contributes crucially to the structural stability, improving the electrochemical performance. These results could provide a general understanding of the intercalation mechanism of host materials containing ammonium ions. In addition, (NH4)2V7O16 intercalates Na+ ions reversibly, implying a potential capability as a host material for other guest ions.

Crystal structure of calcium perchlorate anhydrate, Ca(ClO4)2, from laboratory powder X-ray diffraction data.

The crystal structure of calcium perchlorate anhydrate was determined from laboratory X-ray powder diffraction data. The title compound was obtained by heating hydrated calcium perchlorate [Ca(ClO4)2·xH2O] at 623 K in air for 12 h. It crystallizes in the ortho-rhom-bic space group Pbca and is isotypic with Ca(AlD4)2. The asymmetric unit contains one Ca, two Cl and eight O sites, all on general sites (Wyckoff position 8c). The crystal structure consists of isolated ClO4- tetra-hedra and Ca2+ cations. The Ca2+ cation is coordinated by eight O atoms of eight symmetry-related ClO4- tetra-hedra within a distorted square-anti-prismatic environment.