Synthesis of new mixed valence compounds MV{sup 5+}V{sub 2}{sup 4+}O{sub 7}(M=NH{sub 4},K): Crystal structure of NH{sub 4}V{sub 3}O{sub 7} and electrical properties of KV{sub 3}O{sub 7}

A new mixed valence compound, NH{sub 4}V{sub 3}O{sub 7}, as single crystals, has been synthesized hydrothermally. It crystallizes in the monoclinic system, space group I2/m with lattice parameters a=12.198(1)A, b=3.7530(2)A, c=13.178(1)A, {beta}=100.532(6){sup o}, V=593.11(7)A{sup 3}, Z=4. The crystal structure determined with R=0.038 consists of (V{sub 3}O{sub 7}){sub n} layers linked by ammonium cations. The layer is built up by replication through symmetry elements of three independent distorted octahedra sharing edges and corners. The distortion of vanadium octahedra tends to vary and the coordination number CN is reasonably selected as equal to 5+1. KV{sub 3}O{sub 7}, synthesized by the mild hydrothermal route as a largely pure phase, is isostructural and its semi-conductive character is indicative of the presence of V{sup 4+} and V{sup 5+} sites. These are mixed valence compounds MV{sup 5+}V{sub 2}{sup 4+}O{sub 7}, the vanadium localization on three independent crystallographic sites enabling their electric behavior by electron hopping.