The synthesis, structure, and characterization of a novel 24-layer oxide : Ba8V7O22 with V(III), V(IV), and V(V)

Ba[sub 8]V[sub 7]O[sub 22], the first vanadium oxide in which both VO[sup 4[minus]][sub 4] and VO[sup 8[minus]][sub 6] coordinations and the coexistence and ordering of all the three common oxidation states of vanadium (+3, +4, and +4) are observed, has been synthesized by reducing Ba[sub 2]V[sub 2]O[sub 7] in hydrogen at 1350[degrees]C. Its crystal structure has been solved by single crystal X-ray diffraction, and magnetic and electrical properties have been examined. Ba[sub 8]V[sub 7]O[sub 22] crystallizes in the trigonal system, space group R[bar 3]m (No. 166). The structure consists of face-shared VO[sup 8[minus]][sub 6] and VO[sup 9[minus]][sub 6] octahedra that form V[sub 3]O[sup 14[minus]][sub 12] trimers, corner-shared VO[sup 4[minus]][sub 4] tetrahedra that are connected to the trimers such that they together form pseudo-two-dimensional magnetic layers perpendicular to the c-axis, and isolated VO[sup 3[minus]][sub 4] tetrahedra that are arranged in planes separating the magnetic layers. Vanadium atoms occupy the tetrahedral sites and 3/4 of the octahedral sites in an ordered fashion. The 12-layer structure with (cchh)[sub 3] stacking, previously proposed for BaFeO[sub 3[minus]x] but improperly described in space group R3m, is redescribed in R[bar 3]m. The 24R structure of Ba[sub 8]V[sub 7]O[sub 22] is closely related to the 12R structure. Ba[submore » 8]V[sub 7]O[sub 22] is a semiconductor with three activation energies ranging from 0.31 to 0.48 eV in the temperature range investigated (145-295 K), and a room temperature resistivity of about 12 [Omega] [center dot] cm. Its magnetic behavior is very complicated. Single crystal samples show a broad susceptibility maximum at 280 K, followed by an increasing susceptibility at lower temperatures and additional anomalies at 50 and 20 K.« less