Structure and Physical Properties of La_3.4Ca_0.6V₅Si₄O₂₂: A Novel Mixed-Valent Vanadium(III/IV) Oxosilicate

La_3.4Ca_0.6V₅Si₄O₂₂ crystallizes in the monoclinic system, space group C2/m (No. 12). The unit cell dimensions are a = 13.448(4) Å,b = 5.5879(9) Å, c = 11.070(4) Å, β = 100.57(2)°, V = 817.7(4) ų 123, and Z = 2. Structural refinement with 817 observed reflections (I > 3 σ(I)) and 103 variable parameters resulted in R = 0.034, R_w = 0.037. The structure of La_3.4Ca_0.6V₅Si₄O₂₂ may be described as the intergrowth of vanadium pyrosilicate layers, V^III(Si₂O₇)₂^9-, and rutile-like layers, (VO₂)₄^2.4-. These two layers are interconnected by comer-sharing oxygen atoms between Si₂O₇ groups and VO₆ octahedra in the rutile layers along the c-axis. Distorted octagonal tunnels running along the b-axis are created by the network of polyhedra. The La³⁺ and Ca²⁺ ions located in these tunnels are statistically disordered. The magnetic susceptibility of La_3.4Ca_0.6V₅Si₄O₂₂ follows a Curie-Weiss behavior with an effective moment indicative of two localized unpaired electrons in the V^III(Si₂O₇)₂^9- layer. The (VO₂)₄^2.4- mtile layer forms an extended V-V bond along the b-axis; however, La_3.4Ca_0.6V₅Si₄O₂₂ is an insulator with room-temperature resistivity of the order of 10⁶ Ω cm along the b-axis.