Inorganic Chemistry, Vol.33, No.26, 5994-5998, 1994
Structure and Physical-Properties of La3.4Ca0.6V5Si4O22 - A Novel Mixed-Valent Vanadium(III/IV) Oxosilicate
La3.4Ca0.6V5Si4O22 crystallizes in the monoclinic system, space group C2/m (No. 12). The unit cell dimensions are a = 13.448(4) Angstrom, b = 5.5879(9) Angstrom, c = 11.070(4) Angstrom, beta = 100.57(2)degrees, V = 817.7(4) Angstrom(3), and Z = 2. Structural refinement with 817 observed reflections (I > 3 sigma(l)) and 103 variable parameters resulted in R = 0.034, R(w) = 0.037. The structure of La3.4Ca0.6V5Si4O22 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 corner-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 La3.4Ca0.6V5Si4O22 follows a Curie-Weiss behavior with an effective moment indicative of two localized unpaired electrons in the Vm(Si2O7)2(9-) layer. The (VO2)(4)(2.4-) rutile layer forms an extended V-V bond along the b-axis; however, La3.4Ca0.6V5Si4O(22) is an insulator with room-temperature resistivity of the order of 10(6) Omega cm along the b-axis.