Magnetic and thermal properties of the S=1/2 zig-zag spin-chain compound In2VO5

Static magnetic susceptibility chi, ac susceptibility chi(ac), and specific heat C versus temperature T measurements on polycrystalline samples of In2VO5 and chi and C versus T measurements on the isostructural, nonmagnetic compound In2TiO5 are reported. A Curie-Weiss fit to the chi(T) data above 175 K for In2VO5 indicates ferromagnetic exchange between V4+ (S=1/2) moments. Below 150 K, the chi(T) data deviate from the Curie-Weiss behavior but there is no signature of any long-range magnetic order down to 1.8 K. There is a cusp at 2.8 K in the zero-field-cooled (ZFC) chi(T) data measured in a magnetic field of 100 Oe and the ZFC and field-cooled data show a bifurcation below this temperature. The frequency dependence of the chi(ac)(T) data indicates that below 3 K, the system is in a spin-glass state. The difference Delta C between the heat capacity of In2VO5 and In2TiO5 shows a broad anomaly peaked at 130 K. The entropy up to 300 K is more than what is expected for S=1/2 moments. The anomaly in Delta C and the extra entropy suggest that there may be a structural change below 130 K in In2VO5.