Pseudo-gap and possible spin-Peierls transition in the vanadium oxide VOSb2O4

We report on the magnetic susceptibility and electron spin resonance measur ements on polycrystalline samples of the vanadium oxide VOSb2O4, a quasi-on e-dimensional S = 1/2 Heisenberg system. The susceptibility vanishes at zer o temperature, but there is no cusp at the onset of the susceptibility drop , and the ESR linewidth exhibits an increase characteristic of a phase tran sition at a much lower temperature. We show that this behaviour is consiste nt with the formation of a pseudo-gap in a spin-Peierls system in the adiab atic limit.