  In2VO5, containing magnetically frustrated zig-zag chains, shows a remarkable
magnetic crossover at 120 K between paramagnetic states with positive (17 K)
and negative (-70 K) Weiss temperatures. Magnetic moment and entropy data show
that the V4+ S = 1/2 spins condense into S = 1 triplet dimers below the
crossover. A further freezing of the antiferromagnetically coupled triplet
dimers into a global singlet state is observed at 2.5 K, with no long range
magnetic order down to 0.42 K and in fields up to 9 T. No structural V-V
dimerization is observed by high-resolution X-ray diffraction down to 10 K, but
a subtle lattice anomaly evidences a spin-lattice coupling in the triplet dimer
state. This is assigned to longitudinal oxygen displacement modes that reduce
frustration within the chains and so couple to the spin dimer fluctuations.
