THERMODYNAMIC, TRANSPORT AND MAGNETIC-PROPERTIES OF ALPHA'-NAV2O5

We report on heat capacity, electrical resistance and high-temperature ESR experiments on the spin-Peierls compound alpha '-NaV2O5. The spin susceptibility at high temperatures (T > 250 K) closely follows a Bon ner-Fisher behavior with an exchange interaction J = 578 K, but differ s significantly from this predictions at lower temperatures. The tempe rature depen dence of the heat capacity can be explained assuming a su m of a linear (magnetic) and a Debye (lattice) contribution. The speci fic-heat jump at T-SP approximate to 35 K cannot be described by the o pening of a gap in mean-field approximation. The release of entropy is almost by a factor of 20 too high compared to the MF predictions of t he Bonner-Fisher model for a uniform AFM spin chain with an exchange i nteraction of J = 578 K. The electrical resistance R(T) can roughly be described by variable-range hopping processes and a large anomaly at the transition into the low-temperature dimerized state. From these ob servations we conclude that the phase transition in alpha '-NaV2O5 at 35 K cannot be explained by a spin-Peierls transition alone.