We have measured the specific heats of small (≈1 mg) crystalline flakes of ${\alpha }^{\prime }-{\mathrm{NaV}}_2{\mathrm{O}}_5.$ A large sharp peak is observed at its (33 K) spin-Peierls transition. The size of the peak $\left(\Delta c_P\approx R\right)$ is more than an order of magnitude greater than the value estimated from mean-field theory, consistent with large fluctuations expected from its one-dimensional magnetic structure. However, the peak is somewhat sample dependent and does not have the symmetry about $T_c$ expected for either Gaussian or critical fluctuations, suggesting either that the samples are near a multicritical point or that there is some heterogeneous broadening, in which case we are underestimating the true size of the anomaly. The low-temperature behavior is consistent with a Debye temperature between 345 and 414 K, depending on the size of magnetic contributions to the specific heat.

• Received 17 April 1998

DOI:https://doi.org/10.1103/PhysRevB.58.R2937