Specific heat of ${\alpha }^{\prime }-{\mathrm{NaV}}_2{\mathrm{O}}_5$ in magnetic fields up to 16 T

A detailed investigation of the phase transition in chemically well characterized crystalline samples of ${\alpha }^{\prime }-{\mathrm{Na}}_x{\mathrm{V}}_2{\mathrm{O}}_5(0.95<x<~1)$ by specific-heat measurements in magnetic fields up to 16 T is presented. A single sharp $\lambda$-shaped anomaly close to 33.5 K is observed in $c_p.$ Its shape and height depends sensitively on sodium deficiency in the samples. With magnetic fields along the crystallographic a and b axis the anomaly shifts to lower temperatures $\propto H^2.$ The decrease is identical for both orientations and amounts to only 20% of the decrease predicted by common theories for the spin-Peierls transition. An energy gap in the excitation spectrum of $67\left(5\right)\mathrm{}\mathrm{K}\approx {2T}_{\mathrm{SP}}$ is obtained from the low-temperature specific heat.