Interference of a first-order transition with the formation of a spin-Peierls state in α′-NaV2O5

We present results of high-resolution thermal-expansion and specific-heat measurements on single crystalline ${\ensuremath{\alpha}}^{\ensuremath{'}}$-${\mathrm{NaV}}_{2}$O${}_{5}$. We find clear evidence for two almost degenerate phase transitions associated with the formation of the dimerized state around 33 K: A sharp first-order transition at ${T}_{1}=(33\ifmmode\pm\else\textpm\fi{}0.1)$ K slightly below the onset of a second-order transition at ${T}_{2}^{\mathrm{onset}}\ensuremath{\approx}(34\ifmmode\pm\else\textpm\fi{}0.1)$ K. The latter is accompanied by pronounced spontaneous strains. Our results are consistent with a structural transformation at ${T}_{1}$ induced by the incipient spin-Peierls order parameter above ${T}_{2}{=T}_{\mathrm{SP}}.$