Interference of a first-order transition with the formation of a spin-Peierls state in ${\mathrm{\alpha }}^{\prime }$-${\mathrm{NaV}}_2{\mathrm{O}}_5$

We present results of high-resolution thermal-expansion and specific-heat measurements on single crystalline ${\alpha }^{\prime }$-${\mathrm{NaV}}_2$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 $T_1=(33\mathrm{}\pm 0.1)$ K slightly below the onset of a second-order transition at $T_2^{\mathrm{onset}}\approx (34\mathrm{}\pm 0.1)$ 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 $T_2{=T}_{\mathrm{SP}}.$