Spin-Peierls transition inα′−NaV2O5determined by infrared reflectivity

Polarized infrared reflectivity measurements have been performed on single crystals of the spin-Peierls compound ${\alpha }^{\prime }$-${\mathrm{NaV}}_2{\mathrm{O}}_5$ in the temperature range 20–300 K. Pronounced spectral features associated with the formation of the dimerized phase were detected both in the $\mathbf{a}$ and $\mathbf{b}$ polarizations (perpendicular and parallel to the spin-1/2 chains, respectively). The temperature dependence of a salient spectral line at $718{\mathrm{cm}}^{-1}$ sharply rising below the transition temperature $T_{\mathrm{SP}}$ obeys a $(1\mathrm{}-{T/T}_{\mathrm{SP}}{)}^{2\beta }$ law with $T_{\mathrm{SP}}\simeq 34.3$ K and $\beta \simeq 0.25$. In addition, a continuum signal is observed in the whole temperature range in the $\mathbf{a}$-polarized optical conductivity spectra. In order to interpret these results, calculations of the static dimerization and of the optical conductivity based on a mean-field and a dynamical treatment of the lattice respectively are proposed.