Optical properties and electronic structure of ${\alpha }^{\prime }-{\mathrm{Na}}_{1-x}{\mathrm{Ca}}_x{\mathrm{V}}_2{\mathrm{O}}_5$

The dielectric function of ${\alpha }^{\prime }-{\mathrm{Na}}_{1-x}{\mathrm{Ca}}_x{\mathrm{V}}_2{\mathrm{O}}_5$ $(0\mathrm{}<~x<~20\%)$ was measured for the a and b axes in the photon energy range 0.8–4.5 eV at room temperature. By varying the Ca concentration we control the relative abundancy of ${\mathrm{V}}^{4+}$ and ${\mathrm{V}}^{5+}.$ We observe that the intensity of the main optical absorption peak at 1 eV is proportional to the number of ${\mathrm{V}}^{5+}$ ions. This rules out the interpretation as a ${\mathrm{V}}^{4+}$ $d-d$ excitation, and it establishes that this is the on-rung bonding-antibonding transition.