Temperature dependence of the mid-infrared absorption in La1.84Sr0.16NiO4+δ

The reflectance of a La1.84Sr0.16Ni4+δ crystal has been measured between 115 cm−1 and 5500 cm−1 with the electric-field vector in the ab-plane for several temperatures between 80 and 300 K. On the basis of the Kramers-Kronig derived optical conductivity data we show that it is unlikely that small-polaron absorption can account for the mid-infrared absorption, as had been previously suggested. A doped-semiconductor model is also considered.