Optical spectroscopy of the charge-ordering transition in ${\mathrm{La}}_{1.67}$${\mathrm{Sr}}_{0.33}$${\mathrm{NiO}}_4$

Optical spectra of a single crystal of ${\mathrm{La}}_{1.67}$ ${\mathrm{Sr}}_{0.33}$ ${\mathrm{NiO}}_4$, which undergoes a charge-ordering transition at ${\mathit{T}}_{\mathrm{CO}}$∼240 K, have been investigated for wide ranges of photon energy (0.008–3 eV) and temperature (10–480 K). The opening of a charge gap as well as change of the gap magnitude (2Δ) with temperature is clearly observed below ${\mathit{T}}_{\mathrm{CO}}$, with an anomalously large ratio of 2Δ(T=0)/${\mathit{k}}_{\mathit{B}}$ ${\mathit{T}}_{\mathrm{CO}}$ (∼13). Conspicuous spectral change with temperature is also observed above ${\mathit{T}}_{\mathrm{CO}}$ over the energy region up to 2 eV, which suggests persistent fluctuation of the charge ordering or formation of small polarons above ${\mathit{T}}_{\mathrm{CO}}$. © 1996 The American Physical Society.