Spectroscopic studies on single crystals having the fluorite lattice. Part 1.—The fundamental absorption edge; Urbach's rule and the Debye temperature in CeO2

Abstract

Measurements of the fundamental absorption edge of single crystal CeO₂ up to absorption coefficients α of 120 cm^–1 for 291 < T < 1235 K are presented. The results are analysed in terms of Urbach's rule, α=α₀ exp[–σ(E₀–hν)]/kT. Over this temperature range Urbach's rule is obeyed and its parameters are derived, viz., E₀= 3.78 eV, α₀= 7.6 × 10⁷ cm^–1, σ₀= 0.90 and ħω_p= 70.2 meV; σ increased with temperature more slowly than usual. Urbach's rule has not previously been obeyed up to such high temperatures or investigated for any fluorite lattice compound. Only optical photons are involved in the absorption tail formation. The characteristic Debye temperature for CeO₂ was calculated as 630 and 606 K from specific heat and elastic data respectively. It was concluded that the ceria crystals retained their stoichiometry at all temperatures.