Temperature Dependence of Vibrational Spectra in Calcite by Means of Emissivity Measurement

The emissivities at various temperatures from 500 to 1200 ^∘{}K, together with the reflectivity at 300 ^∘{}K in calcite, were measured on the spectral region 200-4000 cm^-1. It was found by the measurements that conspicuous changes with temperature occurred in the reststrahlen bands of the lattice and molecular vibrations. By analyzing the reststrahlen bands, the temperature dependences of vibration parameters of the lattice vibration Eᵤ₍₃₎, 305 cm^-1 in frequency, and of the molecular vibrations A₂ᵤ₍ₐ₎ and Eu(b), 886 and 1416 cm^-1 in frequency, were obtained in the temperature range 300-1000 ^∘{}K. The results were interpreted in the light of the theories on anharmonic crystals. The damping constants of the lattice and molecular vibrations were found to arise from the quartic as well as the cubic anharmonicities; the contribution of quartic anharmonicity was seen to become prominent with increasing temperatures. The frequency shifts of the lattice and molecular vibrations due to anharmonicity were found to be proportional to -T²; some explanations were given for this. The oscillator strength did not show any change which exceeded the experimental error as expected theoretically.