Mössbauer spectroscopic study of low-frequency vibrational modes in sintered metal powders

Low-frequency vibrational modes in sintered copper and silver powders have been studied by ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$ Mössbauer spectroscopy. A decrease in resonance absorption area has been found for the sinters compared with the values found for bulk materials. In the spectra of a copper sinter of low packing fracton (26.7%), a broad spectral component was clearly observed, superimposed on the normal Mössbauer line. This broad line is interpreted to arise from low-frequency modes due to oscillations of the particles in the sinters. The temperature dependence of the Mössbauer absorption area can be explained by a model based on dispersion theory combined with the Debye model and the fracton model used for describing phonons in a particle and particle oscillations in a sinter, respectively. The energies of the low-frequency modes determined from the Mössbauer measurements range from ≊2 mK to several hundred millikelvin (in equivalent temperature) and coincide within a factor of 6 with the values estimated from ultrasonic data [M. C. Maliepaard et al., Phys. Rev. B 32, 6261 (1985)] using the fracton model, providing support for the localized nature of the modes.