Characteristic Temperatures of the Mössbauer Fraction and Thermal-Shift Measurements in Iron and Iron Salts

Mössbauer spectra have been measured in metallic iron, sodium nitroprusside, sodium ferrocyanide, and potassium ferrocyanide absorbers between 78 and 293 °K. The temperature dependences of the Mössbauer fraction $f_a$ and the resonant velocity $V_0$ were fitted to Einstein and Debye lattice-vibration models. The characteristic temperatures of the models fitted to $f_a$ are consistently lower than those fitted to $V_0$, showing the sensitivity of $f_a$ to low-frequency modes of vibration. The characteristic temperatures obtained from $V_0$ are higher for the salts than for the metal, indicating the presence of higher-frequency modes in the salts. This interpretation is verified semiquantitatively by comparing the thermal-shift Debye temperatures of the salts to their infrared absorption frequencies. The Mössbauer fraction of potassium ferrocyanide shows a weaker temperature dependence than expected for a harmonic solid which suggests potassium ferrocyanide is anharmonic in the temperature range studied.