Effect of Zero-Point Motion on the Mössbauer Spectra of K4Fe(CN)6 and K4Fe(CN)6·3H2O

Abstract

A detailed calculation of the second order Doppler (SOD) shift of the Mössbauer line for some molecular solids demonstrates that the magnitude of this effect is significant in comparison with the full scale of the known isomeric shifts in iron compounds. The main purpose of this paper is to emphasize the importance of the SOD shift and, in particular, of the temperature‐independent portion, which is due to the zero‐point motion of the Mössbauer atom. A simple model for calculation of the SOD shift for some molecular solids is presented, followed by a detailed analysis of the experimental results for K₄Fe(CN)₆ and K₄Fe(CN)₆·3H₂O. An SOD contribution of 0.275 and 0.285 mm/sec to the measured shifts for the two compounds at room temperature is suggested on the basis of the analysis; an isomeric shift of +0.345 mm/sec, with respect to a 310 stainless‐steel absorber at room temperature, is proposed for both compounds. An extrapolated difference of about 0.02 mm/sec at 0°K in the measured thermal‐shift curves of the two compounds is attributed to the change in the zero‐point motion due to modifications in the normal mode distributions that are associated with the water of crystallization. Comparison with results on other compounds shows that differences as high as 0.15 mm/sec may be expected in the contribution of the zero‐point motion to the Mössbauer line positions.