Mössbauer-Effect Study of the Ferroelectric Transitions in Potassium Ferrocyanide Trihydrate and Ferric Ammonium Sulfate Dodecahydrate

The Mössbauer effect has been used to study the ferroelectric transitions in potassium ferrocyanide trihydrate (KFCT) and ferric ammonium sulfate dodecahydrate (FAS). The recoilless fraction and isomer shift of these materials have been observed as a function of temperature. For FAS, the absorption line-width was also observed. For FAS, the recoilless fraction $f$ and absorption linewidth $w$ were found to behave anomalously at the ferroelectric transition temperature $T_c$; $f$ exhibits a minimum at $T_c$, and $w$ has a maximum there. No change in the isomer shift was detected at the FAS transition. The behavior of $f$ is consistent with a description of the transition in FAS in terms of a "soft" transverse optical (TO) lattice mode of zero wave vector (${\omega }_{\mathrm{TO}}\to O$ at $T=T_c$). For KFCT, no anomalous behavior of any Mössbauer-effect parameter was detected. This is contrary to a previous report by other workers of a maximum in $f$ near $T_c$ for KFCT. The lack of any anomaly in KFCT is consistent with an order-disorder model of the KFCT transition, but does not preclude one particular type of displacive transition.