Mössbauer Effect forFe57in Ferroelectric Lead Titanate

Abstract

Mössbauer effect for ${\mathrm{Fe}}^{57}$ in ferroelectric PbTi${\mathrm{O}}_3$ has been investigated over the temperature range 300-1100°K. The quadrupole interaction in the ferroelectric phase remains nearly saturated up to 320°C, beyond which it decreases gradually to disappear at and above the transition temperature $T_C=(480\mathrm{}\pm 2)$ °C. The ratio of the quadrupole interaction sensed by the $\frac{3}{2}$ state of ${\mathrm{Fe}}^{57}$ nucleus in PbTi${\mathrm{O}}_3$ to that in BaTi${\mathrm{O}}_3$ at room temperature is 1.41 ± 0.04, in good agreement with the value 1.46 ± 0.07 obtained through perturbed ${\mathrm{Sc}}^{44}$ $\gamma -\gamma$ angular-correlation studies by Hass and Glass. The numerical value of the electric field gradient, as also that of isomer shift at room temperature, indicates that this lattice is more covalent than BaTi${\mathrm{O}}_3$. Both the center shift and the area under resonance show anomalous temperature variation in the vicinity of the transition temperature. Analysis of the temperature variation of these parameters suggests that the Debye temperature of the lattice decreases considerably on crossing the transition temperature. Analytical form of the Lamb-Mössbauer factor in the presence of the Cochran type of soft mode has been derived. Computer fitting of the theoretical value of the Lamb-Mössbauer factor at various temperatures normalized to that at $T_C$, with the corressponding normalized areas under resonance, has brought out the essential validity of the suggested temperature variation of the soft mode.