${\mathrm{Mn}}^{2+}$ EPR study of phase transitions in dicalcium lead propionate ${\mathrm{Ca}}_2$Pb(${\mathrm{C}}_2$${\mathrm{H}}_5$COO${)}_6$: Determination of critical exponent below the ferroelectric phase transition and comparison with EPR studies on ${\mathrm{Ca}}_2$Ba(${\mathrm{C}}_2$${\mathrm{H}}_5$COO${)}_6$ and ${\mathrm{Ca}}_2$Sr(${\mathrm{C}}_2$${\mathrm{H}}_5$COO${)}_6$

The phase transitions undergone by DLP [dicalcium lead propionate ${\mathrm{Ca}}_2$Pb(${\mathrm{C}}_2$ ${\mathrm{H}}_5$COO${)}_6$] crystal have been studied via the EPR of ${\mathrm{Mn}}^{2+}$. Two phase transitions, one second order at 343±2 K and another first order at 180±2 K, have been deduced to occur, as revealed by the behavior of overall splitting of EPR lines and that of EPR linewidth. In the ferroelectric phase, below $T_{C1}$=343 K, the EPR spectra for the external magnetic field orientation in the {111${\mathrm{\}}}_{\mathrm{C}}$ plane varies with temperature as (T-$T_{C1}$ ${)}^{\beta }$, where β=0.51±0.03. The orientations of the magnetic Z axes corresponding to the various ${\mathrm{Mn}}^{2+}$ complexes above $T_{C1}$ have been determined. As well, the temperature at which the high-temperature hypothetical phase transition to a cubic phase is supposed to take place, has been determined to be 490 K from the temperature variation of the overall splitting of lines for the magnetic field orientations along the magnetic Z and X axes. The ${\mathrm{Mn}}^{2+}$ spin-Hamiltonian parameters in ${\mathrm{Ca}}_2$Pb(${\mathrm{C}}_2$ ${\mathrm{H}}_5$COO${)}_6$ are evaluated above $T_{C1}$ using a rigorous least-squares fitting procedure.