Heat capacity and electrophysical properties of $\mathrm{GdMeFe_2O_5}$($\mathrm{Me}$–$\mathrm{Li}$, $\mathrm{Na}$, $\mathrm{K}$, $\mathrm{Cs}$)-type ferrites

The heat capacity of $\mathrm{GdMeFe_2O_5}$($\mathrm{Me}$–$\mathrm{Li}$, $\mathrm{Na}$, $\mathrm{K}$, $\mathrm{Cs}$) is investigated within the temperature range of $298.15$–$673$ K by a calorimetric method. $\lambda$-points are revealed in the $C_p^\circ(T)$ curves at $448$ K and $598$ K in $\mathrm{GdLiFe_2O_5}$, at $473$ K and $573$ K in $\mathrm{GdNaFe_2O_5}$, at $448$ K and $598$ K in $\mathrm{GdKFe_2O_5}$, and at $448$ in $\mathrm{GdCsFe_2O_5}$ related to a phase transition of the second kind. The temperature dependencies are calculated for the thermodynamic functions $C_p^\circ(T)$, $H^\circ(T)-H^\circ(298.15)$, $S^\circ(T)$, and $\Phi^{**}(T)$. The electrophysical characteristics of the synthesized ferrites are studied within the temperature range of $303$–$493$ K: Effects similar to those found in the $C_p^\circ(T)$ curves are also revealed in the $\log\varepsilon(T)$ and $\log R(T)$ curves. The compositions are shown to exhibit a semiconductor-type electrical conductivity. At the $T_{\text{cond}}$ points of the ferrites, the semiconductor-type conductivity changes the metal type one and both the capacities and permittivities vary sharply, and that may be related to a ferroelectric phase transition (the Curie and the Neel points). These effects provide a certain possibility to clarify the nature of the $\lambda$-points in the $C_p^\circ(T)$ curves.