Influence of the multiplet levels of Sm$^{3+}$ and Eu$^{3+}$ on the heat capacity of multiferroic BiFeO$_{3}$

The temperature dependence of the heat capacity of multiferroics BiFeO$_{3}$, Bi$_{0.90}$Sm$_{0.10}$FeO$_{3}$, and Bi$_{0.90}$Еu$_{0.10}$FeO$_{3}$. has been investigated. It is found that the substitution of bismuth ions with europium and samarium ions in bismuth ferrite leads to the appearance of an additional heat capacity component due to the transitions of 4$f$ electrons of rare earth ions to higher multiplet levels. A relationship is established between the decrease in phonon thermal conductivity and the Schottky effect for the specific heat.