Frustration of Exchange-Striction Ferroelectric Ordering and Growth of Electric Polarization of Phase Separation Domains in R${}_{0.8}$Ce${}_{0.2}$Mn${}_2$O${}_5$ (R = Er, Tb) Solid Solutions

The effect of rare-earth ions (R = Er${}_3^{+}$, Tb${}_3^{+}$, and Ce${}_{3.75}^{+}$) on the dielectric properties and the electric polarization induced by local polar phase separation domains in solid solutions of R${}_{0.8}$Ce${}_{0.2}$Mn${}_2$O${}_5$ (R = Er, Tb) multiferroics has been studied. These parameters are found to qualitatively differ from those of initial RMn${}_2$O${}_5$ (R = Er, Tb) crystals studied before. It is shown that the properties of the polar phase separation domains that form in a subsystem of Mn3+ and Mn4+ ions due to a finite probability of tunneling electrons between these ions with different valences are substantially dependent on the values of crystal fields in which these domains exist. A combined influence of Er${}_3^{+}$, Tb${}_3^{+}$, and Ce${}_{3.75}^{+}$ ions is found to substantially change the crystal field in R${}_{0.8}$Ce${}_{0.2}$ Mn${}_2$O${}_5$ (R = Er, Tb) as compared to RMn${}_2$O${}_5$ (R = Er, Tb).