Study of the temperature behavior of the heat capacity of a relaxor ferroelectric PbNi$_{1/3}$Nb$_{2/3}$O$_3$

The behavior of the heat capacity of single crystals of a relaxor ferroelectric PbNi$_{1/3}$Nb$_{2/3}$O$_3$ (PNN) in the temperature range from 2 to 400 K is considered. The analysis of the results of measurement of the specific heat $C_p$ did not reveal any structural phase transitions in the studied temperature range. In addition, the temperature dependence $C_p/T^3$ demonstrates non-Debye behavior in the low temperature region and the absence of a low-temperature maximum $C_p/T^3(T)$ characteristic of many related materials. The results obtained were discussed within the framework of modern concepts of the physics of relaxor ferroelectrics with magnetic ions in one of the perovskite sublattices.