A thermodynamic investigation of NdMe$_3$Sr$_3$Mn$_4$O$_{12}$ (Me — Li, Na, K) manganites in the range from 298.15 to 673 K

High-temperature synthesis according to ceramic technology is used to prepare new manganites NdMe$_3$Sr$_3$Mn$_4$O${12}$ (Me — Li, Na, K). The X-ray powder method is used to find that the compounds crystallize in tetragonal syngony, and the parameters of their crystal lattices are determined. The method of dynamic calorimetry is used in the range from $298.15$ to $673$ K for determining the heat capacity of manganites; in so doing, the presence of second-order phase transitions is revealed. In view of phase transitions, equations are derived which describe the dependence $C_p^\text{o}\sim f(T)$ in the range from $298.15$ to $673$ K. The temperature dependences of permittivity and electrical resistance of manganites are investigated at $303$–$503$ K; these dependences likewise confirm the presence of second-order phase transitions.