Calorimetric studies of $\rm{LaM}_2\rm{NiMnO}_5\,(\rm{M}$—$\rm{Li}, \rm{Na}, \rm{K})$ nickelite-manganite heat capacity within the temperature range of $298.15$–$673$ K

By means of the calorimetric method, we investigate the $\rm{LaM}_2\rm{NiMnO}_5\,(\rm{M}$—$\rm{Li}, \rm{Na}, \rm{K})$ nickelite-manganite heat capacities within the range of $298.15$–$673$ K. For the studied compounds, within the above temperature range, we reveal the $\lambda$-like effects respective to the second kind phase transition at $323$ K and $498$ K for compounds containing $\rm Li$, $323$ K and $523$ K for compounds containing $\rm Na$, and $448$ K for those containing $\rm K$. Taking into consideration the phase transition temperatures, we derive the equations of the temperature dependence of the compound heat capacity. On the basis of the experimental values of the heat capacities as well as of the calculation data on the nickelite-manganite standard entropy, we calculate the temperature dependences of the heat capacity, the entropy, the enthalpy, and the reduced thermodynamic potential.