## 2 Experimental Part
As shown in Figure 6, for NdK\({}_{2}\)NiMnO\({}_{5}\), a weight loss is observed in the range of 30-187 \({}^{\circ}\)C, caused by the removal of adsorbed moisture, accompanied by an endothermic effect. In the range of 483-581 \({}^{\circ}\)C, an exothermic effect is recorded, likely associated with a first-order phase transition involving structural rearrangements. The compound remains stable and does not decompose up to 1000 \({}^{\circ}\)C. A slight deviation of the TGA curve near this temperature is probably due to slow congruent or incongruent evaporation of the substance. Figure 5: TGA/DTA GA/DTA of the NdNa\({}_{2}\)NiMnO\({}_{5}\) sample. Figure 6: TGA/DTA GA/DTA of the NdK\({}_{2}\)NiMnO\({}_{5}\) sample. The relationship between temperature and heat capacity of NdNa\({}_{2}\)NiMnO\({}_{5}\) and NdK\({}_{2}\)NiMnO\({}_{5}\) compounds was studied using the IT-C-400 dynamic calorimeter (Manufacturer: Instrument-Building Plant, Aktobe, Kazakhstan, 1981). Measurements were conducted in the temperature range from 298.15 K to 673 K. It is worth noting that experimental dynamic calorimetry is a method for measuring thermal effects of chemical reactions or physical processes in real time. This technique is utilized to measure thermal effects occurring during chemical reactions or physical processes within materials. It involves continuous monitoring of the sample's temperature during heating and cooling, allowing for the study of changes in heat capacity and other thermal properties of the material. This type of calorimetry is described in detail in [22]. The IT-C-400 calorimeter operates based on the comparative method of dynamic calorimetry, which includes a thermocouple and an adiabatic shell. During the heating of the sample (in 25 \({}^{\circ}\)C increments), the F136 device and a stopwatch measure the time delay of the ampoule's temperature relative to the base temperature. This unique feature of the instrument enables heat capacity measurements only at every 25 K up to a maximum temperature of 673 K. The initial measurement temperature (298.15 K) allows for determining the standard heat capacity of the compound, which is an important fundamental constant. The measurement range for volumetric heat capacity is no less than 1 \(\times\) 10\({}^{6}\) J/(K-m\({}^{3}\)), and the entire measurement process, including data processing, takes no more than 2.5 h. The measurement error does not exceed \(\pm\)10%, according to the calorimeter's technical specifications. Calibration of the instrument was performed based on the thermal conductivity data of the K\({}_{\mathrm{T}}\) thermocouple. The molar heat capacity was calculated based on the specific heat capacity and the molar mass. At each temperature increment of 25 K, five parallel experiments were conducted. The results were averaged and processed using methods of mathematical statistics. For the averaged values of specific heat capacity at each temperature, the standard deviation \(\left(\overline{\delta}\right)\) was evaluated as described in [23]  
\[\overline{\delta}=\sqrt{\frac{\sum_{i=1}^{n}\left(C_{i}-\overline{C}\right)^{2} }{n-1}} \tag{2}\]  
where \(n\) is the number of experiments, \(C_{i}\) is the measured value of specific heat capacity, and \(\overline{C}\) is the arithmetic mean of the measured specific heat capacity values. To calculate the molar heat capacity of the samples, data on specific heat capacity, along with the molar mass, were utilized. At each temperature increment of 25 K, five parallel measurements were conducted. The results were averaged and processed using mathematical statistical methods. For the averaged values of molar heat capacity, the random component of error was calculated according to [24]  
\[\overset{\circ}{\Delta}=\frac{\delta\times t_{p}}{\overline{C}}\times 100 \tag{3}\]  
where \(\overset{\circ}{\Delta}\) is the random component of error in %, and \(t_{p}\) is the Student's coefficient (for n = 5, \(t_{p}\) = 2.75 at a confidence interval of \(p\) = 0.95).