Ca₂Fe₂O₅ was synthesized using a solid-state reaction method. Stoichiometric amounts of high purity starting materials, including calcium carbonate (CaCO₃) and iron (III) oxide (Fe₂O₃) and manganese oxide (MnO₂), were carefully weighed and thoroughly mixed. The mixtures were ground using an agate mortar and pestle to achieve homogeneous powders.

The resulting powders were pressed into pellets and subjected to a two-step thermal treatment. Initially, the samples were calcined at 1000˚C for 24 hours in MTI muffle furnace, with a ramping rate of 100˚C per hour to ensure controlled heating. After cooling, the pellets were reground, pressed again into pellets, and heated at 1200˚C for an additional 24 hours under the same ramping rate. Following the final heat treatment, the pellets were allowed to cool to room temperature naturally inside the furnace.

The phase purity of the synthesized compounds was confirmed using Bruker D2 phaser X-ray diffractometer (XRD) with Cu Kα1 (λ = 1.54056 Å). Rietveld refinements were conducted using GSAS software and the EXPEGUI interface. The microstructures were studied using high resolution field emission scanning electron microscopy (JEOL-SEM).

The thermoelectric effect was measured by measuring the potential developed by the temperature difference between the two sides of the pellet. A custom designed set up instrument was used to measure it. It contains hollow tubes connected to a hollow cube on two opposite sides for water outlet and inlet. Ice-cold water was run to a metal cubic hollow box through the tube. It keeps the box wall cold all the time. A sample pellet was kept intact with the metal box wall. The temperature of the box wall in contact with the pellet was measured with a k-type thermometer. At the same time the other side (outside) of the pellet was heated with a metal sheet. The metal sheet was heated by an electric heater such as high temperature soldering heater with controller. The temperature of the outside (heated part) of the pellet was measured by another K-type thermometer. Similarly, the potential difference developed by the hot and cold sides was measured using a voltmeter. Thus, the temperature difference between the two sides of a pellet and the potential (voltage) developed were measured at the same time. The whole set-up was diagrammatically shown in Figure 1 .

Ca₂Fe₂O₅ was synthesized using the solid-state synthesis technique. The crystal structure of Ca₂Fe₂O₅ has been previously reported. [4] To further investigate its structural properties, powder X-ray diffraction (XRD), and scanning electron microscopy (SEM) were employed.

Figure 2 presents the powder XRD data for Ca₂Fe₂O₅, confirming its crystal structure through Rietveld refinement. The refinement results were consistent with the previously reported brownmillerite structure characterized by the Pnma space group. The refinement also highlighted the alternating arrangement of tetrahedra and octahedra within orthorhombic unit cells. In this structure, tetrahedra within the same layer are aligned in the same direction, while tetrahedra in alternating layers are oriented oppositely. This is visually depicted in Figure 2 inset, where Blue tetrahedra indicates a rightward orientation, and red tetrahedra indicates a leftward orientation. The refined structural parameters of Ca₂Fe₂O₅ are provided in Table 1 .

Space group = Pnma, a = 5.4024(1) Å, b = 14.7017(3) Å, c = 5.5726(3) Å, wRp =0.0177, Rp = 0.0132.

The scanning electron microscopy (SEM) image of Ca₂Fe₂O₅, shown in Figure 3 , reveals close contact between the grains. The SEM analysis indicates that the material is non-porous. The non-porosity was evaluated using a theoretical density calculation based on the method described in the literature [6] . The experimental density of Ca₂Fe₂O₅ was found to be 98% of the theoretical value, further corroborating the non-porous nature of the bulk material. Despite variations in grain size, larger grains were observed to dominate over smaller ones, as depicted in the SEM image.

The thermoelectric behavior of Ca₂Fe₂O₅ was investigated across a range of temperatures from 7.5˚C to 50˚C. The results, summarized in Table 2 , show the voltage “generated by temperature differences (ΔT) and the corresponding thermoelectric effect,” S = V/ΔT, where V is the measured voltage in millivolts (mV) and ΔT is the temperature difference in Celsius. Seebeck coefficient S indicates the material’s efficiency in converting thermal gradients into electrical voltage.

At lower temperatures (7.5˚C to 17.6˚C), the voltage generated increased with the temperature difference, although this relationship was not linear. For instance, the highest voltage (109 mV) was observed with a small temperature difference of 3˚C between 14.6˚C and 17.6˚C. This suggests that even small temperature gradients can lead to significant voltage generation under certain conditions. As the temperature difference increased in this range (e.g., from 6˚C to 6.3˚C between 7.5˚C and 13.5˚C), the voltage also increased, with a peak value of 120 mV at a difference of 6.3˚C.

However, at higher temperatures (above room temperature), the voltage response began to exhibit more variability. For example, between 30˚C and 47˚C, the voltage generated ranged from 55 mV at a temperature difference of 17˚C to 45 mV at a difference of 25˚C. This suggests that the material’s efficiency in generating voltage diminishes as the temperature rises, possibly due to a reduction in the material’s thermoelectric potential at higher temperatures.

The Seebeck effect refers to the generation of an electromotive force (emf) across two points of an electrically conductive material when there is a temperature gradient between them. This emf is known as the Seebeck emf, or thermoelectric emf. The Seebeck coefficient is the ratio of the emf to the temperature difference. A thermocouple measures the potential difference between the hot and cold ends of two dissimilar materials, and this potential difference is proportional to the temperature difference between the two ends. The value of S shows a decreasing trend with increasing temperature differences. At lower temperatures, for example, when the temperature difference was 3˚C (between 14.6˚C and 17.6˚C), the thermoelectric figure of merit was notably high (36.33), indicating efficient voltage generation with a relatively small temperature gradient. In contrast, at higher temperatures with larger temperature differences, S values were considerably lower. For instance, between 30˚C and 55˚C, the S values decreased significantly, with the lowest value (1.8) recorded for a temperature difference of 25˚C. Table 2 shows the relation between the temperature gradient and Seebeck coefficient. Figure 4 shows the relation between the Seebeck coefficient and the lowest (cold) temperatures at which voltages were measured. Table 3 compares the Seebeck coefficient values with other reported materials.

This trend suggests that while the material is capable of generating relatively high voltage at small temperature differences, its efficiency decreases as the temperature gradient increases. Such behavior may be indicative of the material’s non-linear thermoelectric response, possibly due to intrinsic properties like electron scattering, phonon-electron interactions, or thermal conductivity changes at higher temperatures.

The high voltage at smaller temperature differences and the decrease in S at larger gradients can be attributed to several factors:

1) Non-linear thermoelectric response: The material exhibits a complex thermoelectric behavior, where smaller temperature differences result in a more efficient conversion of thermal gradients into voltage. However, as the temperature difference increases, the voltage generated no longer increases proportionally.

2) Material-specific properties: Ca₂Fe₂O₅ may exhibit a saturation effect in its thermoelectric response at higher temperatures, where the voltage generated reaches a plateau or decreases slightly with further increases in temperature difference. This suggests that the material’s thermoelectric properties are sensitive to both the magnitude and direction of temperature gradients.

3) Thermal conductivity and Seebeck coefficient: The decrease in S at higher temperature differences could also be due to an increase in thermal conductivity, which could reduce the effective temperature gradient across the material, thus lowering its voltage output.