## 3 Results and Discussion
The still very anomalous trend in the curve of FeIII(2) (Figure 5) is attributed to the computer evaluation procedure of the superparamagnetic system. In general, a superparamagnetic system can be easily fitted with the combination of a doublet (or singlet) and a magnetic sextet using Lorentzian line shapes (with some broadening) if the particle size distribution is even (i.e., in a highly polydisperse system). In this case, the particle size fraction of the phase that is at a transitional state between the fast relaxation limit (resulting in a doublet) and the slow relaxation limit (resulting in a sextet) is very low at any temperature. However, if there is a certain particle size with a large contribution to the distribution, then at the blocking temperature that belongs to that particle size, the spectral shape is somewhere in between a doublet and a sextet: it shows up as a very broad line stretched over an extremely wide velocity range. This results in a huge correlation between the baseline parameter and the line width of this relaxed component, and may cause overestimation of the line area. From these experiments, it is clear that the anomaly in the temperature variation of the spectral area of the FeIII(2) doublet is caused by the superparamagnetic behavior of this phase. The non-optimal velocity range of the measurement (too narrow as compared to the large broadening) can result in this virtual phenomenon. However, as can be seen in Figure 6, the superparamagnetism may not always cause a disturbance. In an additional synthesis, we could find a situation where a small amount of iron(III) formed (ca. 13%), first as FeIII(1) (Experiment 6, see later), and after about two weeks aging of the sample (stored in a refrigerator in the opened plastic bag) the iron(III) form was observed as FeIII(2) (Experiment 7). As shown in Figure 6, although the velocity range of the measurement was increased to about \(\pm 6\) mm/s to see any magnetically split component while keeping a reasonably high velocity resolution, there was no sign of any sextets even at 90 K. Most probably, the small amount of the FeIII(2) phase implies small particle size resulting in a blocking temperature of the system below 90 K even for the largest particles. Thus, all FeIII(2) appears as a doublet. Since the analysis of these spectra is not challenged by the superparamagnetic relaxation, one may expect regular behavior of the area vs. temperature curve for the FeIII(2) species (the doublet). However, the trend of the curve is still anomalous (Figure 6). The only reasonable explanation of this finding is that a phase transformation (polymorphism) may occur somewhere in between 170 and 210 K, and the phase existing at the higher temperature has a higher Debye temperature, and, inherently, a larger Mossbauer-Lamb factor. To check this idea, a low-temperature DSC measurement was performed on this sample, and it confirmed our assumption. As shown in Figure 7, a transformation occurs at around 180 K in this sample. The nature of this phase transformation is not known. Unfortunately, these ferrite samples are extremely sensitive to contact with air and humidity, or even to storage at room temperature, which makes other types of measurements like powder-XRD practically unfeasible. Figure 6: Mössbauer spectra of a ferrate(VI) sample containing Fe\({}^{\text{III}}\)(2) degradation product at selected temperatures recorded in \(\pm\)6 mm/s velocity range (Experiment 7), and the temperature variation of the normalized areas for ferrate(VI) (purple squares) and Fe\({}^{\text{III}}\)(2) (orange dots). Figure 7: Low-temperature DSC curve of a ferrate(VI) sample containing degradation product Fe\({}^{\text{III}}\)(2) in fully superparamagnetic form in the studied temperature range. All the experimental findings on ferrate(VI) samples containing FeIII(2) show that determination of the Debye temperature for this degradation product is substantially hindered by superparamagnetic behavior and phase transformation, making this goal almost unattainable.