## 3 Results and Discussion  
In our previous report [17], we showed the results of a large number (64) of measurements, each belonging to individual electrochemical synthesis batches with some variation in the synthesis conditions. We focused mostly on the purity of the product (i.e., ferrate(VI) content). Clear correlation between the synthesis parameters and the form and quantity of the degradation product iron(III) could not be found. In this report, we studied further samples from various synthesis runs, now with every sample measured in a wide temperature range. All samples were first measured at the lowest temperature (85-90 K) to see which degradation product had actually formed. Although the duration of the Mossbauer measurements were typically 1-2 days, the low temperature saved the samples from degradation. The iron(III) forms expected are summarized in Table 1 according to [17]. In these \"trial and error\" synthesis runs, we could confirm the formation of FeIII(1) and FeIII(2) many times. FeIII(3) was not found, but it formed in only 3 cases out of 64 in our previous work [17]. The Mossbauer spectra were first evaluated in a relatively narrow velocity range (as usual) for better velocity resolution. The ferrate(VI) spectral contribution was consequently modeled by a doublet with a small quadrupole splitting \(\Delta=0.12(1)\) mm/s. The isomer shift observed at 90 K was \(-0.83(1)\) mm/s. It should be noted that despite several reports on a singlet assigned to ferrate(VI) with a single cation (Na, K, Rb, or Cs) [23], our experience showed that noticing the small quadrupole splitting is a question of spectrum quality (statistics); therefore, despite the perfect tetrahedral structure of the FeO4\({}^{2-}\) anion, there is some quadrupole splitting that must originate from the non-cubic lattice contribution in the solid potassium ferrate(VI) that has an orthorhombic unit cell (space group Pnam) [26]. This idea is supported by the reports from [PERSON] et al. [38; 39], who synthesized the mixed ferrate(VI) K3 Na(FeO4)2, the unit cell of which is hexagonal (space group P/3m1), and found a different quadrupole splitting of \(\Delta=0.21(1)\) mm/s. The assignments of the degradation product iron(III) species are based on Mossbauer data. FeIII(3) must be octahedrally coordinated iron(III) on the basis of its isomer shift. The zero quadrupole splitting and the colorless nature of this compound support a very stable structure, and the extreme high pH where it exists makes it reasonable to assume that it is hexahydroxo-iron(III). The zero quadrupole splitting also suggests that it readily forms as an amorphous phase which most probably crystallizes quickly; therefore, it turns to FeIII(1) with small quadrupole splitting due to lattice contribution. This also explains why it is hard to catch the FeIII(3) species as the synthesis product. The nature of the crystalline form hosting FeIII(1) is not known. This assignment of FeIII(3) is strongly supported by the findings of [PERSON] et al. [40; 41; 42], who reported the same species in highly alkaline electrolytes (15 M NaOH) containing FeIII. FeIII(2) has Mossbauer parameters somewhat similar to those of lepidocrocite, but we tentatively consider this as an FeIII-oxyhydroxxide or hydroxide. The task to improve the quantitative analysis of an FeVI-FeIII system is therefore to measure the possible differences between the Debye temperatures (Mossbauer-Lamb factors at a particular thermodynamical temperature) of the ferrate(VI) and those of the FeIII(1) and FeIII(2) species. For such an analysis, the spectral area assigned to the particular species is normalized to the baseline of the spectrum (to account for the different measurement durations) and plotted as a function of temperature. According to Equation (3), a monotonously decreasing (closely linear) function is expected with temperature, probably with some different slopes owing to the assumed different Debye temperatures for ferrate(VI) and the degradation product.