## 4 Discussion  
The base glass network (70P\({}_{2}\)O\({}_{5}\)-30 Fe\({}_{2}\)O\({}_{3}\)), according to experimental and theoretical data in the literature [19; 32], is made up of long phosphate chains connected via [FeIIIO4]\({}^{-}\) tetrahedra. These tetrahedra have a negative charge that needs to be compensated for. In pure iron phosphate glasses, the charge compensator may be FeIII or FeII cations, which are placed in the modifier glass network positions. In the positions, the cations are placed in the open spaces within the network. However, they are introduced into the glass as oxides, and the associated oxygen atoms contribute to the glass network depolymerization by disrupting the P-O-P bridges. This leads to an increase in the glass network voids' formation. The modifiers placed in these spaces transform not fully saturated P=O bonds into, e.g., joined P-O-FeII[33]. A similar compensating role may be fulfilled by Sr [20; 34]. It was confirmed experimentally and theoretically that the introduction of SrO into the base glass network initially leads to its gradual depolymerization. Sr cations occupy the open voids of the network, which may be observed as the decrease in molar volume (Figure 5). With an increase, the depolymerization of the phosphate network is almost linear, up to 30 mol% of SrO. This point correlates with the full saturation of all [FeIIIO4]\({}^{-}\) tetrahedra by Sr [34]. Furthermore, Sr and Fe tend to create enriched regions that may act as seeds for the crystallization of strontium-containing iron phosphate compounds like SrFe\({}_{2}\)P\({}_{2}\)O\({}_{7}\)[20]. It was also observed that at low SrO concentration to 30 mol%, the iron phosphate network forms a continuous phase with the Sr cations placed in the open voids. Above the point, the iron phosphate network is no longer continuous and forms separated regions divided by the denser Sr-rich phase [20; 34]. According to the results of the simulations, the self-diffusion coefficients behave differently as a function of temperature, depending on the temperature range (Figure 4, Table 1). In the low-temperature region, up to about 1000 K, the highest coefficient values are observed for modifiers, lower for intermediate FeIII, and the lowest for network P cations. In this temperature range, glass may be considered a solid material. The upper limit of this region, around 1000 K, is close to the glass transition temperature of glasses with similar compositions [35; 36]. In this region, the process of self-diffusion is mostly governed by the migration of modifiers from one open void to another within the glass network. The migration of iron phosphate network species is limited. As the content of glass network modifiers increases, the number of open void sites in the network decreases, limiting the diffusion process of the modifiers. This is evidenced by the decrease in the \(D\) parameter with increasing SrO content in the glass. In the second region, from c.a. 1000 K to c.a. 1900 K, the glass is in liquid form. However, its viscosity remains relatively high. In this case, the self-diffusion process is mostly related to the migration of network species P and FeIII, in contrast to the previous region. This may be related to the decomposition of the iron phosphate network in the temperature range. The rapid decrease in the network species coefficients for x = 40 and 50 (Figure 4) is probably related to changes in the glass network structure discussed above, and the transition from the iron phosphate-dominating phase to the Sr-rich phase. The latter begins to resemble crystalline compounds that likely melt at higher temperatures. As a result, the migration of P and Fe\({}^{\text{III}}\) may be limited to the small space of the residual iron phosphate network, separated by the Sr-rich phase. This process prevents the migration of the species outside the residual regions and limits its self-diffusion. Finally, above c.a. 1900 K, both phases are in a liquid state and the glass viscosity is low.