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Under the extreme conditions of super-Earths’ interior, MgSiO₃ post-perovskite (ppv), the last stable silicate phase in the earth’s mantle, is predicted to dissociate into Mg₂SiO₄ post-post-perovskite (pppv) and SiO₂ (Wu et al., 2013; Umemoto et al., 2017). It should be the primary mantle silicate in super-Earths that recombines and dissociates into other silicates and oxides. Therefore, it is essential to understand the properties of this pppv silicate phase, especially in solid-solution with Fe₂SiO₄. Here we present an ab initio study on the electronic, structural, and vibrational properties of Fe-bearing pppv. We also construct a thermodynamic model to address thermochemical equilibrium among (Mg_2-xFe_x)SiO₄, (Mg_1-xFe_x)SiO_3, and (Mg_1-xFe_x)O phases. These results will help model the mantle of super-Earth-type exoplanets.