In this study, the first of the proposed cycle, the conditions and the mechanism of nucleation and formation of particles of the Pb₂MoO ₅ phase in PbMoO₄ crystals, which are due to the physicochemical features of the heating, melting, and solidification of PbMoO₄, are considered. It is shown that preferred evaporation of one of the composite components of this compound, MoO₃, leads to the violation of the initial stoichiometry and formation of excess PbO in a charge at the synthesis stage and in a melt during the crystal growth. The formation of the Pb₂MoO₅ phase in crystals with excess PbO occurs during their postgrowth cooling at temperatures below 930°C (eutectics between PbMoO₄ and Pb₂MoO₅). The results of the differential thermal analysis, X-ray diffraction analysis, high-temperature powder diffraction study, and annealing of crystals in specific media confirm the solid-state mechanism of the formation of the Pb₂MoO₅ phase in PbMoO₄ crystals.