In this " Master-level " lecture, we introduce the basic thermodynamics of dissolution in the crystalline state, and of partition (segregation or depletion) of an impurity in a crystal grown by solidification from the melt or from a flux. The examples are chosen in the field of high bandgap dielectric crystals doped with RE3+ cations, of great interest for optical or photonic applications, some of which exploit scintillating or laser materials. An increasing difficulty order is adopted. Firstly, the solid state solubility of Nd3+ cations in CaWO4 is explained, starting from a point defects equation. Schottky's building units and gittermolekül are introduced to allow for the definition of correct chemical potentials, permitting in turn to express the equilibrium solubility limit condition. A strategy to displace solubility equilibria is presented from both thermodynamical and kinetic viewpoints. Secondly, the equilibrium partition coefficient of Er3+ cations in Y2SiO5 is explained in the case of a crystal grown from the melt. Thirdly, the equilibrium partition coefficient of Yb3+ cations in Gd2O3 is explained in the case of a crystal grown from a Li6Gd(BO3)3 flux, that is, a melt of completely different composition. All demonstrations start at the point defect level, by means of mass-, charge- and site-balanced dissolution equations.Read less <