Doping of size mismatched aliovalent ions is challenging due to the associated elastic and electronic stress making the thermodynamics unfavorable. Despite such features, its utilization may be viable if such systems can be made metastable by suppressing the kinetics of phase segregation. In light of such a possibility, we utilize sol-gel synthesis for preparing a size mismatched trivalent europium doped MgO (Mg(1-x)Eu(x)O:(x/2)V"(Mg)) system, which can be potentially used in optical applications. It is found that such a doped system can be metastabilized and the extent of metastability can be correlated with critical temperature (Tc) required for phase segregation which decreases with the dopant concentration. For x = 0.005, 0.01, and 0.02, Tc is above 1200 °C, 500-800 °C and less than 500 °C, respectively. As the synthesis temperature is 500 °C, these trends in critical temperatures make it impossible to metastabilize europium in MgO with x > 0.01. Doping is evident from X-ray diffraction data, excitation spectra, high resolution emission spectra, and luminescence lifetimes. A characteristic strong red emission of Eu(3+) has been observed via energy transfer from the MgO matrix to Eu(3+). Density functional theory based simulations suggest stabilization of Eu(3+) in MgO at lower doping concentration through the formation of cation vacancies which is also evident from optical studies. Furthermore, thin films deposited using the e-beam evaporation technique from the Mg(1-x)Eu(x)O:(x/2)V"(Mg) (x = 0.005) system show UV sensitized emission with CIE coordinates (0.26, 0.21).