A careful design of nanocrystal architecture can strongly enhance the nanocrystal function. So far, this strategy faced a synthetic bottleneck in the case of refractory oxides. Here, we demonstrate the epitaxial growth of hafnia shells onto zirconia cores, and pure zirconia shells onto europium doped zirconia cores. The core/shell structures are fully crystalline. Upon shelling, the optical properties of the europium dopant are dramatically improved (featuring a more uniform coordination and a longer photoluminescence lifetime), indicating the suppression of non-radiative pathways. These results launch the stable zirconium and hafnium oxide hosts as alternatives for the established NaYF4 systems.