The SiO2–HfO2 binary system is recognized as a promising candidate for Erbium-doped waveguides amplifiers fabrication. Recently, it was demonstrated that Er3+-activated 70SiO2–30HfO2 planar waveguides with valuable optical and structural properties can be prepared by sol–gel technique with dip-coating processing. The important role played by hafnium in the silica network was evidenced by the particular spectroscopic properties presented by Er3+-ions in the silica–hafnia planar waveguides. In this work we present preliminary results on HfO2–SiO2 bulk xerogels doped with Eu3+ ions, with the aim to go inside the role of hafnium on the rare earth ions local environment. Spectroscopic measurements of the Eu3+ photoluminescence emission are given. Numerical simulations by the molecular dynamics method have been performed showing clearly a phase separation for the HfO2 richer samples. Moreover it is found than the rare earth-doping ions stay preferentially in hafnium rich domains, thus explaining why the rare earth spectroscopic properties are strongly modified.