In the framework of the development of the fourth generation of nuclear reactors, and more specifically Sodium-cooled Fast Reactors (SFR), the sodium coolant must have specified oxygen content in order to limit the corrosion of the steel structures and of the core. A solution to monitor the oxygen content is to use an electrochemical sensor composed of a solid electrolyte. In this field, oxide ceramics with a defective fluorite structure has been extensively investigated. Indeed, MO₂ compounds doped with aliovalent elements generally exhibit high ionic conductivity and are suitable for applications in aggressive media. They should also be compatible with sodium at high temperature (about 400°C) and mechanically resistant. These properties are closely related to their microstructure, thus to their fabrication route. In this frame, hafnium dioxide doped with trivalent elements (i.e. Hf_1-xM^III_xO_2-x/2, with M^III = Y or Gd) has been studied as a potential electrolyte material to be used in liquid sodium, considering its microstructure can be reproduced and controlled with low impurity content, small grain size (< 5 µm) and high density (> 97%dcalc). The starting powder was synthesized by hydroxide precipitation (wet process) and characterized before sintering. First, a multiparametric study was led to specify the operating conditions of the synthesis (aging time, molar ratios of the reactants and doping rate) to fabricate a homogenous powder. Then in order to obtain the desired microstructures, the impact of different sintering parameters such as temperature or dwell time on densification, inter and intra-agglomerate sintering and grain size, were further studied. Materials with high density, very low porosities rate and homogenous composition were finally obtained and will be used as samples for sodium corrosion tests.