This study highlights the possibility to control materials' chemistry simultaneously at the atomic and micrometric scales and to design functional oxide-based multimaterials, thanks to the specificity of Spark Plasma Sintering (SPS). We have used a dual synthesis route, namely chemical and composite, combined with SPS to adjust the oxidation state of intrinsic and dopant ions, the grain boundaries state, and to control interdiffusion in composites made of Mn-doped Ba0.6Sr0.4TiO3 (BST) and MgO. At the atomic level, the Mn substituent valence state can be fixed according to the sintering process: Electron Paramagnetic Resonance evidenced , , and -VO charged defects. We established a link between the nature of the charged defects and the high-frequency dielectric losses. At the microscopic level, control of the grain size, the grain boundaries, and the interdiffusion between the components is achievable using SPS. The composite effect acts at low frequency by efficiently decreasing the extrinsic losses arising from the grain boundaries contribution and by increasing the thermal stability of the permittivity. Such an approach based on SPS, chemical and composite routes, has allowed designing BST-based composites operating in a wide frequency range (kHz–GHz) with low dielectric losses and high electric field tunability.