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[en] This work gives insight in some key aspects for the understanding of the origin of hightemperature superconductivity in the newly discovered class of iron-based materials. In particular, thermodynamic methods, such as SQUID magnetometry, specific heat and dilatometry were used, in order to (i) assess the evolution of electronic correlations in a series of transition metal substitutions of the well-known BaFeAs as a function of 3d band filling and (ii) to re-investigate the phase diagram of Co-doped LaFeAsO on single crystals, with particular interest in the interplay between the nematic/magnetic phase of the parent compound and superconductivity induced by in-plane electron doping. In the first part of this work, the Sommerfeld coefficient (γ) was extracted from the low temperature specific heat data and compared with the theoretical values obtained by band theory calculations (γ), in order to obtain the mass enhancement (m/m) in the series BaTAs (T = Cr, Mn, Fe, Co, Ni, Cu). The results clearly show an overall decrease of the electronic correlations while departing from the half-filled (3d) to the fully filled configuration (3d), thus suggesting a highly correlated 3d state. The evolution of electronic correlations as a function of 3d band filling for n > 5 is in agreement with previous theoretical calculations, underlining the importance of Hund’s coupling (J) in describing the normal-state properties of iron-based superconductors. In addition, it was found that the decrease in m/m for n > 5 follows an increase of the crystal field splitting (Δ), determined by the progressive distortion of the As-T-As angle (α) from the ideal tetrahedral environment. This study reveals a complex interplay between electronic correlations, band filling and crystal structure in determining the physical properties of 122 systems. In the second part, the phase diagram of Co-doped LaFeAsO was re-investigated using single crystals by thermodynamic methods. From magnetic susceptibility studies we track the doping evolution of the antiferromagnetic phase, revealing a continuous decrease of T up to 5% Co doping. In order to study the evolution of the so-called nematic phase, the temperature dependence of the length changes along the a and b orthorhombic directions, ΔL/L, was determined by high-resolution capacitance dilatometry. The results clearly show a gradual reduction of the orthorhombic distortion δ and of T with increasing Co content up to 4.5%, while it is completely suppressed for 7.5% Co. Bulk superconductivity with T = 10.5 K was found in a small doping region around 6% Co content, while both T and the superconducting volume fraction rapidly drop in the neighbouring doping regime. Ultimately, no microscopic coexistence between the superconducting and magnetic phases can be assessed within our resolution limit, in sharp contrast with other iron-pnictide families, e.g., electron- and hole-doped BaFeAs.