We present here a study on the effect of Al doping on the structure of SmBa2Cu3-xAlxO6+δ (Sm-123) superconductor. Electron MicroProbe Analysis (EMPA) and X-Ray Powder Diffraction (XRPD) have revealed that the limit of aluminium solubility x is between 0.5 and 0.6. For further doping BaAl2O4 appears besides the superconducting phase. XRPD analysis on samples annealed in both oxidising and reducing conditions have revealed that the Al doping inhibits the tetragonal to orthorhombic phase transition. Nuclear Magnetic Resonance (NMR) analysis has shown that almost all the Al ions are coordinated tetrahedrally. The comparison between oxygen non-stoichiometry in pure and Al doped SmBa2Cu3-xAlxO6+δ suggests that the Al ions are ordered in clusters. A model is proposed for short-range order around Al doping ions which allows us to interpret the phase transition inhibition.
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