We report on the formation by reactive diffusion of a pure decagonal quasicrystalline film in Al/Co bilayer and multiplayer structures of overall ocmposijtion Al_13Co_4.We show that this mehtod is highly selective since several stable phases which exist around this ocmposition are never observed.the quasicrystal is not the first reaction product but is obtained as a second reaction step by a peritectoid reaction between Al_9Co_2 and the remaining cobalt.DSC analysis allows the determination of both the enthalpy of formaitn of the decagonal quasicrystal (#DELTA#H_f=-31500+-2000 J/g-atom) and its activation energy of growth (2.6 +- 0.5 eV).
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