CeCoAl was synthesized by melting of theelementsin a sealed niobium tube in an inductionfurnace.Annealing of the sample gave access to a singlephase sample. Its structure was refined on the basis of single-crystal X-ray diffractometer data at different temperatures.Above 271 K CeCoAl crystallizes in its own structuretype in the space group C2/m [a = 1107.4(2), b = 440.6(1)and c = 479.6(1) pm, β = 104.6(1)°]. Data obtained at 300 Klead to 511 F 2 values with 20 variables and a residual of[I ≥ 3σ(I)] wR = 0.0539. Below 271 K satellites give rise tothe superspace group C2/m(α0γ)00; α = 2/3, γ = 2/5 with atemperature independent q-vector. For the 90 K data (alsofor 180 and 220 K) the commensurate modulated structurecould be refined with 4817 F 2 values, 129 variables andresiduals of wR = 0.0347 (main), wR = 0.1927 (satellites 1storder), wR = 0.1541 (satellites 2nd order) and wR = 0.1768(satellites 3rd order) [a = 1107.5(1), b = 440.3(1) andc = 479.0(1) pm, β = 104.7(1)°]. For the three temperaturesonly minor variations of the modulation amplitudes areobserved. The relation of the low temperature (3+1)D 3a ×5c approximant and the room temperature 3D structureis discussed on the basis of a group–subgroup relation.By investigation of the heat capacity, the phase transitioncould be identified as a second order one with a transitiontemperature of 271 K. Magnetic measurements clearlyprove the intermediate cerium valence which is in linewith the short Ce–Co distances.< Réduire