[en] An amorphous thin film of a lanthanide aluminium alloy is prepared by thermal evaporation under vacuum of a mixture of the lanthanide and the aluminium metals. The film heated inside the electron microscope with the electron beam yields large crystals of a definite compound of the LnAl₃ type (Ln = Gd, Dy, Tm). The crystals exhibit either pole figures or striations, depending on their crystallographic orientation. For the three lanthanides studied, the LnAl₃ compounds have cubic 3C-type structure. But DyAl₃ and TmAl₃ crystals can have structures which correspond to classical polytypic families built on packing of hexagonal layers with the LnAl₃ chemical composition. They were identified through their electron diffraction patterns. The elongated and striated crystals have diffraction patterns made of numerous aligned spots, The central row shows two intense spots, indexed (00.1), symmetric with respect to the central spot. They correspond to the basic hexagonal layers LnAl₃ whose distances (2.385 A) are the same for all the polytypes. Between these intense spots there are several others. Their distance gives the c parameters which correspond to the crystallographic order for the stacking of n LnAl₃ layers. In many cases the diffraction patterns are made of a large number of closely spaced streaks elongated in the row direction; these can form a continuous row and indexing is then impossible. However, high-resolution lattice images can give the crystal structure. The images show that the crystal is then made of thin lamellae of several different polytypes stacked syntaxically with respect to the c axis. The number and the thickness of the lamellae explain the streaking of the spots. (author)