Coincidence site lattice (CSL) modelling has been applied to determine the orientations and atomic structures of first order twin boundaries in the chalcopyrite structure. A rotation of 250°32′ about [110] generated a CSL similar to that for the Σ = 3 sphalerite boundary, but with one in two cation columns being metal-metal anti-sites. The models predict one coherent boundary type i.e. (112)M - (112 )T, and four lateral types, (110 )M -(118 )T, (1110 )M -(112 )T, (114)M -(114)T, (001)M -(111)T. Un-relaxed atomic models for the boundaries have been constructed. The coherent boundary is shown to differ from that in sphalerite by the inclusion of one in two Se columns having alternate atoms with either three Cu and one In nearest neighbours or vice versa. All of the [110] tilt boundaries in chalcopyrite are expected to be electrically active. A tentative list of the boundaries ranked by energy, electrical and chemical activity is given, along with comments on the influence of preferred orientation in polycrystalline films in the context of solar cells.
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