Beyond Simple Crystal Systems: Identifying Twinning in Body-Centered Tetragonal Nanoparticles

While many elements crystallize in body-centered cubic, hexagonal close packed, or face-centered cubic structures, In and beta-Sn adopt a body-centered tetragonal unit cell. Both elements are gaining popularity as nanoparticles (NPs), with different single crystal and twinned NPs reported. Here, we present a systematic study of the crystal shapes and approaches required to identify the presence of twinning in In and beta-Sn, with applicability to other simple body-centered tetragonal crystals such as compounds and superlattices. Well-faceted crystals could reveal twinning based on shape, but often this is obscured by corner rounding, in which case diffraction or lattice imaging is required. We describe the limited set of low-index orientations that provide either electron diffraction patterns or atomic resolution images capable of distinguishing between single and twinned crystals and find that twins in beta-Sn are easier to identify owing to the additional atoms in the unit cell.