  Colloidal photonic crystals display peculiar optical properties which make
them particularly suitable for application in different fields. However, the
low packing fraction of the targeted structures usually poses a real challenge
in the fabrication stage. Here, we propose a novel route to colloidal photonic
crystals via a binary mixture of hard tetramers and hard spheres. By combining
theory and computer simulations, we calculate the phase diagram as well as the
stacking diagram of the mixture, and show that a colloidal analogue of the
MgCu2 Laves phase -- which can serve as a precursor of a photonic bandgap
structure -- is a thermodynamically stable phase in a large region of the phase
diagram. Our findings show a relatively large coexistence region between the
fluid and the Laves phase, which is potentially accessible by experiments.
Furthermore, we determine the sedimentation behaviour of the suggested mixture,
by identifying several stacking sequences. Our work uncovers a new
self-assembly path towards a photonic structure with a band gap in the visible
region.
