  Depending on the volume fraction and interparticle interactions, colloidal
suspensions can form different phases, ranging from fluids, crystals, and
glasses to gels. For soft microgels that are made from thermoresponsive
polymers, the volume fraction can be tuned by temperature, making them
excellent systems to experimentally study phase transitions in dense colloidal
suspensions. However, investigations of phase transitions at high particle
concentration and across the volume phase transition temperature in particular,
are challenging due to the deformability and possibility for interpenetration
between microgels. Here, we investigate the dense phases of composite
core-shell microgels that have a small gold core and a thermoresponsive
microgel shell. Employing Ultra Small Angle X-ray Scattering, we make use of
the strong scattering signal from the gold cores with respect to the almost
negligible signal from the shells. By changing the temperature we study the
freezing and melting transitions of the system in-situ. Using Bragg peak
analysis and the Williamson-Hall method, we characterize the phase transitions
in detail. We show that the system crystallizes into an rhcp structure with
different degrees of in-plane and out-of-plane stacking disorder that increase
upon particle swelling. We further find that the melting process is distinctly
different, where the system separates into two different crystal phases with
different melting temperatures and interparticle interactions.
