  Between 19 and 54 GPa, potassium has a complex composite incommensurate
host-guest structure which undergoes two intraphase transitions over this
pressure range. The temperature dependence of these host-guest phases is
further complicated by the onset of an order-disorder transition in their guest
chains. Here, we report single crystal, quasi-single crystal, and powder
synchrotron X-ray diffraction measurements of this order-disorder phenomenon in
incommensurate potassium to 47 GPa and 750 K. The so-called chain "melting"
transition is clearly visible over a 22 GPa pressure range, and there are
significant changes in the slope of the phase boundary which divides the
ordered and disordered phases, one of which results from the intraphase
transitions in the guest structure.
