  The sodium reordering in NaxCoO2 in the vicinity of room temperature is
rationalized at high x in terms of phase transitions between square and striped
phases. A striking hexagon-of-hexagons diffraction pattern observed for x=0.78
can be reproduced using coexisting square and striped phases that are related
by simple shear deformations. All compositions exhibit a partial melting
transition to a disordered stripe phase just below room temperature, which
alters the topology of the electrical conduction pathways.
