  We have utilized neutron powder diffraction to probe the crystal structure of
layered Na$_{x}$CoO$_{2}$ near the half doping composition of $x=$0.46 over the
temperature range of 2 to 600K. Our measurements show evidence of a dynamic
transition in the motion of Na-ions at 300K which coincides with the onset of a
near zero thermal expansion in the in-plane lattice constants. The effect of
the Na-ordering on the CoO$_{2}$ layer is reflected in the octahedral
distortion of the two crystallographically inequivalent Co-sites and is evident
even at high temperatures. We find evidence of a weak charge separation into
stripes of Co$^{+3.5+\epsilon}$ and Co$^{+3.5-\epsilon}$, $\epsilon\sim0.06e$
below \Tco=150K. We argue that changes in the Na(1)-O bond lengths observed at
the magnetic transition at \tm=88K reflect changes in the electronic state of
the CoO$_{2}$ layer

Strongly Correlated Electrons;Superconductivity