Magnetic Ordering and Negative Thermal Expansion in PrFeAsO

We report the structure and magnetism of PrOFeAs, one of the parent phases of the newly discovered Fe-As superconductors, as measured by neutron powder diffraction. In common with other REOFeAs materials, a tetragonal-orthorhombic phase transition is found on cooling below 136 K and striped Fe magnetism with $k =$(1,0,1) is detected below $sim$ 85 K. Our magnetic order parameter measurements show that the ordered Fe moment along the a axis reaches a maximum at $sim$ 40 K, below which an anomalous expansion of the c axis sets in, which results in a negative thermal volume expansion of 0.015 % at 2 K. We propose that this effect, which is suppressed in superconducting samples, is driven by a delicate interplay between Fe and Pr ordered moments.

Emergent charge ordering in near half doped Na$_{0.46}$CoO$_{2}$

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$^{+3.5-epsilon}$, $epsilonsim0.06e$ 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