Magnetic ordering and negative thermal expansion in PrFeAsO

We report the structure and magnetism of PrFeAsO, one of the parent phases of the Fe-As family of superconductors, as measured by neutron powder diffraction. In common with other REFeAsO 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 ∼85 K. Our magnetic order-parameter measurements show that the ordered Fe moment along the a axis reaches a maximum at ∼40 K, below which an anomalous expansion of the c axis sets in that 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.