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 ∼ 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, 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.
@article{arxiv.0807.4441,
title = {Magnetic Ordering and Negative Thermal Expansion in PrFeAsO},
author = {S. A. J. Kimber and D. N. Argyriou and F. Yokaichiya and K. Habicht and S. Gerischer and T. Hansen and T. Chatterji and R. Klingeler and C. Hess and G. Behr and A. Kondrat and B. Buechner},
journal= {arXiv preprint arXiv:0807.4441},
year = {2009}
}