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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.
A neptunium analogue of the LaFeAsO tetragonal layered compound has been synthesized and characterized by a variety of experimental techniques. The occurrence of long-range magnetic order below a critical temperature T_N = 57 K is suggested by anomal
The fluorine-doped rare-earth iron oxypnictide series SmFeAsO$_{1-x}$F$_x$ (0 $leq x leq$ 0.10) was investigated with high resolution powder x-ray scattering. In agreement with previous studies, the parent compound SmFeAsO exhibits a tetragonal-to-or
We use powder neutron diffraction to study the spin and lattice structures of polycrystalline samples of nonsuperconducting PrFeAsO and superconducting PrFeAsO0.85F0.15 and PrFeAsO0.85. We find that PrFeAsO exhibits an abrupt structural phase transit
Neutron powder diffraction has been used to investigate the structural and magnetic behavior of the isoelectronically doped Fe pnictide material PrFe1-xRuxAsO. Substitution of Ru for Fe suppresses the structural and magnetic phase transitions that oc
The thermal expansion at constant pressure of solid CD$_4$ III is calculated for the low temperature region where only the rotational tunneling modes are essential and the effect of phonons and librons can be neglected. It is found that in mK region