  We have studied the effects of the isoelectronic substitution of Ru for Fe in
polycrystalline samples of the spin density wave (SDW) material PrFeAsO.
Crystal structures from powder x-ray diffraction at room temperature and
transport properties from 2-300 K are reported. The SDW is completely
suppressed upon Ru substitution. The distortion of the tetrahedral coordination
environment of the transition metal site increases as the Ru concentration
increases, which may be related to the absence of superconductivity above 2 K.
Band structure calculations show that the larger size of Ru 4$d$ orbitals is
primarily responsible for the suppression of magnetism as Ru is substituted
into the Fe layer. The experimental results indicate that long-ranged magnetic
order of Pr moments is suppressed at Ru concentrations as low as 10%.
