  Polycrystalline samples of LaRuAsO, NdRuAsO, SmRuAsO, and GdRuAsO have been
synthesized and studied using powder x-ray diffraction, electrical transport,
magnetization, and heat capacity measurements. Variations in structural
properties across the series reveal a trend toward more ideal tetrahedral
coordination around Ru as the size of the rare earth element is reduced. The
lattice parameters of these Ru compounds show a more anisotropic response to
variation in Ln than their Fe analogues, and significant anisotropy in thermal
expansion is also observed. Transport measurements show metallic behavior, and
carrier concentrations near 10^21 - 10^22 electrons per cm^3 are inferred from
simple analysis of Hall effect measurements. Anomalies in resistivity,
magnetization, and heat capacity indicate antiferromagnetic ordering of rare
earth moments at 5 K for GdRuAsO, 4.5 K for SmRuAsO, and <2 K for NdRuAsO.
Magnetization measurements on LaRuAsO show no evidence of a magnetic moment on
Ru. Observed behaviors are compared to those reported for similar Fe and Ru
compounds.
