Crystallographic and Magnetic Phase Transitions in the Layered Ruthenium Oxyarsenides TbRuAsO and DyRuAsO

The crystallographic and physical properties of TbRuAsO
and DyRuAsO
at and below room temperature are reported, including full structure
refinements from powder X-ray diffraction data and measured electrical
and thermal transport properties, magnetic susceptibility, and heat
capacity. Both compounds are isostructural to LaFeAsO (ZrCuSiAs-type, P4/nmm) at room temperature. However, DyRuAsO
undergoes a symmetry-lowering crystallographic phase transition near
25 K, and adopts an orthorhombic structure (Pmmn)
below this temperature. This structural distortion is unlike those
observed in the analogous Fe compounds. Magnetic phase transitions
are observed in both compounds which suggest antiferromagnetic ordering
of lanthanide moments occurs near 7.0 K in TbRuAsO and 10.5 K in DyRuAsO.
The nature of the structural distortion as well as thermal conductivity
and heat capacity behaviors indicate strong coupling between the magnetism
and the lattice. The behaviors of both materials show magnetic ordering
of small moments on Ru may occur at low temperatures.