Crystallographic and Magnetic Phase Transitions in the Layered Ruthenium Oxyarsenides TbRuAsO and DyRuAsO Michael A. McGuire,* Andrew F. May, and Brian C. Sales Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States * S Supporting Information ABSTRACT: 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. ■ INTRODUCTION Interest in superconductivity has motivated intense study of layered iron-pnictides and chalcogenides since the report of transition temperatures as high as 26 K in fluorine-doped LaFeAsO in 2008. 1 Subsequent research demonstrated not only superconductivity in many structurally related families of materials, but also interesting crystallographic and magnetic properties and phase transitions (for reviews, see refs 2-6). Superconductivity is intimately linked to magnetism in these compounds, and strong coupling between the crystal structure and the magnetism is present as well. Thus, studies of crystal structures and their relationships to physical properties are key in developing a better understanding of these interesting and potentially technologically useful materials. Crystallographic information is particularly important for Fe- based superconductors and related compounds because, in many cases, structural phase transitions are associated with ordering of iron magnetic moments in nonsuperconducting (and some superconducting) compositions. These include ZrCuSiAs-type LnFeAsO (Ln = La-Tb) and AeFeAsF (Ae = Ca, Sr), ThCr 2 Si 2 -type AeFe 2 As 2 (Ae = Ca, Sr, Ba), PbO- or Cu2Sb-type Fe 1+x Q (Q = S, Se, Te), and the ThCr 2 Si 2 derivatives A 1-y Fe 1-x Q 2 (A = K, Rb, Cs, Tl; Q = S, Se, Te). All of these materials are tetragonal at high temperature, and the Fe atoms form a square net in the ab-plane. The Fe net is capped above and below by pnictogens or chalcogens, giving edge-sharing tetrahedral coordination to the Fe sites. The crystallographic phase transitions distort the chemical environ- ment around the Fe atoms in various ways. In LnFeAsO and AeFeAsF, the symmetry is lowered from P4/nmm to Cmme upon cooling. 7-12 The resulting distortion can be visualized as a stretching of the square net of Fe into a rectangular net. A similar distortion occurs in AeFe 2 As 2 which transforms from space group I4/mmm to Fmmm. 13,14 Temperature, pressure, or chemical doping can induce a second type of crystallographic transition in these materials as well, resulting in a collapsed- tetragonal state with a reduced c lattice constant but retaining the parent tetragonal symmetry. 15 Three types of structural transitions occur in Fe 1+x Q. For Q = Se, the distortion is to an orthorhombic structure (Cmme) with a rectangular Fe net as in LnFeAsO and AeFe 2 As 2 . 16 For Q = Te and a small amount of interstitial iron (x ∼ 0.08) the low temperature structure is monoclinic (P2 1 /m), while an orthorhombic (Pmmn) structure is observed at low temperature for larger values of x. 17,18 The orthorhombic distortion in Fe 1+x Te is different from that previously described for the arsenide materials. In this case the Fe net is stretched across the diagonal of the Fe squares. The ternary chalcogenides A 1-y Fe 1-x Q 2 have disordered A and Fe vacancies at high temperatures (>400 K). At lower temper- atures the Fe vacancies are ordered at least to some degree resulting in complex superstructures containing square plaquettes of Fe. 19-22 It is interesting to note that many similar crystallographic distortions can be induced in rare-earth copper pnictides by chemical substitution. 23,24 Importantly, each of the temperature induced crystallo- graphic distortions described above is accompanied by an Fe magnetic ordering transition. A particulary clear example of the interplay between structure, magnetism, and superconductivity can be found in Ba(Fe 1-x Co x ) 2 As 2 . For certain compositions, Received: May 22, 2012 Published: July 26, 2012 Article pubs.acs.org/IC © 2012 American Chemical Society 8502 dx.doi.org/10.1021/ic3010695 | Inorg. Chem. 2012, 51, 8502-8508