Thermal expansion of RFeAsO (R=La,Ce,Pr,Sm,Gd)

Abstract

Abstract. We present measurements of the thermal expansion coefficient α of polycrystalline RFeAsO (R = La,Ce,Pr,Sm,Gd). Anomalies at the magnetic ordering transitions indicate a significant magneto-elastic coupling and a negative pressure dependence of TN. The structural transitions are associated by large anomalies in α. Rare earth magnetic ordering in CeFeAsO, PrFeAsO, and SmFeAsO yields large positive anomalies at low temperatures. Layered Fe2As2-materials have raised enormous attention due to the discovery of superconductivity with transition temperatures TC up to 28 K in LaFeAsO1−xFx. [1] Upon substitution of La by rare earths, TC is increased to above 50 K. [2, 3, 4, 5] Interestingly, evolution of superconductivity is associated to the suppression of a magnetically ordered orthorhombic phase, which has been found in the undoped parent compound. [6] In RFeAsO, both tetragonal distortion and magnetic ordering are observed at intermediate temperatures around ∼ 150 K. A spin density wave (SDW)-type of antiferromagnetic order evolves slightly below the temperature TS of orthorhombic distortion of the tetragonal high temperature phase. [7, 8, 9, 10] Here, we present thermal expansion data of polycrystalline RFeAsO with R = La,Ce,Pr,Sm,Gd. Our measurements yield a very sensitive measure of the volume changes