Structural phase transition and superlattice misfit strain of RFeAsO (R=La,╇Pr,╇Nd,╇Sm)

The tetragonal-to-orthorhombic structural phase transition (SPT) in LaFeAsO (La-1111) and SmFeAsO (Sm-1111) single crystals measured by high-resolution x-ray diffraction is found to be sharp while the RFeAsO (R=La, Nd, Pr, Sm) polycrystalline samples show a broad continuous SPT. Comparing the polycrystalline and the single-crystal 1111 samples, the critical exponents of the SPT are found to be the same while the correlation length critical exponents are found to be very different. These results imply that the lattice fluctuations in 1111 systems change in samples with different surface to volume ratio that is assigned to the relieve of the temperature-dependent superlattice misfit strain between active iron layers and the spacer layers in 1111 systems. This phenomenon that is missing in the AFe2As2 (A=Ca, Sr, Ba) “122” systems, with the same electronic structure but different for the thickness and the elastic constant of the spacer layers, is related with the different maximum superconducting transition temperature in the 1111 (55 K) versus 122 (35 K) systems and implies the surface reconstruction in 1111 single crystals. Disciplines Condensed Matter Physics | Materials Science and Engineering Comments This article is from Physical Review B 82 (2010): 144507, doi:10.1103/PhysRevB.82.144507. Authors A. Ricci, N. Poccia, B. Joseph, L. Barba, G. Arrighetti, G. Clasca, J.-Q. Yan, R. William McCallum, Thomas A. Lograsso, N. D. Zhigadlo, J. Karpinsky, and A. Bianconi This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/ameslab_pubs/48 Structural phase transition and superlattice misfit strain of RFeAsO (R=La, Pr, Nd, Sm) A. Ricci,1 N. Poccia,1 B. Joseph,1 L. Barba,2 G. Arrighetti,2 G. Ciasca,1 J.-Q. Yan,3 R. W. McCallum,3 T. A. Lograsso,3 N. D. Zhigadlo,4 J. Karpinski,4 and A. Bianconi1,* 1Dipartimento di Fisica, Università di Roma “La Sapienza,” P. le Aldo Moro 2, 00185 Roma, Italy 2Institute of Crystallography, National Council of Research, Elettra, 34012 Trieste, Italy 3Divison of Materials Science and Engineering, Ames Laboratory, US-DOE, Ames, Iowa 50011, USA 4Laboratory of Solid State Physics, ETH Zurich, 8093 Zurich, Switzerland Received 5 June 2010; revised manuscript received 18 September 2010; published 11 October 2010 The tetragonal-to-orthorhombic structural phase transition SPT in LaFeAsO La-1111 and SmFeAsO Sm-1111 single crystals measured by high-resolution x-ray diffraction is found to be sharp while the RFeAsO R=La, Nd, Pr, Sm polycrystalline samples show a broad continuous SPT. Comparing the polycrystalline and the single-crystal 1111 samples, the critical exponents of the SPT are found to be the same while the correlation length critical exponents are found to be very different. These results imply that the lattice fluctuations in 1111 systems change in samples with different surface to volume ratio that is assigned to the relieve of the temperature-dependent superlattice misfit strain between active iron layers and the spacer layers in 1111 systems. This phenomenon that is missing in the AFe2As2 A=Ca, Sr, Ba “122” systems, with the same electronic structure but different for the thickness and the elastic constant of the spacer layers, is related with the different maximum superconducting transition temperature in the 1111 55 K versus 122 35 K systems and implies the surface reconstruction in 1111 single crystals. DOI: 10.1103/PhysRevB.82.144507 PACS number s : 74.70.Xa, 62.20. x, 61.05.cp, 61.66.Dk