Tetragonal to orthorhombic phase transition in SmFeAsO: a synchrotron   powder diffraction investigation
  The crystal structure of SmFeAsO has been investigated by means of Rietveld refinement of high resolution synchrotron powder diffraction data collected at 300 K and 100 K. The compound crystallizes in the tetragonal P4/nmm space group at 300 K and in the orthorhombic Cmma space group at 100 K; attempts to refine the low temperature data in the monoclinic P112/n space group diverged. On the basis of both resistive and magnetic analyses the tetragonal to orthorhombic phase transition can be located at T about 140 K. 
A. Martinelli, A. Palenzona, C. Ferdeghini, M. Putti, E. Emerich