On the Structural Transition Driven by Band Nesting in?1111?Pnictides

The lattice critical response in REFeAsO (RE=Pr, Nd and Sm) oxypnictides near the orthorhombic-to-tetrahedral structural phase transition is studied using ultra-high-resolution X-ray diffraction in the temperature range 4 to 250 K. The critical exponent of the structural phase transition ?? is found to decrease with decreasing RE size. The ?? values determined to be 0.212, 0.209 and 0.194, respectively, for PrOFeAs, NdOFeAs and SmOFeAs. Temperature hysteresis is observed for all the REFeAsO, with ??T being of the order of 2 to 3 K. The results are discussed in the frame of Fermi surface nesting induced structural transition in the pnictides.