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We report the temperature dependent x-ray powder diffraction of thequaternary compound NdOFeAs (also called NdFeAsO) in the range between 300 Kand 95 K. We have detected the structural phase transition from the tetragonalphase, with P4/nmm space group, to the orthorhombic or monoclinic phase, withCmma or P112/a1 (or P2/c) space group, over a broad temperature range from 150K to 120 K, centered at T0 ~137 K. Therefore the temperature of this structuralphase transition is strongly reduced, by about ~30K, by increasing the internalchemical pressure going from LaOFeAs to NdOFeAs. In contrast thesuperconducting critical temperature increases from 27 K to 51 K going fromLaOFeAs to NdOFeAs doped samples. This result shows that the normal stripedorthorhombic Cmma phase competes with the superconducting tetragonal phase.Therefore by controlling the internal chemical pressure in new materials itshould be possible to push toward zero the critical temperature T0 of thestructural phase transition, giving the striped phase, in order to getsuperconductors with higher Tc.

The effect of internal pressure on the tetragonal to monoclinic structural phase transition in ReOFeAs: the case of NdOFeAs