Verwey transition under oxygen loading in RBaFe2O5+w (R = Nd and Sm)

Double-cell perovskites RBaFe2O5+W, (R = Nd and Sm) are synthesized in the -0.03 < w < 0.47 portion of the nonstoichiometry range. The ideal composition with w = 0 has di- and trivalent iron in equal proportions and exhibits a charge-ordering Verwey transition upon cooling, preceded by a weaker premonitory transition. Both transitions are detectable by differential scanning calorimetry. The changes in entropy, volume, orthorhombic distortion and electrical conductivity at the main transition are larger for the variant with the smaller Sm atom than for R = Nd. The discontinuity of the SmBaFe2O5+W,, transition also persists into much higher levels of the oxygen nonstoichiometry w than for the R = Nd variant. Whereas 3D-charge-ordered SmBaFe2O5.00 is isostructural with TbBaFe2O5.00 adopting space group Pmma, NdBaFe2O5.00 undergoes a more profound structural change upon entering the charge-ordered state, the structure of which is refined in the P2(1)ma symmetry from high-resolution X-ray synchrotron diffraction data. (C) 2002 Elsevier Science (USA).