Studies on phase transition temperature of rare earth niobates Ln 3 NbO 7 ( Ln  = Pr, Sm, Eu) with orthorhombic fluorite-related structure

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Abstract The phase transition of ternary rare earth niobates Ln 3 NbO 7 ( Ln  = Pr, Sm, Eu) was investigated by the measurements of high-temperature and low-temperature X-ray diffraction, differential scanning calorimetry (DSC) and differential thermal analysis (DTA). These compounds crystallize in an orthorhombic superstructure derived from the structure of cubic fluorite (space group Pnma for Ln  = Pr; C 222 1 for Ln  = Sm, Eu). Sm 3 NbO 7 undergoes the phase transition when the temperature is increased through ca. 1080 K and above the transition temperature, its structure is well described with space group Pnma . For Eu 3 NbO 7 , the phase transition was not observed up to 1273 K Pr 3 NbO 7 indicates the phase transition when the temperature is increased through ca. 370 K. The change of the phase transition temperature against the Ln ionic radius for Ln 3 NbO 7 is quite different from those for Ln 3 MO 7 (M = Mo, Ru, Re, Os, or Ir), i.e., no systematic relationship between the phase transition temperature and the Ln ionic radius has been observed for Ln 3 NbO 7 compounds.

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