Crystal structure of monoclinic samarium and cubic europium sesquioxides and bound coherent neutron scattering lengths of the isotopes 154Sm and 153Eu

Abstract

Abstract The crystal structures of monoclinic samarium and cubic europium sesquioxide, Sm2O3 and Eu2O3, were reinvestigated by powder diffraction methods (laboratory X-ray, synchrotron, neutron). Rietveld analysis yields more precise structural parameters than previously known, especially for oxygen atoms. Interatomic distances d(Sm–O) in Sm2O3 range from 226.3(4) to 275.9(2) pm [average 241.6(3) pm] for the monoclinic B type Sm2O3 [space group C2/m, a = 1418.04(3) pm, b = 362.660(7) pm, c = 885.48(2) pm, β = 100.028(1)°], d(Eu–O) in Eu2O3 from 229.9(2) to 238.8(2) pm for the cubic bixbyite (C) type [space group Ia3̅, a = 1086.87(1) pm]. Neutron diffraction at 50 K and 2 K did not show any sign for magnetic ordering in Sm2O3. Isotopically enriched 154Sm2O3 and 153Eu2O3 were used for the neutron diffraction work because of the enormous absorption cross section of the natural isotopic mixtures for thermal neutrons. The isotopic purity was determined by inductively coupled plasma – mass spectrometry to be 98.9% for 154Sm and 99.8% for 153Eu. Advanced analysis of the neutron diffraction data suggest that the bound coherent scattering lengths of 154Sm and 153Eu need to be revised. We tentatively propose b c(154Sm) = 8.97(6) fm and b c(153Eu) = 8.85(3) fm for a neutron wavelength of 186.6 pm to be better values for these isotopes, showing up to 8% deviation from accepted literature values. It is shown that inaccurate scattering lengths may result in severe problems in crystal structure refinements causing erroneous structural details such as occupation parameters, which might be critically linked to physical properties like superconductivity in multinary oxides.