Crystal chemistry of GdScO₃, DyScO₃, SmScO₃ and NdScO₃

The crystal chemistry of GdScO₃ and, for the first time, of DyScO₃, SmScO₃ and NdScO₃ has been investigated using single crystals. The structure of the Czochralski grown crystals was refined from single crystal X-ray diffraction data, their chemical compositions were analysed by inductively coupled plasma optical emission spectrometry (ICP OES). Orthorhombic distorted perovskite structure types with the space group Pnma could be confirmed in all cases. The B-site of the lanthanoid scandates shows with 2.090 to 2.116 Å typical bond lengths for octahedrally coordinated scandium. The distortion of the B-site is rather small. The A-site is occupied by the lanthanoid (Ln) and the Ln—O bond distances vary from 2.233 to 3.722 Å. This indicates its high distortion and makes an assignment of the coordination number difficult. However, an 8-fold coordination for the A-site has to be assumed. The Ln-scandates show a continuous structural evolution with the size of the lanthanoid. A discontinuity within the intermediate members of the Ln-scandates — which had previously been described — was not observed. The ICP OES investigation of these samples results in non-stoichiometric chemical compositions: the chemical analyses show always a depletion in the lanthanoid. Site occupancy refinements based on single crystal diffraction data support the idea of the Ln-depletion on the A-site (inducing O-defects on the O2-position). Several substitution mechanisms are discussed, but the authors favour that vacancies on the A-site coupled with oxygen vacancies cause the Ln-deficiencies. The crystallochemical formula of the investigated Ln-scandates may be written as: (∀_0.056Dy_0.944)ScO_2.916,(∀_0.048Gd_0.952)ScO_2.928, (∀_0.045Sm_0.955)ScO_2.933 and (∀_0.033Nd_0.967)ScO_2.951.