Short-range order in heavily doped CaF2:Ln3+ fluorites: A powder neutron diffraction study
Abstract The Ca0.68Ln0.32F2.32 series (Ln=La, Nd, Tb, Ho, Er, Yb, Lu) of anion-excess fluorite-related disordered phases has been studied by a time-of-flight powder neutron diffraction technique. All the structure refinements taking into account the information provided by Fourier-difference maps analysis, clearly indicate a steady evolution of the defect structure. With increasing dopant cation size, F' interstitials ( 1 2 , u, u; u≅0.37) prominent for low-size cations (Lu, Yb) are progressively replaced by F″ ones (v, v, v; v≅0.41) and relaxed normal fluorine F″″ atoms (w, w, w; w≅0.31). These results can be explained by the presence for high-size dopant cations (La→Tb) of small 1 : 0 : 3 cluster involving two ninefold coordinated Ln3+ cations, and for low- and medium-size dopant cations (Ho→Lu) of a mixture of small 1 : 0 : 3 clsters and large cuboctahedral 8 : 12 : 1 ones, the number of the latter increasing with the decrease of the dopant cation radius.