Stuffed rare earth pyrochlore solid solutions

Synthesis and crystal structures are described for the compounds Ln₂(Ti_2-xLn_x)O_7-x/2, where Ln=Tb, Dy, Ho, Er, Tm, Yb, Lu, and x ranges from 0 to 0.67. Rietveld refinements of X-ray powder diffraction data indicate that in the Tb and Dy titanate pyrochlores, the extra Ln³⁺ cations mix mainly on the Ti⁴⁺ site with little disorder on the original Ln³⁺ site. For the smaller rare earths (Ho-Lu), stuffing additional lanthanide ions results in a pyrochlore to defect fluorite transition, where the Ln³⁺ and Ti⁴⁺ ions become completely randomized at the maximum (x=0.67). Initial magnetic characterization for the fully stuffed x=0.67 samples for Ln=Tb-Yb shows no long range ordering down to 2 K, and only partial saturation of the full expected magnetic moment under applied fields up to 5 T. In all of these Ln-Ti-O pyrochlores, the addition of magnetic Ln³⁺ in place of non-magnetic Ti⁴⁺ adds edge sharing tetrahedral spin interactions to a normally corner sharing tetrahedral network of spins. The increase in spin connectivity in this family of solid solutions represents a new avenue for investigating geometrical magnetic frustration in the rare earth titanate pyrochlores.