Average and Local Crystal Structure of β-Er:Yb:NaYF4 Upconverting Nanocrystals Probed by X-ray Total Scattering

The average and local structures of similar to 22 nm beta-Er:Yb:NaYF4 upconverting nanocrystals were probed using a dual-space approach combining Rietveld and pair distribution function analysis of X-ray total scattering. Comparison of the fits provided by the structural models derived from P (6) over bar 2m, P (6) over bar, and P6(3)/m space groups demonstrates that the latter yields a crystallochemically meaningful description of the nanocrystals' average and local structures. This result is in line with those previously reported for bulk beta-Na-3x-RE2-xF6 (x similar to 0.45; RE = Y, Er, Tm, Yb) using powder X-ray diffraction,(1) and for beta-NaREF4 (RE = Y, Lu) nanocrystals using solid-state NMR;(2) however, it differs from those reported in studies of beta-NaREF4 (RE = La, Gd, Er) single crystals, which favored the structural model derived from the P (6) over bar space group.(3) The proposed structural model features sodium cations distributed in four translation-equivalent chains, each shifted relative to the others along the c axis. Interestingly, the structural model derived for-the mineral gagarinite (NaCaREF6, P6(3)/m), in which sodium cations are distributed in eight chains, also provides an adequate fit to the X-ray scattering data. The potential implications of this finding are discussed from the perspective of the size dependence of the crystal structure of beta-NaREF4.