151Eu-Mössbauer spectroscopic and X-ray diffraction study of the Eu 2(Ce 1− xZr x) 2O 7 and LnEuZr 2O 7 (Ln=lanthanide) systems

Abstract

We have performed a systematic 151Eu-Mössbauer and powder X-ray diffraction (XRD) study on two types of zirconia-based defect-fluorite-type solid solutions (a) Eu 2(Ce 1− x Zr x ) 2O 7 (0≤ x≤1.0) and (b) LnEuZr 2O 7 (Ln=La, Nd, Sm, Gd, Dy, Ho, Y, Er, Tm) at ideal-pyrochlore composition (A 3+ 2B 4+ 2O 7). The XRD results show that with increasing ratio of average A, B cations' radii ( r A/ r B), the crystal structure of both of these two systems changes from the disordered defect-fluorite (DF) type to the ordered pyrochlore (P) type at around r A/ r B=1.435 ( x=0.85) for (a) and 1.46 (between Gd and Dy) for (b). The Eu 3+ Mössbauer parameters of these two systems, isomer shift (IS) and quadrupole splitting (QS), have been found to exhibit a clear tendency that with increasing r A/ r B both Mössbauer parameters decrease first gradually in the DF-phase region and then much more steeply in the P-phase region. The more steep change of both the IS and QS in the P-phase region coincides with the XRD-obtained crystallographic data that both the average Eu–O bond length and the distortion of oxygen coordinated polyhedron around Eu 3+ increase with increase in r A/ r B.