151Eu-Mössbauer spectroscopic and X-ray diffraction study of the Eu2(Ce1−xZrx)2O7 and LnEuZr2O7 (Ln=lanthanide) systems

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) Eu2(Ce1−xZrx)2O7 (0≤x≤1.0) and (b) LnEuZr2O7 (Ln=La, Nd, Sm, Gd, Dy, Ho, Y, Er, Tm) at ideal-pyrochlore composition (A3+2B4+2O7). The XRD results show that with increasing ratio of average A, B cations' radii (rA/rB), 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 rA/rB=1.435 (x=0.85) for (a) and 1.46 (between Gd and Dy) for (b). The Eu3+ Mössbauer parameters of these two systems, isomer shift (IS) and quadrupole splitting (QS), have been found to exhibit a clear tendency that with increasing rA/rB 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 Eu3+ increase with increase in rA/rB.