A general and Eu specific perspective on lattice dynamics in pyrochlore and defect fluorite (Eu_1-xNd_x)₂Zr₂O₇

Lattice dynamics in pyrochlore and defect fluorite type (Eu_1-xNd_x)₂Zr₂O₇ were investigated using nuclear inelastic scattering by the 151Eu Mössbauer resonance for x = 0, 0.5, and 0.75 and using inelastic neutron scattering for x = 1. On the basis of the generalized density of phonon states obtained by inelastic neutron scattering, the vibrational entropy of the disordered defect fluorite Nd₂Zr₂O₇ is found to be 12(3)% smaller than the one of the ordered pyrochlore counterpart. Accordingly, the loss of specific Zr-O coordination in the defect fluorite structure leads to a strong broadening of phonon modes. The Eu sublattice in both structure types was further characterized by thermodynamic quantities calculated using the Eu partial density of phonon states. The Eu framework is shown to be significantly softer than the other sublattices.