Covalency-Driven Structural Evolution in the Polar Pyrochlore Series Cd2Nb2O7-xSx

The arrangement of cations on the triangular pyrochlore lattice leads to a wealth of interesting physical phenomena influenced by geometric frustration. Although uncommon, several pyrochlore materials overcome this frustration and exhibit polar structures. Unraveling the origin of such behavior is key to understanding how broken inversion symmetry arises in complex crystal structures. Here we investigate the effect of varying degrees of covalency in the pyrochlore lattice through a detailed structural and lattice dynamical analysis of the pyrochlore oxysulfide series Cd2Nb2O7-xSx above and below the ferroelectric transition temperatures (Tc) using synchrotron x-ray diffraction (SXRD) and first principles calculations. All compositions exhibit the cubic Fd-3m pyrochlore aristotype above Tc, whereas the amplitude and character of various structural distortions are found to be composition dependent below Tc. For x = 0, large Cd and Nb cation displacements occur to produce the polar Ima2 structure accompanied...