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 and first principles calculations. All compositions exhibit the cubic Fd3 m 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 by a change in translational symmetry. Our symmetry and lattice dynamical calculations indicate that Cd2Nb2O7 undergoes a proper ferroelectric transition through TC. Analysis of the sulfur-substituted niobates indicates that although the polar space group Fdd2 is adopted by the nominal x = 0.25 sample, the transition into the polar phase is improper. For the nominally x = 0.7 composition, the lattice remains nearly cubic, but exhibits a high degree of structural disorder in the pyrochlore channel, with a deviation from the linear Cd-X′-Cd bond by nearly 15° to accommodate the large size of S while preventing extreme stretching of the Nb-O bond. This highly distorted Cd-X′ network is accompanied by a highly distorted NbO6 network, which is accommodated by the polarizable NbO6 coordination environment. This sheds light on the limited existence of oxysulfide pyrochlores; for example, the lack of reported S substitution in the case of the similar yet less-polarizable Cd2Ta2O7. Our work provides a new understanding of how inversion-symmetry lifting displacements arise and how anion substitution, which tunes covalent cation-anion interactions, is a useful strategy for manipulating polar behavior in a pyrochlore lattice