Frustrated Ferroelectricity in Niobate Pyrochlores

TL;DR

This study reports the discovery of frustrated ferroelectricity in niobate pyrochlores, revealing short-range ordered B-site displacements that create electric dipoles, analogous to magnetic frustration in spin systems.

Contribution

It is the first to observe B-site displacements and electric dipoles in niobate pyrochlores, demonstrating geometric frustration in their dielectric state.

Findings

B-site displacements create electric dipoles

Displacements are short-range ordered, indicating frustration

Charge singlets are likely present in these materials

Abstract

The crystal structures of the A2B2O7-x Niobium-based pyrochlores Y2(Nb0.86Y0.14)2O6.91, CaYNb2O7, and Y2NbTiO7 are reported, determined by powder neutron diffraction. These compounds represent the first observation of B-site displacements in the pyrochlore structure: the B-site ions are found to be displaced from the ideal pyrochlore positions, creating electric dipoles. The orientations of these dipoles are fully analogous to orientations of the magnetic moments in Ising spin based magnetically frustrated pyrochlores. Diffuse scattering in electron diffraction patterns shows that the displacements are only short range ordered, indicative of geometric frustration of the collective dielectric state of the materials. Comparison to the crystal structure of the Nb5+ (d0) pyrochlore La2ScNbO7 supports the prediction that charge singlets, driven by the tendency of Nb to form metal-metal bonds, are present in these pyrochlores. The observed lack of long-range order to these singlets suggests that Nb4+-based pyrochlores represent the dielectric analogy to the geometric frustration of magnetic moments observed in rare earth pyrochlores.