Lone-Pair-Driven Vacancy Formation in Lead Niobate

Materials exhibiting tunable polar distortions are desirable in various applications, including sensing, non-volatile memory, energy harvesting, and energy storage. The most well-studied and broadly understood polar materials belong to the perovskite family, but a wide variety of crystal structure types have demonstrated the ability to host polar distortions. A better understanding of polar distortions beyond perovskites is necessary to both broaden the suite of materials available for technological applications and to improve our understanding of the fundamental mechanisms responsible for driving structural phase transitions. One such exotic and understudied materials family is the pyrochlore crystal structure (chemical formula A₂B₂O₆O’), which can host ferroelectricity via a second-order Jahn-Teller active B site, as demonstrated in Cd₂Nb₂O₇. However, replacing Cd with Pb on the A-site of Cd₂Nb₂O₇ in fact destabilizes the otherwise polar ground state. Whereas Pb (and its associated lone pair) is known to help stabilize polar ground states in other materials, such as the perovskite PbTiO₃, this report illustrates how the introduction of Pb to the pyrochlore structure enforces nonstoichiometric crystallization and results in the elimination of long-range polar order.