Emerging scenario on displacive cubic bismuth pyrochlores (Bi,M)MNO7-δ (M = transition metal, N = Nb, Ta, Sb) in context of their fascinating structural, dielectric and magnetic properties

Among the vast pyrochlore group, oxide bismuth pyrochlores are quite interesting owing to the presence of stereochemically active lone pair electrons of bismuth and prominent Jahn-Teller interactions of the pyrochlore constituent cations leading to interesting structural modifications as well as emerging physical phenomenon of fundamental and technological significance. Particularly the bismuth pyrochlores nominally denoted by (Bi,M)MNO7-δ (M = transition metal, N= Nb, Ta, Sb) crystallizing with cubic symmetry at ambient conditions are intriguing due to the presence of ionic displacements and mixed cation occupancy especially at Bi site leading to non-stoichiometry and tunability of the physical properties with composition. Magnetic properties of the pyrochlore lattice has also garnered much attention as spin polarization on the frustrated square lattice indicated distinctive magnetic ground states such as spin glass, spin ice and spin liquid. In these cubic pyrochlores large cationic displacements coupled with uncontrolled random hopping, mixed cation crystallographic site occupancy and compositional variability gave rise to tunable dielectric relaxation. The cubic pyrochlore have been well known for showing low temperature as well as high temperature dielectric relaxation. Although long-range magnetic order was not supported on a bismuth pyrochlore lattice, recent findings reflected polar nature at room temperature in some of these disordered cubic pyrochlores comprising of highly polarizable cations. Herein we summarize the state of art in the exploration of cubic pyrochlores of Bi, and Nb/Ta/Sb in combination with various transition metals occupying multiple crystallographic sites. Their interesting structural, dielectric, magnetic properties and device applications are highlighted. Current challenges in the understanding of these materials are discussed and conclusions are put forth.