Lone Pairs in the Halide Perovskites, Hidden or Otherwise

The ABX₃ halide perovskites that have attracted so much renewed attention usually have the B site occupied by divalent Sn or Pb, with ns² lone pairs of electrons. These lone pairs are frequently hidden, in the sense that the crystal structures are not consistent with the presence of a stereochemically active lone pair, that would normally be associated with certain characteristic distortions of the BX₆ polyhedra in the structure. Such hidden lone pairs can be seen in many of the perovskites including the hybrids with methylammonium and formamidinium ions on the A site. These lone pairs are associated with proximal instabilities that can profoundly influence material properties. We will discuss the understanding that we have developed from extensive real and k-space studies of local and average structure in these materials using synchrotron and neutron scattering. Density Functional Theory-based electronic structure calculations and Nuclear Magnetic Resonance studies complement the structural studies to obtain a coherent picture of what is happening, structurally and electronically, in these materials.