The ABX3 halide perovskites that have attracted so much renewed attention mostly possess B= Sn2+ or Pb2+,
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 for example result in certain characteristic distortions of MX6 polyhedra in the structure. Such hidden lone pairs can be seen in the hybrids with methylammonium [A= CH3NH3+] and formamidinium [A= CH(NH2)2+], where the lone pairs are associated with proximal instabilities that can profoundly influence materials 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. DFT calculations and NMR studies complement the structural studies to obtain a coherent picture of what is happening, and where all the red herrings are.
Ram Seshadri has been on the faculty of the University of California, Santa Barbara since 2002, where he is currently Professor of Materials and Professor in the Department of Chemistry and Biochemistry. In addition, he Directs the NSF Materials Research Science and Engineering Center (MRSEC) at UCSB: The Materials Research Laboratory. Seshadri obtained his BSc. (Hons.) in Chemistry from St. Stephens College (Delhi University) and a PhD in Solid State Chemistry from the Indian Institute of Science, Bangalore, where he carried out research under the supervision of Professor C. N. R. Rao FRS. Following postdoctoral stays at the CRISMAT lab in Caen, France, and the University of Mainz in Germany, he started his independent career in the Indian Institute of Science, Bangalore as a faculty member before moving to Santa Barbara. The research carried out in his group combines aspects of physics, chemistry, and materials science, within the broad theme of crystal chemistry, and crystal-structure–property relations in functional inorganic materials. Seshadri has over 300 publications associated with these topics. He currently serves as an Associate Editor of the ACS journal Chemistry of Materials, and of Annual Reviews of Materials Research.