Active materials can interchange types of energy. For example, piezoelectrics interchange mechanical and electrical energy, and multiferroics also convert these to magnetic energy and vice versa. Ferroelectrics in addition have a spontaneous polarization that can be manipulated and used to store information, just as ferromagnetics have a net magnetic moment. Most useful active materials and ferroelectrics are oxide perovskites, but new material classes may have new applications that can be exploited. We designed new polar molecular crystals computationally. We find that polar metallocenes form an exciting new class of molecular crystals that may be useful multiferroics. In a new cover article in the journal molecules, Geophysical Lab staff member Ronald Cohen and his colleague Haiwu Zhang report on predictions of a new class of polar metallocene crystals, which may be useful as active materials. The crystals are built from polar metallocene molecules. Their computations for the structural and magnetic properties of known metallocenes using Density Functional Theory (DFT) computations with non-local van der Waals interactions agree well with experiments. They predict new materials including metallocene sandwiches with furan or pyrrole, and find Mn-azametallocene Mn(C4H4N)2 to have a polar and ferromagnetic ground state, making it particularly interesting for further study. Other compounds have antiferromagnetic or non-magnetic ground states, and/or are antipolar. Some antipolar compounds have metastable polar or ferroelectric states under imposed electric fields. They predict significant polarizations of greater than 5 μC/cm2.
