Our Group in the News: Exxon Mobil Award
, NSF CAREER Award
, New Materials for Energy-Efficient Refrigeration
1. Molecular Magnetism and Photomagnetism.
Certain complexes of d4
transition metals exhibit magnetic bistability, a transition between the low-spin and high-spin states commonly referred to as spin crossover (SCO). They represent great interest with regard to the development of new paradigms in molecular electronics and high-density data storage. Our group is exploring hybrid materials that combine such complexes with (a) conducting charge-transfer salts, (b) luminescent quantum dots, or (c) divergent linkers that can lead to the formation of multinuclear spin-crossover complexes or coordination polymers. To achieve these goals, we use a combination of synthesis, crystal structure analysis, and magnetic measurements, as well as various spectroscopic techniques to create novel materials with magnetically bistable Fe(II) centers. On this NSF-funded project, the most important achievements are: (1) functionalization of well-known Fe(II) SCO complexes with tetrathiafulvalene (TTF), a ubiquitous component of synthetic organic conductors. We now work on obtaining conducting SCO materials via electrooxidation of the obtained TTF-containing Fe(II) complexes; (2) preparation of several multinuclear clusters, in which Fe(II) SCO centers are bridged by cyanide or dicyanometallate linkers; (3) discovery of a robust ligand set that contains 2,2′-biimidazole and preserves SCO at the Fe(II) center upon alkylation of the ligand: this allows functionalization of the SCO complexes, facilitating their assembly on various substrates, such as thin films, nanotubes, or nanoparticles; (4) preparation of a complex with an alkylated derivative of biimidazole; the complex exhibits an abrupt SCO with thermal hysteresis and a photoinduced transition between the spin states at low temperature.
2. Discovery of New Soft and Hard Magnets.
Itinerant magnets represent a peculiar class of materials, whose magnetic behavior is strongly dependent on the nature of electronic structure in the vicinity of the Fermi level, and therefore can be effectively tuned by controlling the band structure of the solid. While such materials traditionally have been a focus of condensed matter physics, the fast-paced development of methods for electronic structure calculations and appearance of reliable user-friendly codes allow solid state chemists to become actively involved in this area of research. This trend has been reinforced by the recent discovery of FeAs-based superconductors, which caused a renewed interest to the ThCr2
structure type in the solid-state chemistry community. Our research thus far has focused on elucidating correlations between the crystal and electronic structures and magnetic properties of RCo2
materials (R = rare earth; Pn = P, As) to achieve better understanding and control over the type and temperature of magnetic ordering in these systems. We are also interested in exploring R-rich sections of the R-T-X phase diagrams (T = transition metal; X = Si, Ge, Sn) for the discovery of new 3d-4f
coupled itinerant magnets. Summarized below are the most important achievements on this project (funded by the NSF CAREER award): (1) control of ferromagnetic ordering temperature in La1
via modification of interlayer Co-Co interactions; (2) stabilization of a homogeneous mixed-valence state of Eu by means of chemical compression that results in the change from antiferromagnetism in PrCo2
to ferromagnetism in Pr0.8
; (3) generation of a spin-glass state in LaFex
, with a possible formation of a rare Griffiths-like state; (4) synthesis of ternary RCo2
from Bi flux and the first report of their magnetic properties; (5) a detailed structural analysis of a giant unit cell compound Sm117
and its rare-earth analogs, and elucidation of their magnetic behavior.