Prof. Daniel G Nocera

The Nocera group focuses on basic mechanisms of energy conversion in biology and chemistry. A signature of the program is the ability to make and measure. Because the group is well versed in a variety of synthetic methodologies, we can design molecules and solids, ranging from organic supramolecular assemblies to inorganic coordination and organometallic compounds that are precisely targeted to investigate the energy conversion issues at hand. Expertise in a host of steady state and time-resolved spectroscopies permits us to define the critical physical and chemical phenomena, which in turn guide the further design of new systems with desired reactivity.

Investigations have been completed of vanadium-based jarosites, a new class of jarosites dicovered in this CMSE program. The fourth, and last publication on V-jarosites has been submitted to Phys. Rev. B, and it is currently under the review process.

Investigations in the area of iron jarosites have continued during the past year. The H3O+ jarosite had previously caught the attention of the condensed matter physics community because it is the only jaorsite to not exhibit long-range order. However, our studies indicate that this anomaly is not due to fundamentally new spin physics but instead arises from a chemical disorder problem. Comparison of pure jarosites prepared by our new synthetic methods with studies published in the literature revealed the presence of the H2O bridges (vs the normal OH-- bridge) on Fe3+ triangles of the kagome intralayers. We have shown that proton transfer from the H3O+ ion to the OH-- bridges is the source for the chemical disorder. The role of protons in altering the magnetic properties of iron jarosite has been investigated by preparing a series of solid solutions of the general formula HxK1-xFe3(OH)6(SO4)2, and by following the magnetic properties by SQUID measurements. Increase of the amount of the H3O+ ions in the interlayer space causes a roughly proportional depression of the ordering temperature TN.

Research has sought to chemically modify the iron jarosite lattice to further probe the origin of long-range order (LRO) in jarosites. We have prepared non-alkali iron jarosites having Ag+, Tl+, and _Pb2+ as interlayer cations. This work was undertaken to study magneto-structural and magneto-electronic correlations in jarosites, specifically geared towards those factors giving rise to TN. We find no single geometric parameter that accounts for the small variation in TN (56-65 K) in iron jarosites. Moreover, we find remarkable structural similitude, especially true of the Fe3_(OH)3 triangles comprising the kagomé lattice, regardless of the electronic nature of the interlayer cation. We have also synthesized the selenate capped analogs of sodium and potassium iron jarosites with the goal of finding larger Fe3_(OH)3 triangles due to the longer bond Se--O bond length (1.6 Å compared to 1.4 Å for SO42--). Rather, we find the only structural difference is in the Fe--O--Se angle, which must clamp down in order to preserve the rigid intralayer structure of the kagomé lattice. Most important, the magnetism of selenate-capped iron jarosites is again the same as that found in their sulfate counterparts. The observations that the magnetic properties of iron jarosites are unperturbed by both interlayer cation changes or by capping group changes leads us to conclude that the magnetism of pure iron jarosites is determined by a single kagomé layer and that LRO follows directly from in-plane coupling and the anisotropy developed by the Dzyaloshinky-Moriya interaction.

Investigations of magnetic properties of KFe3(OH)6(SO4)2 single crystals continued in collaboration with the group of Dr. Y. S. Lee. Interesting results were obtained using Dr. Young Lee’s new low temperature heat capacity instrument. The results of these studies are reported in Dr. Lee’s progress report.

As the electron occupancy of the d-orbitals controls the magnetic properties, the scope of jarosite research was expanded by replacing Fe3+ ions with Cr3+ ions. A series of ACr3(OH)6(SO4)2 jarosites was prepared (A = Na+, K+, Rb+, NH4+, ND4+, D3O+). All of the compounds were prepared in single crystalline forms, and their single-crystal structures were determined. All the representatives were investigated by means of SQUID magnetometry. Results revealed antiferromagnetic nearest-neighbor coupling (negative Weiss constants in all cases) with a strong ferromagnetic component and strong tendency to retain magnetization in the absence of magnetic field. This tendency holds true for all representatives except D3O+, which further reveals the impact of proton exchange on magnetic properties. Some of the representatives were prepared in crystal size large enough to permit single crystal magnetic measurements. These investigations are currently still underway.