Ronald Cohen

The Geophysical Laboratory has a long and distinguished history of determining the stability and physical properties of materials experimentally. This work continues today. The materials investigated include Earth's minerals, materials that form the other planets, those used in high technology, and those that are interesting because of the problems they pose to physics and chemistry. One goal of this research is to understand what the underlying physics is that gives rise to the materials' behavior. Cohen and his research group look at the problem by starting with electrons and nuclei; they try to predict and understand mineral and material behavior using fundamental physics.

Three major projects are currently under way in Cohen's lab. The first explores the question: How do transition metal compounds behave at high pressures? Whereas electronically simple compounds like MgO and SiO₂ are well understood and their properties can be predicted quite accurately, transition metal oxides such as FeO are at the frontier of solid-state physics and are poorly understood. Experiments have not yet been able to clarify the properties and physical origins of the behavior of FeO. At high pressures and temperatures (approximately 100 GPa and 1000 K), a NiAs-type structure forms. The reanalysis of the experimental data shows that this product is most likely a polytype of NiAs with anti-NiAs structures (with Fe on the Ni or As sites respectively).¹ Interestingly, the researchers predict the NiAs part to be metallic, and the anti-NiAs part to be insulating. Since this phase may form in the deep Earth near the core-mantle boundary, this behavior could be very important in helping control the variations in the Earth's magnetic field.

Currently, Steve Gramsch, a CHiPR fellow, is trying to understand possible high-spin low-spin and metal insulator transitions in the rock salt (NaCl) structure of FeO. He is applying a newly developed method, which includes local electronic correlations, and finds that with increasing pressure a metal insulator transition occurs before a high-spin low-spin transition. This previously had been found theoretically with more conventional methods.²

In the second area of research, Cohen is working on materials used in high technology. In particular, he is trying to understand piezoelectricity in ferroelectric perovskites. These materials are used in sonar and medical imaging as well as in dielectrics and nonvolatile computer memories. Postdoctoral associates Henry Fu and Sergei Stolbov are working with Cohen to understand the large strains found in a new class of single-crystal piezoelectrics that will revolutionize the above applications.

Finally, active research is also under way in Cohen's lab to understand the thermoelastic, melting, and rheological behavior of minerals, metals, and compressed gases at high pressures and temperatures.

¹ Mazin, I. I, Y. Fei, R. Downs, and R. E. Cohen, Possible polytropism in FeO at high pressures, Am. Mineral. 83, 451-457, 1998.

2 Cohen R. E., Y. Fei, R. Downs, I. I. Mazin, and D. G. Downs, Magnetic collapse and the behavior of transition metal oxides: FeO at high pressures, in High-Pressure Materials Research, Materials Research Society Proceedings, Vol. 499, R. Wentzcovitch, R. J. Hemley, W. J. Nellis, and P. Yu, eds. 1998.