http://SaturnianCosmology.Org/ mirrored file For complete access to all the files of this collection see http://SaturnianCosmology.org/search.php ========================================================== Alkali-C_60 Compounds **************************** (Aside: my policy on scientific explanations.) You may remember from the pure C_60 page that combining alkali metals with buckyballs was a natural combination chemically. Part of my own research focused on these materials, and this page is a bit biased toward the areas I am most familiar with. Of particular interest are the A_3 C_60 (A is one of the alkalis: K, Rb, Cs, Na) compounds, which turn out to be superconductors. There are a few caveats regarding the Na and Cs compounds that I don't want to get into, so for the rest of this article, assume I am talking about K- and Rb-C60 materials. As a class of materials, the A_3 C_60 's have the second-highest superconducting transition temperatures ever observed (after the cuprates), ranging from 19 to 40 K. What aspects of these materials are interesting to study? Much remains unknown about the alkali-C_60 compounds. I could not possibly tell you about all the research currently under investigation, but I can tell you a small piece of it: The alkali compounds tend to form crystals with a significant amount of /disorder/, which is a catch-all term for imperfections in the crystal lattice structure. There is always some disorder-- atoms in the wrong place, planes of atoms misaligned, impurity atoms-- even in very pure single crystals. But in the alkali fullerenes (and in /many/ materials, actually) the structure is much more disrupted than this. There are small regions, called "grains" or "crystallites," where the crystal is relatively intact. A grain can be anywhere from 25 angstroms (very small-- roughly 20 atoms in each direction) up to several microns (more than ten thousand atoms) in size. The macroscopic "crystal" that you see is made of zillions of these grains all jumbled together, where each grain may not have any special orientation relative to grains around it. To add to the confusion, there is a tendency for impurities to pile up at the boundaries between grains. Some materials grow into beautiful ordered crystals with no effort on our part, such as those nifty angled stones you can buy in nature stores. Other materials can be grown into nice crystals only if we work at it, say by condensing them from vapor into solid very slowly at high temeprature. The alkali-C60 compounds are especially stubborn in this regard, forming large-scale regular structure only reluctantly. Why are some materials prone to developing disordered structures? That's a whole essay in itself; it has to do with the energies and orientations of the bonds between the atoms, as well as other interactions, such as magnetism (although alkali-C60 materials are not especially magnetic). Anyway, it's more than I can get into in this space. So, one can ask, how does such a mess of atoms conduct an electric or thermal current? Where do the electrons want to be relative to such a disordered collection of nuclei? We have been seriously theorizing about and experimentally probing disordered compounds for about 40 years. It's a complicated problem, but we have learned a few things. The C_60 compounds conform to only some of these things, showing that we don't completely understand such materials. I worked on learning more about the thermal and transport properties of alkali fullerenes. But I still haven't told you the whole story. Besides the disorder, there is another curious aspect to these materials. Without going into detail, it is true that in many compounds the electrons behave as if they do not interact with each other very much (in fact they do, but they /behave/ as if they don't). So in these materials, we can pretend that the electrons happily fill their orbitals, as dictated by the Pauli Exclusion Principle, then leave each other alone. We can predict properties of these materials even if we completely ignore electron-electron interaction forces. This is a good thing-- such forces are /very difficult/ to describe theoretically. Because physicists are bored by systems that behave normally, we deliberately seek out "anomalous" behavior, such as materials in which electron-electron interactions really /are/ important. The C_60 compounds display this behavior. They are part of a class of systems labeled "strongly correlated," and we currently have no complete theory of such materials. What did I work on? I measured the *heat capacity of thin films* of C_60 and alkali-C_60 . I have also made some measurements on endohedral fullerenes . Much of my work involved making the first of these measurements ever accomplished. (Note: since finishing my Ph.D., I have moved out of the fullerene field. But I am still interested in and acquainted with some of the results. You can send me email about this field if you want, and I'll do my best to answer you). Materials can be made in thin-film and/or single-crystal form; most people study one or the other but not both (they require somewhat different equipment). Each form yields different information about the material, so it's important to study both if possible and compare/contrast the results. It happens that fullerenes can be made in both forms, but I only study the thin films. What I gain in this case is being able to observe the material out of equilibrium. If you like jargon, single crystals exist in a much more limited region of phase space than thin films. (If you don't know what heat capacity is, you can read my page entitled What is Microcalorimetry? <../Microcal/heatcap.html>) Of course heat capacity alone can never tell the whole story, but it's an important part. The goal in my work was to elucidate the underlying electronic and vibrational behavior of alkali-C_60 by observing how it reacts to heat. From my measurements, I figured out how important electrons are compared to lattice vibrations in carrying heat, how much heat you have to apply to see a certain change in temperature, and various other important properties. I have published a paper on my results in Physical Review B. Here's the reference: Specific heat of C60 and K3C60 thin films for T=6-400K, Phys. Rev. B, Volume 60, Number 16, p. 11,765 (1999) Who Cares? OK, that's a valid question. (Obviously /I/ care, since I worked on this for my Ph.D., but some sort of broader justification is in order). The usual reason for studying new materials is that they may be technologically useful-- is that true for the fullerenes? Maybe. See the page called What Are These Things Good For? But even if the first fullerenes studied are turning out not be technologically useful, look at what has come from the research. Because we studied buckyballs, we discovered carbon nanotubes . Nanotubes have greater potential to be useful in industry; they are already being used in some applications (for example, strengthening polymer beams). If we hadn't committed some resources to studying fullerenes, we would not have discovered nanotubes so quickly. And we may not have figured out the relatively cheap-and-easy way of producing them. This is a good example of basic research leading to valuable applications (which in turn pay for the basic research through (1) direct revenues and (2) saved R&D time-- the basic research often yields "shortcuts" that you wouldn't have thought of if you had just been doing directed research). The fundamental (non-applied) aspect of fullerene research is also a worthy endeavor revolving around extension of knowledge. Many new materials of the last decade-and-a-half, including fullerenes and the cuprates (one class of high-Tc materials), have embarrassed physicists by exposing how little we understand about highly correlated systems and about the phenomenon of superconductivity. Actually, "embarrassed" is not the right word-- as I mentioned above, physicists delight in finding anomalous behaviors, but only to a point. When /every/ new system that we come across seems to have its own weird set of anomalies and deviations from the "standard," one must ask just how standard the standard is. Paradoxically, physicists also seek unification-- being able to understand seemingly diverse behavior with a single, elegant description (a good example is the unification of electricity and magnetism, accomplished by Maxwell in the late 19th century. It can be shown that all magnetic and electric effects arise from the same fundamental interaction, even though they seem rather different in everyday life). We would like such a unified view of many phenomena in the physics of solids. I think it is fair to say that we have a good theory which applies in the case of a perfect crystal with no electron-electron interactions. Clearly this is a limited view; it's time to extend our understanding to systems that more closely resemble /real solids/. Perhaps the big surprise in our research so far is that the theory we use now works so well (and it really does!). By continuing basic research on such materials as C_60 , we can improve our theories and use that knowledge to make even better and more useful materials in the future. Basic research has led to many breakthroughs in technology. /Return to the Main Fullerene Page / Copyright © 1997-present Kim Allen **************************** <../../../../../> <../../../../../> <../../../../../> Email: kimall (at symbol) mindspring.com