Alkali-C60 Compounds
(Aside: my policy
on scientific explanations.)
You may
remember from the pure C60 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 A3C60
(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 A3C60'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-C60
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 C60
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 C60 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 C60 and alkali-C60. 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?)
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-C60
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 C60, 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