Cluster Solids: Ionic, Covalent, or Both?
We all want faster computers, more powerful solar cells, and
more sources of energy that don’t contribute to pollution. In
order to develop these technologies, scientists and engineers
research and create new materials. Current work focusing on
a new type of solid, called a cluster solid, is allowing scientists
to create materials with precise magnetic and electronic
properties. Because of the control scientists have over these
properties, cluster solids may be part of a supercomputer or
highly efficient solar panel someday.
Cluster solids are materials that are made up of really big
particles called cluster units. These cluster units are
composed of many atoms held together by covalent or ionic
bonds (or a mixture of both), depending on the identity of
the cluster unit. Cluster units are capable of bonding to each
other like individual atoms do in ionic solids by transferring
electrons. They can be a full order of magnitude (up to 10
times!) bigger than an atom. For example, fullerene or C60
(Figure 1) is a cluster unit made up of 60 carbon atoms
organized in the shape of a soccer ball. Fullerene cluster
units are one nanometer in diameter—almost four times
bigger than the diameter of a single carbon atom, as shown
in the figure. To get an idea of how those compare think about a tenth grader and a two story
building. If an atom were the size of a tenth grader, a fullerene cluster unit would be as big as a
two story building! The difference in size between cluster units and atoms is enormous and
leads to some of their interesting properties.
Cluster solids can be made up of many different types of cluster units. Depending on the identity
of the cluster units the resulting cluster solids have varying amounts of magnetism and varying
abilities to conduct electricity. This means that their magnetism and electrical conductivity are
“tunable”—they can be controlled by varying the components of the cluster solid.
Scientists at Columbia University created a variety of cluster solids to investigate their tunable
properties. To make the cluster solids, they first created several different transition
metal-containing cluster units. Each of these transition metal-containing cluster units was
combined with C60 cluster units to make a different cluster solid. They found that an ordered
ionic structure formed in all of these cluster solids, but the exact arrangement of the cluster units
and the properties that resulted varied depending on the atoms in the transition metal cluster
units. For example, one difference between these solids is that they combine in different ratios
with C60 depending on how many electrons were transferred. This is similar to the way that
calcium combines with two chlorides to make CaCl2, but sodium only combines with one
chloride to make NaCl. When a nickel-containing cluster unit was mixed with C60, it made a 1:1
structure that is analogous to the structure of sodium chloride, which contains one sodium ion
for every chloride ion (see Figure 2).
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