Like an ice cube on a warm day, most materials melt -- that is,
change from a solid to a liquid state -- as they get warmer. But
a few oddball materials do the reverse: They melt as they get
cooler. Now a team of researchers at MIT has found that silicon,
the most widely used material for computer chips and solar
cells, can exhibit this strange property of "retrograde melting"
when it contains high concentrations of certain metals dissolved
in it. The material, a compound of silicon, copper, nickel and
iron, "melts" (actually turning from a solid to a slush-like mix
of solid and liquid material) as it cools below 900 degrees
Celsius, whereas silicon ordinarily melts at 1414 degrees C. The
much lower temperatures make it possible to observe the behavior
of the material during melting, based on specialized X-ray
fluorescence microprobe technology using a synchrotron -- a type
of particle accelerator -- as a source. The material and its
properties are described in a paper just published online in the
journal Adv