Each diffusion
path for an oxygen atom (red) moving across a graphene ring composed of carbon
atoms (gray ) is
considered a “distortion” and is indexed by a unique “distortion symmetry
group”
indicated below
each image. The symmetry group contains all the essential information about the
properties of the
material system as the diffusion occurs, including the ability to help
determine the
minimum energy
pathway. In this case, the minimum energy pathway is when oxygen moves
around the ring
(right image) rather than across it (left image). Image: Penn State
(November 17, 2015) A
new symmetry operation developed by Penn State researchers has the potential to
speed up the search for new advanced materials that range from tougher steels
to new types of electronic, magnetic, and thermal materials. With further
developments, this technique could also impact the field of computational
materials design.
"In the physical sciences, making measurements can be
time consuming and so you don't want to make unnecessary ones," said
Venkat Gopalan, professor of materials science and engineering. "This is
true for any material property -- mechanical, electrical, optical, magnetic,
thermal or any other. Knowing the symmetry group of a material can greatly
reduce the number of measurements you have to make. "
Symmetry is pervasive throughout the physical universe and
underlies the basic laws of physics. Gopalan gives a simple but scientifically
accurate definition. "Symmetry is when doing something looks like doing
nothing."
A circle has perfect symmetry, because if you rotate it by
any number of degrees, it will look the same. Similarly, rotating a hexagon by
sixty degrees leaves it exactly the same, but rotating it by a different amount
does not. Anything that can be done that leaves an object looking the same is a
symmetry operation.
In crystals, atoms are arranged in symmetrical patterns,
like a cube of salt or a crystal of sugar or quartz. Symmetry groups tell
scientists in how many different ways atoms can arrange in repeating patterns.
If they know which symmetry group a material falls into, they already know a
great deal about the properties -- mechanical, thermal, electrical and so forth
– that material will have. There are precisely 230 groups that explain how atoms
can be arranged in space. These are symmetry "boxes" a material will
fit into. If scientists are looking for a material with a certain property,
such as the ability to be electrically polarized, they can look at materials
only in that symmetry box and ignore all the boxes that cannot possibly contain
polar materials.