Cation Order
A central theme of
our research is establishing the effect of cationic order and extended
defects on
the electronic properties of oxide systems. By controlling the
bulk
chemistry, the method of synthesis, and the details of the thermal
processing
it is possible to induce variable degrees of nano-level ordering in
many oxide
systems.
Our research focuses on the
characterization of the ordering and
defects using high resolution TEM, and x-ray/neutron diffraction, and
examining
the effect of solid solution formation on the behavior of the various
systems. These data are coupled to an understanding of the
energetics of
the different systems to permit tailoring of the atomistic level
structure and
control of the properties. More details can be found in our
publications
on microwave ceramics and ferroelectrics and in the general
publications list.
In
our
most recent work we have focused on an unusual family of perovskites
containing
under-sized cations such as Li on the A-site (e.g. Nd2/3-xLi3x)TiO3).
These systems undergo a series of ordering and phase separation
reactions resulting
in the formation of nanoscale "chessboard" structures. The nanostructural modulations arise
from a periodic separation into domains of two phases with nanometer
dimensions, one Li-rich with a composition (Nd1/2Li1/2)TiO3
and the other Li-free with a composition Nd2/3TiO3.
A remarkable feature of this structure is the phase separated domains
form a
superstructure that extends across the entire crystal, with a
periodicity that
can be tuned by changing the overall composition. This is
the first
observation of periodic phase separation in any inorganic solid.
The
structures offer the potential for new classes of perovskites that
could serve
as templates for the assembly of nanoscale objects or molecular
overlayers. This work has been
featured in Nature
and Nature Materials.
Currently this work is supported
by the National Science Foundation (Ceramics Division) under
Grant #DMR0704255.
Any opinions, findings and conclusions or recommendations expressed in
this material are those of the authors and do not necessarily reflect
the views of the National Science Foundation (NSF)