Symmetry Switching of Negative Thermal Expansion by Chemical Control

The
layered perovskite Ca3–xSrxMn2O7 is shown
to exhibit a switching from a material exhibiting uniaxial negative
to positive thermal expansion as a function of x.
The switching is shown to be related to two closely competing phases
with different symmetries. The negative thermal expansion (NTE) effect
is maximized when the solid solution is tuned closest to this region
of phase space but is switched off suddenly on passing though the
transition. Our results show for the first time that, by understanding
the symmetry of the competing phases alone, one may achieve unprecedented
chemical control of this unusual property.