Tolerance Factor Control of Uniaxial Negative Thermal Expansion in a Layered Perovskite

By
tuning the tolerance factor, t, of the Ruddlesden–Popper
oxide Ca2MnO4 through isovalent substitutions,
we show that the uniaxial coefficient of linear thermal expansion
(CLTE) of these systems can be systematically changed through large
negative to positive values. High-resolution X-ray diffraction measurements
show that the magnitude of uniaxial negative thermal expansion (NTE)
increases as t decreases across the stability window
of the NTE phase. Transitions to phases with positive thermal expansion
(PTE) are found to occur at both the high-t and low-t limits of stability. First-principles calculations demonstrate
that reducing t enhances the contribution to thermal
expansion from the lowest frequency phonons, which have the characteristics
of octahedral tilts and have negative mode Grüneisen parameter
components along the NTE axis. By tuning t to the
lower edge of the NTE phase stability window, we are hence able to
maximize the amplitudes of these vibrations and thereby maximize NTE
with a CLTE of −8.1 ppm/K at 125 K. We also illustrate, at
the other end of the phase diagram, that an enhancement in compliance
of these materials associated with the rotational instability provides
another mechanism by which NTE could be yet further enhanced in this
and related systems.