Tolerance factor control of uniaxial negative thermal expansion in a layered provskite

DOI: 10.1021/acs.chemmater.9b04512

Authors: Chris Ablitt (Imperial College London; University of Warwick) , Harriet Mccay (University of Oxford) , Sarah Craddock (University of Oxford) , Lauren Cooper (University of Warwick) , Emily Reynolds (University of Oxford) , Arash A. Mostofi (Imperial College London) , Nicholas C. Bristowe (Imperial College London; University of Kent) , Claire A. Murray (Diamond Light Source) , Mark S. Senn (University of Oxford; University of Warwick)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Chemistry Of Materials

State: Published (Approved)
Published: December 2019
Diamond Proposal Number(s): 18786

Abstract: By tuning the tolerance factor, $t$, of the Ruddlesden--Popper oxide Ca$_2$MnO$_4$ 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 character of octahedral tilts and have negative mode Gr\"uneisen parameter components along the NTE axis. By tuning $t$ to the lower edge of the NTE phase stability window, we are hence able to maximise the amplitudes of these vibrations and thereby maximise 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.

Subject Areas: Materials, Chemistry


Instruments: I11-High Resolution Powder Diffraction

Added On: 06/01/2020 14:31

Discipline Tags:

Physical Chemistry Chemistry Materials Science Chemical Engineering Engineering & Technology Perovskites Metallurgy

Technical Tags:

Diffraction X-ray Powder Diffraction