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SUMMARY:Uni-axial Negative Thermal Expansion in Layered Perovskites - Prof
 . Arash Mostofi\, Imperial College London
DTSTART:20181025T131500Z
DTEND:20181025T141500Z
UID:TALK108853@talks.cam.ac.uk
CONTACT:Bartomeu Monserrat
DESCRIPTION:Using first-principles calculations we explain why pronounced 
 uniaxial negative thermal expansion (NTE) is found in certain layered pero
 vskites\, such as Ruddlesden-Popper oxides\, but elusive in bulk ABO3 pero
 vskites. We show that there are two key ingredients that determine the ext
 ent of NTE: (1) the proximity to a phase transition driven by a soft a pho
 non mode with negative Gruneisen parameter\; and (2) the presence of large
  off-diagonal components of the elastic compliance tensor that predispose 
 the material to the deformations required for NTE. Whilst both ABO3 and la
 yered perovskites often possess the former\, it is the unusually high elas
 tic compliance unique to layered perovskites of certain symmetry that allo
 ws NTE to persist over a large temperature range. We show that the anisotr
 opy in compliance in Ruddlesden-Popper oxides increases as a function of t
 he density of interface layers in the material and describe a model for th
 e atomic mechanism that is responsible for this feature. These insights op
 en up the possibility of high-throughput screening of NTE materials using 
 the elastic tensor as a descriptor.
LOCATION:TCM Seminar Room\, Cavendish Laboratory
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