Negative Thermal Expansion in Hybrid Improper Ferroelectric Ruddlesden-Popper Perovskites by Symmetry Trapping
M  S Senn, A Bombardi, C A Murray, C Vecchini, A Scherillo, X Luo, and S  W Cheong
Phys Rev Lett 114, 035701 (2015)

The high temperature phase of the ferroelectric ground state Ruddlesden-Popper Ca3Mn2O7 is shown to exhibit large uniaxial negative thermal expansion. We show, by symmetry analysis, how the “incompatibility” of the high temperature and ferroelectric phase (as evident by the lack of group-subgroup relationship between them) leads to the trapping of a soft mode with transverse vibrational character accounting for the observed negative thermal expansion. Our observations lead us to propose a general strategy for designing and search for materials exhibiting NTE.

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