title: Negative thermal expansion in hybrid improper ferroelectric Ruddlesden-Popper perovskites by symmetry trapping.
abstract: We present new results on the microscopic nature of the ferroelectricity mechanisms in Ca3 Mn2O7 and Ca3Ti2O7. To the first approximation, we confirm the hybrid improper ferroelectric mechanism recently proposed by Benedek and Fennie for these Ruddlesden-Popper compounds. However, in Ca3Mn2O7 we find that there is a complex competition between lattice modes of different symmetry which leads to a phase coexistence over a large temperature range and the "symmetry trapping" of a soft mode. This trapping of the soft mode leads to a large uniaxial negative thermal expansion (NTE) reaching a maximum between 250 and 350 K (3.6×10^(-6)  K^{-1}) representing the only sizable NTE reported for these and related perovskite materials to date. Our results suggest a systematic strategy for designing and searching for ceramics with large NTE coefficients.
authors: Diamond Light Source Limited, Harwell Science and Innovation Campus, Didcot OX11 0DE, United Kingdom and Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QR, United Kingdom.
authors: M S
authors: MS
authors: Senn
authors: Diamond Light Source Limited, Harwell Science and Innovation Campus, Didcot OX11 0DE, United Kingdom.
authors: A
authors: A
authors: Bombardi
authors: Diamond Light Source Limited, Harwell Science and Innovation Campus, Didcot OX11 0DE, United Kingdom.
authors: C A
authors: CA
authors: Murray
authors: National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom.
authors: C
authors: C
authors: Vecchini
authors: ISIS, STFC, Rutherford Appleton Laboratory, Didcot OX11 0QX, United Kingdom.
authors: A
authors: A
authors: Scherillo
authors: Laboratory for Pohang Emergent Materials and Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea.
authors: X
authors: X
authors: Luo
authors: Laboratory for Pohang Emergent Materials and Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Korea and Rutgers Center for Emergent Materials and Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA.
authors: S W
authors: SW
authors: Cheong