Multiple instabilities inBi4Ti3O12: A ferroelectric beyond the soft-mode paradigm

Abstract

The Aurivillius compound Bi 4 Ti 3 O 12 exhibits a single phase transition from a tetragonal high-temperature phase to a ferroelectric state. By means of first-principles calculations guided by a complete symmetry analysis of the experimental distortion, we confirm B1a1 as the ͑monoclinic͒ symmetry of the room-temperature phase but show that the transition involves a complex interplay of six different normal modes belonging to four different irreducible representations. Three of them are necessary for the observed symmetry break. We do not find any feature in the energy landscape that would force the simultaneous condensation of these three order parameters and cause such an avalanche transition, so we suggest that further experimental work might be able to detect intermediate phases. On the other hand, the necessary condensation of pairs of normal modes of the same symmetry already puts Bi 4 Ti 3 O 12 outside the standard soft-mode paradigm and has important structural implications. In particular, the approximate rigidity of the BiO 6 octahedra is due to anharmonic couplings of these modes. In addition, a comparison of the energy calculations and the behavior of the bond-valence global instability index is presented.

Ferroelectricity from coupled cooperative Jahn-Teller distortions and octahedral rotations in ordered Ruddlesden-Popper manganates

“…Proposed improper mechanisms based on electronic degrees of freedom include magnetic cycloidal order, 6 charge ordering, 7 and spin-Peierls distortions. More recently several (hybrid) improper ferroelectric mechanisms have been proposed that rely on lattice anharmonicities between two or more displacive modes (Q) of the crystal, [9][10][11][12] Useful anharmonicities in the free energy potential are of the form λP n Q m , λP n Q m 1 Q 2 , . .…”

Multiferroic behavior of Aurivillius Bi4Mn3O12from first principles

“…So, based on an exhaustive study of the BIT phonon modes by Perez-Mato et . al , 23 we restrict our discussion to zone center polar modes with E u symmetry and zone boundary antiferrodistortive modes with X + 3 symmetry. We find that two of the 11 bi-dimensional E u modes of BIM have imaginary frequency, similarly to what was obtained for BIT.…”

Temperature-driven phase transitions in SrBi2Ta2O9 from first-principles calculations

“…They are ferroelectric at room temperature, but in addition to the polar distortion, they have other nonpolar antiferrodistortive distortions, which involve large tiltings of the perovskitelike octahedra. Understanding the properties of Aurivillius compounds is important for future device applications but presents a challenge to theory and modelling because of the complex interplay between the different structural instabilities, which suggests that the phase transition sequence in these materials is not as simple as considered until now [2].…”

“…However, due to the limitations of the powder method for diffraction analysis, which is the only method used up to now at high temperatures, it cannot be ruled out that intermediate phases indeed exist. Recently, it was shown that the ferroelectric transition in BIT involves the interplay of six different normal modes belonging to four different irreducible representations, indicating that the apparent absence of intermediate phases remains to be explained [2].…”

“…While first-principles calculations have contributed to the understanding of the electronic structure and structural instabilities of SBT [4,5,14] and other Aurivillius [2,15], these methods are restricted to studying zero-temperature properties. Nowadays, the combination of first-principles calculations with effective Hamiltonian [16], or shell-model [17] techniques offers a multiscale approach to investigate the various functional properties of ferroelectric oxides.…”