Improper Inversion Symmetry Breaking and Piezoelectricity through Oxygen Octahedral Rotations in Layered Perovskite Family, LiRTiO₄ (R = Rare Earths)

Abstract

A signifi cant amount of research in the piezoelectric and ferroelectric communities is presently being dedicated to fi nding new electronic materials for applications in sensors, actuators, and transducers. This fi eld of research has long been dominated by AB O 3 perovskite oxides in which the inversion centers in the crystal structures are removed by atomic displacements due to the second-order Jahn-Teller (SOJT) effect. [1][2][3] There are only a few SOJT-active cations such as Ti 4+ and Zr 4+ with empty n d ( n = 3, 4) orbitals, and Pb 2+ and Bi 3+ with n s ( n = 6) lone-pair electrons, which limit the choices of chemical elements that can be used to design noncentrosymmetric materials. On the other hand, the most ubiquitous distortion in the perovskite oxides is the rotation or tilting of B O 6 oxygen octahedra enclosing the B-site cations. [ 4 ] These rotations do not break the inversion symmetry in simple AB O 3 perovskites because of the nature of octahedral connectivity. Further, the oxygen octahedral rotations (OORs) encompassing A-site cation displacements suppress the instability of polar distortions in most perovskites that are available in nature. [4][5][6] Though OORs cannot break inversion symmetry in simple perovskites, they can break inversion symmetry in layered oxide structures with different topologies (arising from different connectivities), such as Ruddlesden-, and perovskite superlattices (PS) [ABO 3 ] m /[A′B′O 3 ] n . [7][8][9][10][11][12][13][14][15] In addition to topology , layered oxides provide three additional tuning knobs for realizing new properties: dimensionality (subscript integers m and n above), chemical ordering (cations A, A′, B, and B′), and strain such as when grown as an epitaxial fi lm on a single crystal substrate.Benedek et al. [ 8,12 ] and Rondinelli et al. [ 13 ] have proposed that the most abundant OOR, represented by a − a − c + in the Glazer notation, [ 16 ] can remove inversion centers in n = 2 RP phase and AB O 3 / A ′ B O 3 PS, respectively. This idea of breaking inversion symmetry by OORs has been developed in other layered perovskites such as RP phases with and without A-site cation An improper mechanism for breaking inversion symmetry is revealed and thus inducing piezoelectricity in the family of layered perovskites, Li R TiO 4 ( R = rare earths), which are previously reported as centrosymmetric. Noncentrosymmetry in this family of compounds arises from TiO 6 octahedral rotation represented by a − b o c o / b o a − c o in the perovskite blocks between R O rock salt and LiO antifl uorite layers. X-ray diffraction and optical second harmonic generation complemented by density functional theory predictions are crucial in determining the new structures. High transition temperature ( T ac ) of up to 1200 K from noncentrosymmetric to centrosymmetric phase is observed. Piezoelectric coeffi cients ( d 36 ) of up to −15 pC/N are predicted, and piezoelectric force microscopy experiments confi rm a piezoelectric response. The demonstrated improper mechanism in this...

Learning from data to design functional materials without inversion symmetry

“…As a general observation, we note that the NCS P 2 1 m space group that we predict for 13 out of 19 compositions from DFT is also one of the most commonly observed experimental ground states 16 17 (also see Fig. 4 ) for the n =1 RP compounds.…”

“…1b ). In the literature, the family of cation-ordered Na R TiO 4 and Li R TiO 4 (found only recently), where R =La, Nd, Dy, Gd, Sm, Ho, Eu and Y, have been experimentally shown 16 17 to have the piezoelectric P 2 1 m space group [ ⊕ ( η 1 , η 1 ; η 1 )]. The nominal electronic configuration of Ti 4+ in these compounds is d 0 .…”

“…We apply it to the two-dimensional n =1 RP structure family ( Fig. 1a ), for which to date few compositions exist in NCS crystal classes 15 16 17 . Nonetheless, the chemical search space is ( Fig.…”

Hybrid Improper Ferroelectricity in (Sr,Ca)3Sn2O7 and Beyond: Universal Relationship between Ferroelectric Transition Temperature and Tolerance Factor in n = 2 Ruddlesden–Popper Phases

“…The phonon dispersion curves were drawn for a standard primitive cell 64,65 along a q-space path based on crystallography 66 . Our systematic stable structure exploration was conducted in a previously reported way 37,[67][68][69][70] .…”

Ferroelectricity of Dion-Jacobson layered perovskites CsNdNb$_2$O$_7$ and RbNdNb$_2$O$_7$

“…13 It was also reported that acentricity is induced by oxygen octahedral rotations in A-site ordered n = 1 RP titanates ARTiO 4 (A = H, Li, Na, K; R = rare earth). [14][15][16][17] Hybrid improper ferroelectricity is also possible for n = 2 Dion-Jacobson (DJ) phases, 18,19 which is another type of layered perovskites consisting of primitive stacking of adjacent perovskite blocks, in contrast to bodycentered stacking in RP phases. In spite of the difference in stacking fashions, ferroelectricity induced by oxygen octahedral rotations was also theoretically predicted for the n = 2 DJ phases ARB 2 O 7 (A = Rb, Cs, R = La, Nd, B = Nb, Ta), 18,19 and indeed, these oxides were proven to be polar by means of synchrotron x-ray diffraction, neutron diffraction, and optical second harmonic generation measurements.…”