Crystal-Chemistry Guidelines for Noncentrosymmetric A₂BO₄Ruddlesden−Popper Oxides

Abstract

Noncentrosymmetric (NCS) phases are seldom seen in layered A2BO4 Ruddlesden-Popper (214 RP) oxides. In this work, we uncover the underlying crystallographic symmetry restrictions that enforce the spatial parity operation of inversion and then subsequently show how to lift them to achieve NCS structures. Simple octahedral distortions alone, while impacting the electronic and magnetic properties, are insufficient. We show using group theory that the condensation of two distortion modes, which describe suitable symmetry unique octahedral distortions or a combination of a single octahedral distortion with a "compositional" A or B cation ordering mode, is able to transform the centrosymmetric aristotype into a NCS structure. With these symmetry guidelines, we formulate a data-driven model founded on Bayesian inference that allows us to rationally search for combinations of A- and B-site elements satisfying the inversion symmetry lifting criterion. We describe the general methodology and apply it to 214 iridates with A(2+) cations, identifying RP-structured Ca2IrO4 as a potential NCS oxide, which we evaluate with density functional theory. We find a strong energetic competition between two closely related polar and nonpolar low-energy crystal structures in Ca2IrO4 and suggest pathways to stabilize the NCS structure.

Designing new ferroelectrics with a general strategy

“…15 The hybrid improper ferroelectricity (HIF) was recently discovered in the artificial superlattice PbTiO 3 /SrTiO 3 , 16 where the ferroelectricity is induced by a trilinear coupling (PQ 1 Q 2 ) between the FE mode (P) and two oxygen octahedral rotational modes (Q 1 and Q 2 , respectively). The HIF was also found in the double-layered Ruddlesden-Popper (RP) perovskite A 3 B 2 O 7 (A=Ca, Sr; B=Mn,Ti), [17][18][19] the 1:1 A-cation ordering perovskite-type superlattice, [20][21][22][23] A-cation ordering RP NaRTiO 4 (R = Y, La, Nd, Sm-Ho), 24 the 2:2 B-cation ordered superlattice, 25 and metal-organic perovskite material. 26 In the above-mentioned theoretically designed FEs, one usually starts from a high-symmetry structure (e.g., cubic perovskite structure), then the effect of atomic substitution and soft phonon modulations are examined to see whether the ferroelectricity can be induced or not.…”

“…But for the improper FEs, the FE mode is no longer the primary order parameter, i.e., the polarization is induced by one or two rotational modes. [13][14][15][16][17][18][19][20][21][22][23][24][25] To verify whether the ½111 -superlattice LaCoO 3 /LaAlO 3 is an improper FE or not, the stability of the FE mode will be examined. We adopt the ISOTROPY software 33 to obtain the symmetry-adapted phonon modes in the low-symmetry FE structure.…”

Polymorphism and Structural Distortions of Mixed-Metal Oxide Photocatalysts Constructed with α-U3O8 Types of Layers

“…The apical O atoms of the single layers of edge-shared Nb/Ta pentagonal bipyramids form the intervening coordination environments (from above and below) for the layers of Nb/Ta octahedra and A-site cations. Each BO 6 octahedral environment can accommodate either three (m = 2) or six (m = 1) A-site cations that are 2, 6, 7, or 8-coordinate depending on their oxidation state, size, and coordination preference. Linearly coordinated cations have solely been observed for Cu-containing compounds (e.g., Cu 2 Ta 4 O 11 ) where Cu vacancies are found to occur over 33% of the available Cu sites [40].…”

“…Alternating octahedral and pentagonal bipyramidal layers are mirror images of one another; thus, upper and lower faces of the BO 6 (B = Nb, Ta) octahedron cavity are formed from three oxygen atoms in the layer above and from three oxygen atoms in the layer below [36]. The layers are bridged through the apical vertices of BO 7 (B = Nb, Ta) pentagonal bipyramids that are perpendicular to BO 6 (B = Nb, Ta) octahedral layers. The interatomic distances for 7-coordinate Nb/Ta polyhedra are included in Table S1.…”

“…Structural families of metal-oxides that have been commonly explored in the literature are ABO 3 pervoskites [1][2][3][4][5], layered Ruddelsden-Popper pervoskites [6][7][8], and spinel structures [9][10][11]. Metal-oxide materials can also be classified by their properties that can include superconductors [12][13][14][15], transparent conducting oxides [16][17][18][19][20], and photocatalysts [21][22][23][24][25][26].…”