Interplay of Octahedral Rotations and Lone Pair Ferroelectricity in CsPbF₃

Abstract

CsPbF3 is the only experimentally synthesized ABF3 fluoride perovskite with a polar ground state. We use CsPbF3 as a guide in our search for rules to rationally design new ABX3 polar fluorides and halides from first-principles and as a model compound to study the interactions of lone pairs, octahedral rotations, and A- and B-site driven ferroelectricity. We find that the lone pair cation on the B-site serves to stabilize a polar ground state, analogous to the role of lone pair cations on the A-site of oxide perovskites. However, we also find that the lone pair determines the pattern of nonpolar structural distortions, rotations of the PbF6 octahedra, that characterize the lowest energy structure. This result is remarkable since rotations are typically associated with bonding preferences of the A-site cation (here Cs(+)), whereas the Pb(2+) cation occupies the B site. We show that the coordination requirements of the A-site cation and the stereoactivity of the B-site lone pair cation compete or cooperate via the anionic displacements that accompany polar distortions. We consider the generalizability of our findings for CsPbF3 and how they may be extended to the oxide perovskites as well as to the organic-inorganic hybrid halide perovskite photovoltaics.

Dynamic Stereochemical Activity of the Sn2+ Lone Pair in Perovskite CsSnBr3

“…Geometrical considerations of the local atomic interactions could help elucidate the origin of the observed asymmetry in the Sn-Br correlation. For example, the enlarged perpendicular anisotropic ADPs of the Br atoms could result in a distribution of Sn-Br correlations at and above the nominal Sn-Br bond distance, which would appear as peak tailing to high r. Alternatively, a rhombohedral off-centering of the Sn 2+ (towards the face of the SnBr 6 octahedra along the [111] crystallographic direction) would result in two Sn-Br correlation distances. If this off-centering is dynamic, this could appear as an asymmetric peak at the nominal Sn-Br correlation distance.…”

“…Ca 2+ is chosen for comparison because of its identical ionic radius to Sn 2+ in octahedral coordination, as calculated from bond valence sums in oxides. 77,78 Because effective ionic size depends on the ligands, CsSnBr (200) is presented in Figure 8(b), for the case where Sn 2+ is displaced along [111]. An asymmetric lobe of elevated localization opposite to the direction of cation dis-placement is indicative of the broken inversion symmetry of the lone pair that has become stereochemically active.…”

Strain-Engineered Multiferroicity inPnmaNaMnF3Fluoroperovskite

“…Nonetheless, none of the reported fluoride candidates belong to the most claimed perovskite family. Recently, we have shown that even if none of the fluoroperovskites is reported with a FE ground state (except CsPbF 3 [11,12]) they have nevertheless the propensity to have a FE instability in their high symmetry cubic reference structure [13]. We have identified that the FE instability of the fluoroperovskites is related to a steric geometric effect when small cations lie at the A-site.…”

Effect of chemical nature of atoms on the electronic, dielectric, and dynamical properties of ABX3 halide perovskite

“…Thus, the presence of a lone pair allows the cation to displace off‐center of the polyhedron . Furthermore, Smith et al explained the important role of stereoactivity of Pb lone pair in making CsPbF 3 ferroelectric at room temperature; the authors compare this compound with the nonferroelectric CsSrF 3 , where Sr 2+ has almost the same ionic radius as Pb 2+ but no lone pair.…”