First-principles indicators of metallicity and cation off-centricity in the IV-VI rocksalt chalcogenides of divalent Ge, Sn, and Pb

Abstract

We use first-principles density functional theory to calculate the phonon frequencies, electron localization lengths, Born effective charges, dielectric response, and conventional electronic structures of the IV-VI chalcogenide series. The goals of our work are twofold: first, to determine the detailed chemical composition of lone pairs and, second, to identify the factors that cause lone pairs to favor high-or low-symmetry environments. Our results show that the traditional picture of cation s-p mixing causing localization of the lone pair lobe is incomplete, and instead the p states on the anion also play an important role. In addition these compounds reveal a delicate balance between two competing instabilities-structural distortion and tendency to metallicity-leading, at the same time, to anomalously large Born effective charges as well as large dielectric constants. The magnitude of the LO-TO splitting, which depends on the relative strength of both instabilities, shows a trend consistent with the structural distortions in these compounds.

Energy Landscape of Molecular Motion in Cubic Methylammonium Lead Iodide from First-Principles

“…41,42 The calculated Born effective charge tensors for the Pb 2+ ions of cubic CH 3 NH 3 PbI 3 is predicted to be nearly isotropic with values of the averaged trace around 4.9 e. Well above the nominal value of +2 e, the high values for the Born effective charges suggest a highly polarizable Pb s-lone pair. Figure 6(b,c,d), showing the summed partial charge density near the top of the valence bands, reveals the interplay between the molecular dipole and the Pb s-lone pair.…”

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

“…CsPbBr 3 is further from this instability as expected from the lesser tendency for lone pair stereochemical activity of Pb 2+ . 13 Importantly, this feature is absent in the perovskite CsCaBr 3 , which lacks a lone pair on the divalent cation. 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.…”

“…13 Based on these factors, we selected CsSnBr 3 for study as an exemplar of the delicate balance of structural tendencies in the main-group halide perovskites. In CH 3 NH 3 SnBr 3 , the larger A-site cation expands the lattice and leads to coherent Sn 2+ displacements in the orthorhombic phase 35 and to local displacements in the cubic phase.…”

“…This can be achieved by alloying of a lighter halogen, a lighter carbon-group element, or a larger A-site cation such as methylammonium or formamidinium which will increase the propensity for lone pair stereochemical activity. 13 Because of the importance of defect screening for carrier transport, such tuning can be used to enhance carrier mobilities, as has been shown in pseudo-Jahn-Teller perovskite 27 oxides. 15 In the high performance perovskite halide CH 3 NH 3 PbI 3 , the large polarizability associated with the lone pair and with molecular reorientation may explain why carrier scattering is dominated by phonons rather than charged impurities.…”

Magnetoelectric Ordering of BiFeO3 from the Perspective of Crystal Chemistry

“…These transitions are related to the structural changes. It was concluded on the basis of the performed studies [37][38][39][40][41][42][43][44] that the main reason of the ferroelectric distortion in BiFeO 3 , just like in PbTiO 3 and BiMnO 3 , consists in the stereochemical activity of the so-called lone pairstwo valence s-electrons of Bi 3+ and Pb 2+ cations. The polarization analysis performed in [42] demonstrates that partial contributions to polarization from the Fe and O atoms almost cancel each other and the net polarization present in BiFeO 3 originates mainly (>98%) from Bi atoms.…”