Electronic Origins of Structural Distortions in Post-Transition Metal Oxides: Experimental and Theoretical Evidence for a Revision of the Lone Pair Model

Abstract

Structural distortions in post-transition metal oxides are often explained in terms of the influence of sp hybrid "lone pairs." Evidence is presented here showing that this model must be revised. The electronic structures of prototypically distorted alpha-PbO and alpha-Bi2O3 have been measured by high-resolution x-ray photoemission and soft x-ray emission spectroscopies. In contrast with the expectations of the lone pair model, a high density of metal 6s states is observed at the bottom of the valence band. The measurements are consistent with the results of density functional theory calculations.

The electronic structure of the antimony chalcogenide series: Prospects for optoelectronic applications

“…Sb 2 Te 3 thus takes on a symmetric structure and the Sb 5p do not mix with the states at the top of the VB. The results of the Sb chalcogenide series are in support of previous studies of the revised lone pair model [58,59,60,62,63,64,113]. A similar reduction in lone pair activity from O to Te was observed previously for the Sn monochalcogenide series and from PbO to PbS [62].…”

“…The distorted α-PbO structure was found to arise from the stabilisation of the anti-bonding Pb (6s) -O (2p) interaction at the top of the valence band by the Pb 6p states. This theory was later supported by experimental studies [60,61].…”

“…The change from a highly distorted Sb 2 O 3 structure to the hexagonal symmetric Sb 2 Te 3 structure can be explained by the formation of asymmetric density or lone pairs, on the Sb III cations. The lone pair formation is a result of the stabilisation of the Sb 5s-anion p anti-bonding states by the Sb 5p states at the top of the VB, similar to other lone pair materials [58,59,60,62,64,113]. The asymmetric density on the Sb III cations weakens down the series due to the increase in energy gap between the Sb 5s and Ch p states, as also observed for the Sn monochalcogenide series [63].…”

High thermoelectric performance of oxyselenides: intrinsically low thermal conductivity of Ca-doped BiCuSeO

“…In addition, the lone pair of electrons on Bi possibly lead to more asymmetric electron density and a stronger lattice vibration energy. 52 We speculate that the very low thermal conductivity and high Gruneisen parameter (g) of 1.5 in the BiCuSeO system are due in part to the increased bond anharmonicity associated with the presence of trivalent Bi. 50 In addition to the Young's modulus and Gruneisen parameter (g) for the BiCuSeO system, other possible reasons include the layered structure, as the phonons can be confined to scattered layer interfaces, 53 and the presence of heavy elements, 1,2 among others.…”

Quasi-particle electronic band structure and alignment of the V-VI-VII semiconductors SbSI, SbSBr, and SbSeI for solar cells

“…As with other group 14 and 15 cations (e.g., Pb 2þ , Sn 2þ , Bi 3þ ), a second-order Jahn-Teller instability can exist, which is associated with the change from a symmetric to asymmetric coordination environment of the cation with on-site s-p hybridisation. 15,16 SbSI exhibits an optical band gap of $2 eV, and the value can be tuned by the choice of chalcogen and halide. 17 The reference optical band gaps of SbSI, SbSBr, and SbSeI fall in the range 1.7-2.2 eV.…”