Structural and optical high-pressure study of FASnBr 3 (FA = formamidinium) revealed a cubic to orthorhombic phase transition near 1.4 GPa accompanied by a huge band gap red-shift from 2.4 to 1.6 eV, which is followed by a blue-shift of ∼0.2 eV upon further pressure increase. DFT calculations indicate that the variation in band gap is related to changes in Sn–Br bond length alternation, with an equalization of such difference predicted at high pressure. Extending the calculations to analogous lead-free systems provides a unifying mechanistic picture of pressure-induced band gap tuning in tin halide perovskites, which is correlated to the variation of specific structural parameters. These results represent a solid guide to predict and modulate the pressure-response of metal halide perovskites based on the knowledge of their structural properties at ambient pressure.
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Band gaps of crystalline solids from Wannier-localization–based optimal tuning of a screened range-separated hybrid functional
Accurate prediction of fundamental band gaps of crystalline solid-state systems entirely within density functional theory is a long-standing challenge. Here, we present a simple and inexpensive method that achieves this by means of nonempirical optimal tuning of the parameters of a screened range-separated hybrid functional. The tuning involves the enforcement of an ansatz that generalizes the ionization potential theorem to the removal of an electron from an occupied state described by a localized Wannier function in a modestly sized supercell calculation. The method is benchmarked against experiment for a set of systems ranging from narrow band-gap semiconductors to large band-gap insulators, spanning a range of fundamental band gaps from 0.2 to 14.2 electronvolts (eV), and is found to yield quantitative accuracy across the board, with a mean absolute error of ∼0.1 eV and a maximal error of ∼0.2 eV.
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- Award ID(s):
- 2015991
- PAR ID:
- 10301002
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 118
- Issue:
- 34
- ISSN:
- 0027-8424
- Page Range / eLocation ID:
- e2104556118
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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null (Ed.)Accurate computational predictions of band gaps are of practical importance to the modeling and development of semiconductor technologies, such as (opto)electronic devices and photoelectrochemical cells. Among available electronic-structure methods, density-functional theory (DFT) with the Hubbard U correction (DFT+U) applied to band edge states is a computationally tractable approach to improve the accuracy of band gap predictions beyond that of DFT calculations based on (semi)local functionals. At variance with DFT approximations, which are not intended to describe optical band gaps and other excited-state properties, DFT+U can be interpreted as an approximate spectral-potential method when U is determined by imposing the piecewise linearity of the total energy with respect to electronic occupations in the Hubbard manifold (thus removing self-interaction errors in this subspace), thereby providing a (heuristic) justification for using DFT+U to predict band gaps. However, it is still frequent in the literature to determine the Hubbard U parameters semiempirically by tuning their values to reproduce experimental band gaps, which ultimately alters the description of other total-energy characteristics. Here, we present an extensive assessment of DFT+U band gaps computed using self-consistent ab initio U parameters obtained from density-functional perturbation theory to impose the aforementioned piecewise linearity of the total energy. The study is carried out on 20 compounds containing transition-metal or p-block (group III-IV) elements, including oxides, nitrides, sulfides, oxynitrides, and oxysulfides. By comparing DFT+U results obtained using nonorthogonalized and orthogonalized atomic orbitals as Hubbard projectors, we find that the predicted band gaps are extremely sensitive to the type of projector functions and that the orthogonalized projectors give the most accurate band gaps, in satisfactory agreement with experimental data. This work demonstrates that DFT+U may serve as a useful method for high-throughput workflows that require reliable band gap predictions at moderate computational cost.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1073/pnas.2104556118
