Abstract
We demonstrate that DFT calculations performed with the local density approximation (LDA) allow for significantly better reproduction of lattice constants, the unit cell volume and the density of Ag(II)SO4 than those done with generalized gradient approximation (GGA). The LDA+U scheme, which accounts for electronic correlation effects, enables the accurate prediction of the magnetic superexchange constant of this strongly correlated material and its band gap at the Fermi level. The character of the band gap places the compound on the borderline between a Mott insulator and a charge transfer insulator. The size of the band gap (0.82 eV) indicates that AgSO4 is a ferrimagnetic semiconductor, and possibly an attractive material for spintronics. A bulk modulus of 27.0 GPa and a compressibility of 0.037 GPa–1 were determined for AgSO4 from the third-order Birch–Murnaghan isothermal equation of state up to 20 GPa. Several polymorphic types compete with the ambient pressure P-1 phase as the external pressure is increased. The P-1 phase is predicted to resist pressure-induced metallization up to at least 20 GPa.
Despite its more simplified character, the local density approximation allows for much better reproduction of many microscopic parameters of Ag(II)SO4 than the more sophisticated generalized gradient approximation
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Notes
U is a quasi-arbitrary parameter. It is a common practice to vary U over a broad energy range when trying to reproduce either a crystal structure, magnetic moments, or a band gap, etc. Unfortunately, different U values will perform best for each of these properties. In addition, in most cases, unrealistically large values of U are needed to make the band gap broad enough to match the experimental value. With this in mind, we have decided to use certain arbitrarily chosen U values. A typical value for Cu(II) is 9–10 eV, and it is reduced considerably if one moves to the next period (larger, softer cation). The values of U and J used in this paper for Ag(II) have in the past yielded reliable results for other compounds of Ag(II), such as KAgF3 and both polymorphic forms of K2AgF4 [8, 9]. In particular, the crystal and electronic structures of Cs2AgF4 have been screened very carefully by a number of theoretical groups, and the values of U and J used in these works for Ag match our values well. Concerning the oxide dianion, it is reasonable to assume that strong correlations are comparable to those for the isoelectronic fluoride one. In this way, we can improve the description of electronic correlation compared to that usually employed for, say, oxocuprates, where U is typically set for a transition metal only.
Sometimes LDA outperforms GGA in the prediction of lattice constants and dynamic properties, as found, for example, for alkali metal hydrides [12].
It also turns out that also the phonon spectra of AgSO4 are reproduced by LDA calculations with excellent accuracy (Derzsi M et al., submitted to Vibr Spectrosc).
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Acknowledgments
The project “Quest for Superconductivity in Crystal-Engineered Higher Fluorides of Silver” is operated by the Foundation for Polish Science’s TEAM program, and co-financed by the EU European Regional Development Fund.
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Dedicated to the memory of Professor Andrzej Sadlej, an outstanding Polish quantum chemist.
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Derzsi, M., Stasiewicz, J. & Grochala, W. Crystal and electronic structures and high-pressure behavior of AgSO4, a unique narrow band gap antiferromagnetic semiconductor: LDA(+U) picture. J Mol Model 17, 2259–2264 (2011). https://doi.org/10.1007/s00894-010-0950-y
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DOI: https://doi.org/10.1007/s00894-010-0950-y