An ambient conditions alpha polymorph of solid AgF2 with a layered AgIIF2 structure recently received attention due to numerous structural and electronic similarities with oxocuprate precursors of high-temperature superconductors [1]. The individual [AgIIF2] layers here are isoelectronic with undoped [CuO2] sheets in oxocuprates and both systems are antiferromagnetic semiconductors with charge-transfer band gap. Interestingly, diamagnetic mixed-valence AgIAgIIIF4 beta polymorph was also reported [2]. Notably, β form undergoes a rapid exothermic conversion to the α form (AgIAgIIIF4 à 2AgIIF2) when the temperature is raised to 0 °C. This observation is important, since a direct relation between the disproportionation reaction and unconventional superconductivity is well documented for charge-ordered insulator BaBi3+/5+O3; this compound can be converted to superconductor by a non-isovalent substitution leading to Ba1-xKxBiO3 [3]. In order to better understand the thermodynamics and electron-lattice interplay in the two AgF2 phases, we have calculated their lattice vibrations and related electronic structures using various Density Functional Theory exchange-correlation functionals and we have evaluated phonons in quasi-harmonic approximation. Additionally, we have evaluated anharmonicity effects in lattice dynamics of the alpha phase by applying a new developed method based on a nonperturbative approach of probing the crystals potential-energy landscape in the multidimensional space of atomic displacements [4]. This permitted us to calculate the alpha-beta (p,T) phase diagram. Importantly, for alpha-AgIIF2 we have identified Ag-F bond stretching modes of B2g symmetry which induce modulated intervalence charge transfer (charge density wave) with unusually strong response to on-site Coulombic correlation [5].
Acknowledgment: K.T. and M.D. acknowledges the ERDF, Research and Innovation Operational Programme, for project No. ITMS2014+: 313011W085; the Slovak Research and Development Agency, grant No. APVV-18-0168 and Scientific Grant Agency of the Slovak Republic, grant No. VG 1/0223/19. WG acknowledges Polish National Science Center for Maestro grant (UMO-2017/26/A/ST5/00570). The calculations were carries out using Aurel supercomputing facility in CC of Slovak Academy of Sciences acquired in projects ITMS 26230120002 and 26210120002 funded by ERDF and the Interdisciplinary Centre for Mathematical and Computational Modelling, University of Warsaw under grant no. ADVANCE++ (GA76-19).
References:
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[2] C. Shen, B, Žemva, G. M. Lucier, O. Graudejus, J. A. Allman, N. Bartlett, Inorg. Chem., 38, 4570 (1999).
[3] C. Franchini, G. Kresse, and R. Podloucky, Phys. Rev. Lett. 102, 256402 (2009).
[4] K. Parlinski, Phys. Rev. B, 98, 054305 (2018).
[5] K. Tokár, M. Derzsi, W. Grochala, arXiv:2008.03081 (2020).
