Fate of doped carriers in silver fluoride cuprate analogs

Abstract

AgF2 is a correlated charge-transfer insulator with properties remarkably similar to insulating cuprates which have raised hope that it may lead to a new family of unconventional superconductors upon doping. We use ab initio computations to study doping strategies leading to metallization. Because the upper Hubbard band is very narrow electron doping leads to undesired strongly selftrapped states (polarons). For the hole-doped case, polaron tendency is stronger than for cuprates but still moderate enough to expect that heavily doped compounds may become metallic. Since the strong electron lattice coupling originates in the strong buckling we study also an hypothetically flat allotrope and show that it has excellent prospect to become metallic. We compare the AgF2 behavior with that for the hole-doped conventional cuprate La2CuO4 and electron-doped Nd2CuO4. Our results show a clear path to achieve high temperature superconductivity in silver fluorides.

Defect Trapping and Phase Separation in Chemically Doped Bulk AgF2

“…In our recent study of polarons in AgF 2 , lattice self-trapping of defects induced by the modified electron count in the unit cell was also observed for bulk AgF 2 but, importantly, to a lesser extent in hypothetical polymorphs with flat monolayers. 6 This is also in line with strong covalence of Ag−F bonds, well documented in previous studies, 35 which leads to strong vibronic coupling and in turn increases the tendency for trapping of defects. Another thing to consider is that the type of doping studied here, while in principle similar to chemical modifications of cuprates, is by definition more local: the fluorine defects/interstitials ultimately have to occupy a particular position in the unit cell, different from any of the original F atoms in the stoichiometric AgF 2 because of symmetry constraints (or rather, the appearance of the said modifications breaks some of those constraints).…”

“…Our conclusions reached here using "chemical" models are worth to be compared to a parallel study on polaronic states in stoichiometric AgF 2 polytypes using external charge doping. 6 Moreover, there are also different pathways toward cupratelike properties in AgF 2 that are currently being exploredin particular, a flat AgF 2 monolayer stabilized and doped by using an appropriate substrate for epitaxy. 7 In addition, because our models always contain the inversion center and because we discuss only the lowest energy solutions, it is not improbable that in the real-life experiment, some metastable metallic structures could be obtained (cf.…”

“…This has already been attempted by our group in bulk AgF2 and in hypothetical monolayers. [6] The latter, although as yet unknown, could in principle be obtained by epitaxy. [7] Another approach is to synthesize a material with a modified (e.g.…”

“…This has already been attempted by our group in bulk AgF 2 and in hypothetical monolayers. 6 The latter, although as yet unknown, could in principle be obtained by epitaxy. 7 Another approach is to synthesize a material with a modified (e.g., nonstoichiometric) composition, leading to mixed (or intermediate) valence of one of its constituent elements.…”

Theoretical study of ternary silver fluorides AgMF4 (M = Cu, Ni, Co) formation at pressures up to 20 GPa

“…Recent theoretical investigations suggest that electron doping to AgF 2 would not lead to a stable metallic phase due to a lattice polaronic effect and charge localization. 6 Therefore the remaining way to achieve electron doping in AgF 2 is to try partial isostructural substitution of Ag( ii ). By this, we mean to replace square planar Ag( ii ) in AgF 2 with another redox-active metal cation, M( ii ), so that the layered structure is preserved but the electrons may partly flow from M( ii ) to Ag( ii ).…”