Crystal, electronic, and magnetic structures of M₂AgF₄ (M = Na–Cs) phases as viewed from the DFT+U method

Abstract

Theoretical investigations of the magneto-structural correlations of MAgF (M = Na-Cs) compounds show that they adopt two polymorphs, the layered perovskite and post-perovskite structures, which differ greatly in the connectivity of the Ag/F sub-lattice and hence in their magnetic properties. With the use of the DFT+U method, the relative stabilities of various MAgF phases were established and the collective JT effect within the Ag/F sub-lattice of these systems was modelled. Calculations show that for all studied stoichiometries, the preferred scenario of the collective JT effect in the layered perovskite phase corresponds to an antiferrodistortive order of elongated octahedra, which leads to 2D ferromagnetic coupling, in agreement with the experimental findings for the M = Cs, and Rb systems. The layered perovskite phase is found to be progressively destabilized with respect to the post-perovskite structure when moving from Cs to Na, again in agreement with the experimental findings. Our results strongly indicate that the layered polymorph of KAgF should not exhibit a ferrodistortive order of compressed octahedra, which contradicts the previous experimental results.

Changing the Usual Interpretation of the Structure and Ground State of Cu2+-Layered Perovskites

“…The present ideas can also be of interest to explain the properties of Ag 2+ layered compounds, like Cs2AgF4, deeply investigated in the last years 36,[83][84][85] . Calculations carried out on Cs2AgF4 support that the local structure and ground state of the AgF6 4complex arise from the same causes discussed in this work for K2CuF4 involving an axial internal electric field and a subsequent orthorhombic distortion in the layer plane.…”

Jahn–Teller and Non-Jahn–Teller Systems Involving CuF64– Units: Role of the Internal Electric Field in Ba2ZnF6:Cu2+ and Other Insulating Systems

“…In other words, similarly to Eq. (3) we can write for these cases 10Dq = 10Dqint(R) + 10Dqext (10) where the intrinsic contribution 10Dqint(R) is sensitive to the actual value of the metalligand distance, R.…”

“…The Jahn-Teller (JT) effect has been widely invoked in the last fifty years for explaining the properties of systems involving d 9 , d 7 and d 4 cations under octahedral coordination. Accordingly, experimental results on Cu 2+ systems like superconducting copper oxides or copper oxyfluorides and on silver(II) fluorides have often been analyzed assuming the existence of a JT effect [3][4][5][6][7][8][9][10][11] . Nevertheless, this assumption implies that there exist a high symmetry reference conformation possessing an orbitally degenerate electronic state, but this crucial condition has not been proved in all cases 2,12 .…”

The Jahn-Teller Distortion at High Pressure: The Case of Copper Difluoride

“…The Jahn-Teller effect, also present in compounds containing the iso-electronic Ag 2+ cation (4d 9 ) [5], has a large impact on material properties such as magnetism [6], and electronic structure [7][8][9]. Even subtle distortions in the first coordination sphere of the JT-active cation can lead to large changes in material properties, as exemplified by the case of Ag 2+ -bearing fluorides [10,11]. Due to the fluxional nature of the Jahn-Teller effect many studies have been devoted to tuning this distortion either by chemical substitution [12][13][14][15][16], or by high external pressure [17][18][19][20][21][22].…”