New Ideas for Understanding the Structure and Magnetism in AgF₂: Prediction of Ferroelasticity

Abstract

In the search for new high‐temperature superconductors, it has been proposed that there are strong similarities between the fluoroargentate AgF2 and the cuprate La2CuO4. We explored the origin of the possible layered structure of AgF2 by studying its parent high‐symmetry phase and comparing these results with those of a seemingly analogous cuprate, CuF2. Our findings first stress the large differences between CuF2 and AgF2. Indeed, the parent structure of AgF2 is found to be cubic, naturally devoid of any layering, even though Ag2+ ions occupy trigonal sites that, nevertheless, allow the existence of a Jahn‐Teller effect. The observed Pbca orthorhombic phase is found when the system is cooperatively distorted by a local E⊗e trigonal Jahn‐Teller effect around the silver sites that creates both geometrical and magnetic layering. While the distortion implies that two Ag2+−F− bonds increase their distance by 15 % and become softer, our simulations indicate that covalent bonding and interlayer electron hopping is strong, unlike the situation in cuprate superconductors, and that, in fact, exfoliation of individual planes might be a harder task than previously suggested. As a salient feature, these results prove that the actual magnetic structure in AgF2 is a direct consequence of vibronic contributions involved in the Jahn‐Teller effect. Finally, our findings show that, due to the multiple minima intrinsic to the Jahn‐Teller energy surface, the system is ferroelastic, a property that is strongly coupled to magnetism in this argentate.

Red Shift in Optical Excitations on Layered Copper Perovskites under Pressure: Role of the Orthorhombic Instability

“…A similar situation holds [45] for CdCl 2 :Cu 2 + where a trigonal host lattice still keeps a degenerate electronic ground state that makes possible the development of a static JT effect. [28,68] The experimental g-tensor at 77 K for NaCl:Cu 2 + (g k = 2.373, g ⊥ = 2.070) [37] clearly proves that the ligand octahedron is elongated and thus the unpaired electron is placed in a j x 2 À y 2 i molecular orbital of the CuCl 6 4À unit, transforming like x 2 -y 2 , for the center whose main axis is parallel to Z (Figure 2). [17,27,66] Above 100 K the EPR spectrum is isotropic with g = (2 g ⊥ + g k )/3 as a result of incoherent hopping among the three equivalent distortions.…”

Pressure Effects on 3dn (n=4, 9) Insulating Compounds: Long Axis Switch in Na3MnF6 not Due to the Jahn‐Teller Effect

“…In the study of a d n (n=4, 9) compounds it has proved to be useful to determine the actual structure once each cation is replaced by another one with a similar ionic radius but a more spherical electronic density [24,28,29,33,51] . As during this process, giving rise to the so‐called parent phase, the space group does not change, the symmetry of the parent phase is necessarily equal or higher than that of the initial d n (n=4, 9) compound.…”

“…Results on the parent phase Na 3 FeF 6 pure and doped with Mn 3 + at zero pressure In the study of a d n (n = 4, 9) compounds it has proved to be useful to determine the actual structure once each cation is replaced by another one with a similar ionic radius but a more spherical electronic density. [24,28,29,33,51] As during this process, giving rise to the so-called parent phase, the space group does not change, the symmetry of the parent phase is necessarily equal or higher than that of the initial d n (n = 4, 9) compound. For example, for CuF 2 (monoclinic P2 1 /n space group) the Cu 2 + !Zn 2 + substitution leads to a ZnF 2 parent phase belonging to the tetragonal P4 2 /mnm space group.…”

“…[9,10] Indeed, the JT effect requires the existence of an initial geometry where the ground state of the complex is actually degenerate. This condition is fulfilled under cubic or trigonal symmetry, [9] and thus a static JT effect has been observed in cases like KZnF 3 : Cu 2 + , [11][12][13] d 9 ions (Ni + , Cu 2 + , Ag 2 + ) doped NaCl, [14][15][16][17][18][19][20] A-(H 2 O) 6 SiF 6 : Cu 2 + (A = Mg, Zn) [21][22][23]17] or AgF 2 [24,25] but not in K 2 ZnF 4 :Cu 2 + [26] where the host lattice is tetragonal. [27] This key argument thus makes difficult to assume the existence of a JT effect in crystals like Na 3 MnF 6 , (NH 4 ) 2 Cu(H 2 O) 6 (SO 4 ) 2 , CuF 2 (H 2 O) 2 (pyz), or CuWO 4 , whose structures are either monoclinic or triclinic.…”

“…For this reason, in the study of Na 3 MnF 6 it is also useful to explore how the structure evolves when all Mn 3 + cations are replaced by a cation with a similar ionic radius but a more spherical electronic density, keeping the same space group. This is just the so-called parent phase previously used [24,28,29] as a good starting point for understanding the structure and ground state of compounds like AgF 2 , CuF 2 , CrF 2 , or K 2 CuF 4 .…”