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

Novel ternary AgIICoIIIF5 fluoride: synthesis, structure and magnetic characteristics

We present a new compound in the silver-cobalt-fluoride system, featuring paramagnetic silver (d9) and high-spin cobalt (d6), synthesized by solid-state method in an autoclave under F2 overpressure. Based on powder X-ray diffraction, we determined that AgIICoIIIF5 crystallizes in a monoclinic system with space group C2/c. The calculated fundamental band-gap falls in the visible range of the electromagnetic spectrum, and the compound has the character of charge-transfer insulator. AgCoF5 is likely a ferrimagnet with one predominant superexchange magnetic interaction constant between mixed spin cations (Ag-Co) of -62 meV (SCAN result). Magnetometric measurements conducted on a powdered sample allowed the identification of a transition at 128 K, which could indicate magnetic ordering.

A Unique Two-Dimensional Silver(II) Antiferromagnet Cu[Ag(SO4 )2 ] and Perspectives for Its Further Modifications

Copper(II) silver(II) sulfate crystallizes in a monoclinic CuSO4 -related structure with P21 /n symmetry. This quasi-ternary compound features Ag(SO4 )2 2- layers, while the remaining cationic sites may be occupied either completely or partially by Cu2+ cations, corresponding to the formula of (Cux Ag1-x )[Ag(SO4 )2 ], x=0.6-1.0. CuAg(SO4 )2 is antiferromagnetic with large negative Curie-Weiss temperature of -140 K and shows characteristic ordering phenomenon at 40.4 K. Density functional theory calculations reveal that the strongest superexchange interaction is a two-dimensional antiferromagnetic coupling within Ag(SO4 )2 2- layers, with the superexchange constant J2D of -11.1 meV. This renders CuAg(SO4 )2 the rare representative of layered Ag2+ -based antiferromagnets. Magnetic coupling is facilitated by the strong mixing of Ag d(x2 -y2 ) and O 2p states. Calculations show that M2+ sites in MAg(SO4 )2 can be occupied with other similar cations such as Zn2+ , Cd2+ , Ni2+ , Co2+ , and Mg2+ .

Unexpected Coexisting Solid Solutions in the Quasi-Binary Ag(II) F2 /Cu(II) F2 Phase Diagram

High-temperature solid-state reaction between orthorhombic AgF2 and monoclinic CuF2 (y=0.15, 0.3, 0.4, 0.5) in a fluorine atmosphere resulted in coexisting solid solutions of Cu-poor orthorhombic and Cu-rich monoclinic phases with stoichiometry Ag1-x Cux F2 . Based on X-ray powder diffraction analyses, the mutual solubility in the orthorhombic phase (AgF2  : Cu) appears to be at an upper limit of Cu concentration of 30 mol % (Ag0.7 Cu0.3 F2 ), while the monoclinic phase (CuF2  : Ag) can form a nearly stoichiometric Cu : Ag=1 : 1 solid solution (Cu0.56 Ag0.44 F2 ), preserving the CuF2 crystal structure. Experimental data and DFT calculations showed that AgF2  : Cu and CuF2  : Ag solid solutions deviate from the classical Vegard's law. Magnetic measurements of Ag1-x Cux F2 showed that the Néel temperature (TN ) decreases with increasing Cu content in both phases. Likewise, theoretical DFT+U calculations for Ag1-x Cux F2 showed that the progressive substitution of Ag by Cu decreases the magnetic interaction strength |J2D | in both structures. Electrical conductivity measurements of Ag0.85 Cu0.15 F2 showed a modest increase in specific ionic conductivity (3.71 ⋅ 10-13 ±2.6 ⋅ 10-15  S/cm) as compared to pure AgF2 (1.85 ⋅ 10-13± 1.2 ⋅ 10-15  S/cm), indicating the formation of a vacancy- or F adatom-free metal difluoride sample.

Superexchange interactions in AgMF4 (M = Co, Ni, Cu) polymorphs

Magnetic properties of silver(II) compounds have been of interest in recent years. In covalent compounds, the main mechanism of interaction between paramagnetic sites is the superexchange via the connecting ligand. To date, little is known of magnetic interactions between Ag(II) cations and other paramagnetic centres. It is because only a few compounds bearing a Ag(II) cation and other paramagnetic transition metal cation are known from experimental work. Recently the high-pressure synthesis of ternary silver(II) fluoridometallates with 3d metal cations AgMF4 (M = Co, Ni, Cu) was predicted to be feasible. Here, we investigate the magnetic properties of these compounds in their diverse polymorphic forms. Using well-established computational methods we predict superexchange pathways in AgMF4 compounds, evaluate coupling constants and calculate the impact of the Ag(II) presence on superexchange between the other cations. The results indicate that the low-pressure form of AgCuF4, the only one composed of stacked layers like the parent AgF2, would show mainly Ag–Ag and Cu–Cu superexchange interactions. Upon compression, or with the nickel(II) cation, the Ag–M interactions in AgMF4 compounds are intensified, which is emphasized by an increase of Ag–M superexchange coupling constants and Ag–F–M angles. All the strongest Ag–M superexchange pathways are quasi-linear, leading to the formation of antiferromagnetic chains along the crystallographic directions. The impact of Ag(II) on M–M superexchange turns out to be moderate, due to factors connected to the crystal structure.