Polymorphism of Fluoroargentates(II): Facile Collapse of a Layered Network of α-K2AgF4 Due to the Insufficient Size of the Potassium Cation

Mechanochemical Synthesis and Magnetic Properties of the Mixed-Valent Binary Silver(I,II) Fluorides, AgI2AgIIF4 and AgIAgIIF3

Fluoridoargentates(II) represent a fascinating class of silver(II) compounds with structural and magnetic similarities to cuprate superconductors. However, their synthesis is challenging, leaving their properties largely underexplored and hindering the discovery of new phases. This study introduces mechanochemistry as a novel approach for the synthesis of fluoridoargentates(II), avoiding the use of anhydrous HF or elemental fluorine and employing readily available equipment. Notably, ball milling of commercially available precursors successfully produced the long-sought-after first two examples of binary mixed-valent silver(I,II) phases, AgI2AgIIF4 (Ag3F4) and AgIAgIIF3 (Ag2F3). While the AgI2AgIIF4 phase was obtained at room temperature, the AgIAgIIF3 phase is metastable and required milling under cryogenic conditions. Characterization by synchrotron powder X-ray and neutron diffraction revealed that AgI2AgIIF4 crystallizes in the P21/c space group and is isostructural to β-K2AgF4. In this crystal structure, [AgIIF2F4/2]2- distorted octahedral units with 4 + 2 coordination, extend parallel to a-crystallographic axis giving a quasi-one-dimensional canted antiferromagnetic character, as shown by magnetic susceptibility. The triclinic perovskite AgIAgIIF3 phase adopts the P1̅ space group, is isostructural to AgCuF3 and also shows features of a one-dimensional antiferromagnet. This mechanochemical approach, also successfully applied to synthesize β-K2AgF4, is expected to expand the field of silver(II) chemistry, accelerating the search for silver analogs to cuprate superconductors and potentially extending to other cations in unusual oxidation states.

Structural Phase Transitions and Magnetic Superexchange in MI AgII F3 Perovskites at High Pressure

Pressure-induced phase transitions of MI AgII F3 perovskites (M=K, Rb, Cs) have been predicted theoretically for the first time for pressures up to 100 GPa. The sequence of phase transitions for M=K and Rb consists of a transition from orthorhombic to monoclinic and back to orthorhombic, associated with progressive bending of infinite chains of corner-sharing [AgF6 ]4- octahedra and their mutual approach through secondary Ag⋅⋅⋅F contacts. In stark contrast, only a single phase transition (tetragonal→triclinic) is predicted for CsAgF3 ; this is associated with substantial deformation of the Jahn-Teller-distorted first coordination sphere of AgII and association of the infinite [AgF6 ]4- chains into a polymeric sublattice. The phase transitions markedly decrease the coupling strength of intra-chain antiferromagnetic superexchange in MAgF3 hosts lattices.

Phase Transitions and Amorphization of M2AgF4 (M = Na, K, Rb) Compounds at High Pressure

We report the results of high-pressure Raman spectroscopy studies of alkali metal fluoroargentates (M2AgF4, where M = Na, K, Rb) combined with theoretical and X-ray diffraction studies for the K member of the series. Theoretical density functional calculations predict two structural phase transitions for K2AgF4: one from low-pressure monoclinic P21/c (beta) phase to intermediate-pressure tetragonal I42d structure at 6 GPa, and another to high-pressure triclinic P1 phase at 58 GPa. However, Raman spectroscopy and X-ray diffraction data indicate that both polymorphic forms of K2AgF4, as well as two other fluoroargentate phases studied here, undergo amorphization at pressures as low as several GPa.

Fluorides of Silver Under Large Compression*

The silver-fluorine phase diagram has been scrutinized as a function of external pressure using theoretical methods. Our results indicate that two novel stoichiometries containing Ag⁺ and Ag²⁺ cations (Ag₃ F₄ and Ag₂ F₃ ) are thermodynamically stable at ambient and low pressure. Both are computed to be magnetic semiconductors under ambient pressure conditions. For Ag₂ F₅ , containing both Ag²⁺ and Ag³⁺ , we find that strong 1D antiferromagnetic coupling is retained throughout the pressure-induced phase transition sequence up to 65 GPa. Our calculations show that throughout the entire pressure range of their stability the mixed-valence fluorides preserve a finite band gap at the Fermi level. We also confirm the possibility of synthesizing AgF₄ as a paramagnetic compound at high pressure. Our results indicate that this compound is metallic in its thermodynamic stability region. Finally, we present general considerations on the thermodynamic stability of mixed-valence compounds of silver at high pressure.

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

Theoretical analysis of various polymorphic forms of a series of four alkali metal fluoroargentates(ii), M₂AgF₄ (M = Na–Cs), helped to establish clear trends of crystal structures and magnetic properties across the alkali metal series.

Crystal and electronic structures and high-pressure behavior of AgSO4, a unique narrow band gap antiferromagnetic semiconductor: LDA(+U) picture

We demonstrate that DFT calculations performed with the local density approximation (LDA) allow for significantly better reproduction of lattice constants, the unit cell volume and the density of Ag(II)SO(4) than those done with generalized gradient approximation (GGA). The LDA+U scheme, which accounts for electronic correlation effects, enables the accurate prediction of the magnetic superexchange constant of this strongly correlated material and its band gap at the Fermi level. The character of the band gap places the compound on the borderline between a Mott insulator and a charge transfer insulator. The size of the band gap (0.82 eV) indicates that AgSO(4) is a ferrimagnetic semiconductor, and possibly an attractive material for spintronics. A bulk modulus of 27.0 GPa and a compressibility of 0.037 GPa(-1) were determined for AgSO(4) from the third-order Birch-Murnaghan isothermal equation of state up to 20 GPa. Several polymorphic types compete with the ambient pressure P-1 phase as the external pressure is increased. The P-1 phase is predicted to resist pressure-induced metallization up to at least 20 GPa.