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.

Crystal structure, lattice dynamics and superexchange in MAgF3 1D antiferromagnets (M = K, Rb, Cs) and a Rb3Ag2F7 Ruddlesden–Popper phase

With the use of lattice dynamics calculation within the hybrid HSE06 framework we were able to understand the vibrational spectra of MAgF3 (M = K, Rb, Cs) compounds.

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.

Local structure and excitations in systems with CuF64− units: lack of Jahn–Teller effect in the low symmetry compound Na2CuF4

The lack of a Jahn–Teller effect in Na₂CuF₄ is illustrated by the anisotropy of the Na₂ZnF₄ parent lattice.

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.