Significance: Antiferromagnets can host strong quantum fluctuations in their ground state if they combine both low dimensionality and low spin. Materials based on copper oxides (spin-1/2 ions in layered or 1D structures) are unique in optimizing the tendency to strong quantum fluctuations. As a bonus, they show extremely large magnetic interactions, which lead to interesting quantum effects at relatively high temperatures as anomalous transport properties and high- T c superconductivity in doped systems. Obtaining similar features with other ions has been a long-standing goal. We show that silver and fluorine (which are next to copper and oxygen in the periodic table) in the commercial compound A g F 2 reach the goal, paving the way for a different generation of quantum materials.

Crystal structure and magnetic properties of K₂AgF₄, related to recently studied Cs₂AgF₄, have been scrutinized. It crystallizes orthorhombic (Cmca No.64) with a = 6.182(3) Å, b = 12.632(5) Å, c = 6.436(3) Å (Z = 4, V = 502.6(7) ų). K₂AgF₄ exhibits slightly puckered [AgF₂] sheets and a compressed octahedral coordination of Ag(II) and it is not isostructural to related Cs₂AgF₄. Violet–coloured K₂AgF₄ orders ferromagnetically below 26 K. The DFT calculations reproduce semiconducting properties and ferromagnetism of K₂AgF₄ at the LSDA + U level but only if substantial values of Mott–Hubbard on-site electron–electron repulsion energies for Ag and F are used in calculations. We have also succeeded to solve the crystal structure of a brown KAgF₃ (1D antiferromagnet below 64 K; GdFeO₃–type, PnmaNo. 62, a = 6.2689(2) Å, b = 8.3015(2) Å, c = 6.1844(2) Å, Z = 4, V = 321.84(2) ų) and to prepare K₃Ag₂F₇, a novel KAgF₃/K₂AgF₄ intergrowth phase and a member of the Ruddlsden–Popper K_nAgF_{n +2} series (n = 1.5). Dark brown K₃Ag₂F₇ crystallizes orthorhombic (K₃Cu₂Cl₇-type, CccaNo. 68, setting 2) with a = 20.8119(14) Å, b = 6.3402(4) Å, c = 6.2134(4) Å (Z = 4, V = 819.87(9) ų).