Experimental realization of a unique class of compounds: XY-antiferromagnetic triangular lattices, KAg₂Fe[VO₄]₂ and RbAg₂Fe[VO₄]₂, with ferroelectric ground states

AAg₂Fe[VO₄]₂ with A = K or Rb was synthesized by solid state methods and characterized by thermodynamic properties measurements (Néel order at T_N ≈ 3.0 K). The respective nuclear (a^K,Rb = 5.48 Å, c^K = 7.212 Å, c^Rb = 7.357 Å, P3¯, Z = 1) and magnetic structures were refined using neutron diffraction in applied magnetic fields. The results indicate enhanced structural stability of the P3¯ structure and the realization of a frustrated triangular lattice with antiferromagnetic XY-anisotropies. Two magnetic structures were identified: a helical and chiral Y-phase with a propagation vector of (1/3, 1/3, ≈ 0.39) and a commensurate up-up-down phase. These unique compounds offer convenient experimental access for optimizing the features and properties of ferroaxial multiferroic materials.