Frustration and 120 degrees Magnetic Ordering in the Layered Triangular Antiferromagnets AFe(PO3F)(2) (A = K, (NH4)(2)Cl, NH4, Rb, and Cs)

A new family of oxofluorophoshates, AFe(PO3F)(2) (A = K, (NH4)(2)Cl, NH4, Rb, and Cs), was synthesized via ionothermal methods using PF6 ionic liquids. Single-crystal and powder X-ray diffraction reveal that AFe(PO3F)(2) with A = (NH4)(2)Cl crystallizes in a trigonal structure, while AFe(PO3F)(2) with A = NH4, Rb, and Cs crystallizes in a triclinic structure. Dimorphic KFe(PO3F)(2) crystallizes in both the trigonal and triclinic forms. The structures of all compounds feature Yavapaiite-like Fe(PO3F)(2) slabs, which are characterized by triangular Fe layers, planar in the case of the trigonal structure and undulated in the case of the triclinic one. Magnetization measurements reveal all compounds to order antiferromagnetically at low temperatures. The trigonal phases AFe(PO3F)(2) (A = K and (NH4)(2)Cl) display complex magnetic H-T phase diagrams. The observation of magnetization plateaus at M-sat/3 (M-sat = saturation magnetization) indicates the existence of the up-up-down (UUD) and V-phases at applied magnetic fields in the magnetically ordered state. Powder neutron diffraction measurements of KFe(PO3F)(2) confirm the 120 degrees spin structure at zero fields. Along c, the magnetic moments form a commensurate spiral since the spins in each plane are rotated by 90 degrees with respect to the adjacent one. To our knowledge, this is the first time such a non-centrosymmetric version of the 120 degrees spin structure with a 90 degrees rotation between nearest planes has been reported.