Crystal structure of KMnPO 4 F with short- and long-range order inside the layered magnetic system.

Potassium manganese fluoride phosphate, KMnPO 4 F, has been obtained through mild hydrothermal synthesis and characterized by scanning electron microscopy, microprobe analysis and X-ray diffraction. The compound possesses an orthorhombic symmetry and chiral space group P 2 1 2 1 2 1 with a = 4.7884(2), b = 9.0172(4), c = 9.5801(4) Å, and Z = 4. Its crystal structure is composed of Mn 3+ O 4 F square pyramids sharing vertices with PO 4 tetrahedra. This anionic framework is neutralized by K + cations. As the temperature decreases, a short-range correlation state ( T max ∼ 35 K) of KMnPO 4 F is formed, followed by the establishment of antiferromagnetic (AFM) long-range order at T N = 25 K. The latter is marked by sharp λ-type anomalies in both Fisher's specific heat d( χ ‖ T )/d T and heat capacity C p . Pulsed magnetic field measurements on the single crystals identify the a axis as the easy magnetic axis and reveal a spin-flop transition at μ 0 H spin-flop = 19 T. Density functional theory indicates that in variance with the three-dimensional network of KMnPO 4 F, it is a two-dimensional Ising magnetic system represented by buckled layers of integer spins S = 2 of Mn 3+ ions. The strongest AFM exchange interaction, J 1 ∼ -13 K, couples Mn 3+ ions into linear chains running along the a axis. The chains themselves are ferromagnetically connected ( J 3 ∼ -4 K) within the ab plane. The interplane AFM exchange interaction ( J 2 ∼ -1 K) is weak and frustrated.