Crystal structure of KMnPO4F with short- and long-range order inside the layered magnetic system

Abstract

Potassium manganese fluoride phosphate, KMnPO₄F, 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 P2₁2₁2₁ with a = 4.7884(2), b = 9.0172(4), c = 9.5801(4) Å, and Z = 4. Its crystal structure is composed of Mn³⁺O₄F square pyramids sharing vertices with PO₄ tetrahedra. This anionic framework is neutralized by K⁺ cations. As the temperature decreases, a short-range correlation state (T_max ∼ 35 K) of KMnPO₄F 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)/dT 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 μ₀H_spin–flop = 19 T. Density functional theory indicates that in variance with the three-dimensional network of KMnPO₄F, it is a two-dimensional Ising magnetic system represented by buckled layers of integer spins S = 2 of Mn³⁺ ions. The strongest AFM exchange interaction, J₁ ∼ −13 K, couples Mn³⁺ ions into linear chains running along the a axis. The chains themselves are ferromagnetically connected (J₃ ∼ –4 K) within the ab plane. The interplane AFM exchange interaction (J₂ ∼ −1 K) is weak and frustrated.