Magnetic properties of KNa M Si₄ O₁₀ compounds (M = Mn, Fe, Cu)

In the present work the synthesis and magnetic properties of three compounds with formula KNa M Si₄ O₁₀(M = Mn, Fe, Cu) are described. These compounds are synthetic analogs to natural occurring minerals: fenaksite-Fe^{2 +}, litidionite-Cu^{2 +} and manaksite-Mn^{2 +}. The crystal structure consists of complex silicate chains interconnected by edge-sharing M O₅ square pyramids dimerized in M₂ O₈ units. This charged metal-silicate framework is compensated by monovalent alkali metals (K⁺, Na⁺). Despite the isostructural nature of these compounds, and the consequent similarity of the M-O topology, that rules the magnetic properties, these are quite different. While there are antiferromagnetic (AF) interactions within the Mn and Cu dimers (exchange interaction J = - 3.83 (1) and - 2.86 (3) K, respectively) with no long range order, a ferromagnetic interaction within Fe dimers (J =+ 7.6 (1) K) is observed with a three-dimensional transition at 9 K to an AF ground state. The magnetic behavior is analyzed using the HDVV (Heisenberg-Dirac-Van Vleck) formalism and discussed in the light of the crystal structure.