New manganese pyrophosphates: The syntheses, crystallographic characterizations and magnetic properties of BaMnP2O7 and CaMnP2O7

The new compound BaMnP₂O₇ was obtained in two crystallographic modifications, (monoclinic-1) and (triclinic-2), by heating mixtures of BaCO₃, P₂O₅ and MnO₂ to 1100°C and to 1000°C for 72 h, respectively. CaMnP₂O₇ (3) was obtained by heating a mixture of CaCO₃, P₂O₅ and MnO₂ to 1050°C for 48 h. Both crystallographic forms of BaMnP₂O₇ and CaMnP₂O₇ (3) were investigated by single-crystal X-ray diffraction analysis. The high-temperature monoclinic form of BaMnP₂O₇ could not be obtained free of the low-temperature triclinic form in the bulk form. In monoclinic-1 the manganese ions exist in a distorted MnO₆ octahedron surrounded by five closely and one remotely positioned oxygen atoms. In triclinic-2 and -3 forms the manganese ions are associated in pairs by the formation of Mn₂O₁₀ units that share one edge of two adjacent octahedra. The magnetic properties of the triclinic-2 and -3 forms were also investigated. The effective magnetic moments, μ_eff, are 5.7 B.M. and 5.8 B.M./Mn atom for triclinic-2 and -3, respectively, and are consistent with a high-spin Mn²⁺ ion in an octahedral environment with five unpaired electrons. The temperature-dependent magnetic measurements of 2 and 3 have revealed a combination of short-range antiferromagnetic coupling, J, between the two Mn ions within the Mn₂O₁₀ units and a longer range weaker antiferromagnetic coupling, J′, between the neighbouring Mn₂O₁₀ units, |J′/J| = 0.18 and 0.074 for 2 and 3, respectively.