New manganese pyrophosphates: The syntheses, crystallographic characterizations and magnetic properties of BaMnP 2O 7 and CaMnP 2O 7

Abstract

The new compound BaMnP 2O 7 was obtained in two crystallographic modifications, (monoclinic- 1) and (triclinic- 2), by heating mixtures of BaCO 3, P 2O 5 and MnO 2 to 1100°C and to 1000°C for 72 h, respectively. CaMnP 2O 7 ( 3) was obtained by heating a mixture of CaCO 3, P 2O 5 and MnO 2 to 1050°C for 48 h. Both crystallographic forms of BaMnP 2O 7 and CaMnP 2O 7 ( 3) were investigated by single-crystal X-ray diffraction analysis. The high-temperature monoclinic form of BaMnP 2O 7 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 MnO 6 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 Mn 2O 10 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 Mn 2+ 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 Mn 2O 10 units and a longer range weaker antiferromagnetic coupling, J′, between the neighbouring Mn 2O 10 units, | J′/ J| = 0.18 and 0.074 for 2 and 3, respectively.