NEW MANGANESE PYROPHOSPHATES - THE SYNTHESES, CRYSTALLOGRAPHIC CHARACTERIZATIONS AND MAGNETIC-PROPERTIES OF BAMNP2O7 AND CAMNP2O7

The new compound BaMnP2O7 was obtained in two crystallographic modific ations, (monoclinic-1) acid (triclinic-2), by heating mixtures of BaCO 3, P2O5 and MnO2 to 1100 degrees C and to 1000 degrees C for 72 h, res pectively. CaMnP2O7 (3) was obtained by heating a mixture of CaCO3, P2 O5 and MnO2 to 1050 degrees C for 48 h. Both crystallographic forms of BaMnP2O7 and CaMnP2O7 (3) were investigated by single-crystal X-ray d iffraction analysis. The high-temperature monoclinic form of BaMnP2O7 could not be obtained free of the low-temperature triclinic form in th e bulk form. In monoclinic-1 the manganese ions exist in a distorted M nO6 octahedron surrounded by five closely and one remotely positioned oxygen atoms. In triclinic-2 and -3 forms the manganese ions are assoc iated in pairs by the formation of Mn2O10 units that share one edge of two adjacent octahedra. The magnetic properties of the triclinic-2 an d -3 forms were also investigated. The effective magnetic moments, mu( eff), are 5.7 B.M. and 5.8 B.M./Mn atom for triclinic-2 and -3, respec tively, and are consistent with a high-spin Mn2+ ion in an octahedral environment with five unpaired electrons. The temperature-dependent ma gnetic measurements of 2 and 3 have revealed a combination of short-ra nge 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.