Characterization of M(H2PO4)2·2H2O (M = Mn, Co, Ni) and their in situ thermal decomposition by magnetic measurements

It has been established that M(H2PO4)2·2H2O (M = Mn, Co, Ni) are paramagnetics between 173 and 353 K with weak antiferromagnetic exchange interaction between the metal ions. In situ magnetic measurements during the thermal decomposition of the salts show that the oxidation state and the octahedral coordination of M2+ are preserved. From the data obtained it could be supposed that in M(H2PO4)2·2H2O (M = Co, Ni) this process is topotactic with no long-range diffusion transport. In Mn(H2PO4)2·2H2O, the formation of the large variety of intermediate products probably requires more drastic rearrangement and diffusion of the manganese ions during the complex transformations, which reflect on both the value and the sign of the θ constants. M2P4O12 (M = Mn, Co, Ni), which are the final decomposition products of the corresponding dihydrogen phosphates are paramagnetics in the temperature range of 295–573 K with antiferomagnetic interactions between the metal ions. The lattice parameters of Ni(H2PO4)2·2H2O have been calculated. It crystallizes in the monoclinic system with a = 7.228(1) Å; b = 9.778(1) Å; c = 5.306(1) Å; β = 94.50(1)°, SG P21/n with Z = 2.