On the Ordered State of Nickelous Formate Dihydrate

The behavior of the magnetic ions in nickelous formate dihydrate, Ni(HCOO)(2)center dot 2H(2)O, below its transition temperature is discussed theoretically. Expectation values of the spins at the two inequivalent sites in this crystal are calculated and expressions for paramagnetic and antiferromagnetic susceptibilities are derived. Crystal-field parameters and the exchange integral which appear in the spin Hamiltonian for the present system are estimated from experimental data. If the same approximation as was used in the case of dihydrated formates of Mn(++) and Fe(++) is adopted, it is concluded that the spins for Ni (HCOO)(2)center dot 2H(2)O at the two inequivalent sites in its antiferromagnetic state have different expectation values, 0.75 and 0.40. The coexistence of both paramagnetic and antiferromagnetic states is shown to be impossible.