An ab initio study of the magnetic coupling in bi-metallic CrIII cyanides

A number of bi-metallic CrIII cyanides exhibiting high magnetic ordering temperatures have recently been reported. Here, first-principles theory based on the unrestricted Hartree-Fock approximation, has been applied to the idealised compounds KMII [CrIII(CN)6] (MII  V, Mn, Ni) and CrIII [CrIII(CN)6] in order to gain insight into the magnetic interactions. The computed magnetic ground state and ordering energies are consistent with recent experimental observations of the parent materials. It is found that the metal-ligand interactions are ionic in nature. The large magnetic coupling between the metal d-orbitals can be understood in terms of a superexchange model in which the orthogonality constraint is relaxed through delocalisation of the d-orbitals and polarisation of the CN group. The contribution of covalent interactions to the magnetic coupling is small.