AN AB-INITIO STUDY OF THE MAGNETIC COUPLING IN BIMETALLIC CR-III CYANIDES

A number of bi-metallic Cr-III cyanides exhibiting high magnetic order ing temperatures have recently been reported. Here, first-principles t heory based on the unrestricted Hartree-Fock approximation, has been a pplied to the idealised compounds KM(II)[Cr-III(CN)(6)] (M(II) = V, Mn , Ni) and Cr-III[Cr-III(CN)(6)] in order to gain insight into the magn etic interactions. The computed magnetic ground state and ordering ene rgies are consistent with recent experimental observations of the pare nt materials. It is found that the metal-ligand interactions are ionic in nature. The large magnetic coupling between the metal d-orbitals c an be understood in terms of a superexchange model in which the orthog onality constraint is relaxed through delocalisation of the d-orbitals and polarisation of the CN group. The contribution of covalent intera ctions to the magnetic coupling is small. (C) 1997 Elsevier Science B. V.