Tuning of the exchange interaction and the Curie temperature by mixed crystal formation of the bridging anionic ligands.

Mixed crystals with the composition [Co(NCS)x(NCSe)2-x(pyridine)2]n were prepared from solution and by annealing of Co(NCS)x(NCSe)2-x(pyridine)4. With increasing selenocyanate content, an increase of the magnetic exchange constant and of the critical temperature (Curie temperature) is observed, which offers a rational route to control these parameters in detail, without changing the metal cations.