ar X iv : c on d - m at / 9 90 61 84 v 1 1 3 Ju n 19 99 K S Krishnan and the early experimental evidences for the Jahn - Teller Theorem

Jahn-Teller theorem, proposed in 1937, predicts a distortional instability for a molecule that has symmetry based degenerate electronic states. In 1939 Krishnan emphasized the importance of this theorem for the arrangement of water molecules around the transition metal or rare earth ions in aqueous solutions and hydrated saltes, in a short and interesting paper published in Nature by pointing out atleast four existing experimental results in support of the theorem. This paper of Krishnan has remained essentially unknown to the practitioners of Jahn-Teller effect, eventhough it pointed to the best experimental results that were available, in the 30’s and 40’s, in support of Jahn-Teller theorem. Some of the modern day experiments are also in conformity with some specific suggestions of Krishnan. Jahn-Teller effect is a beautiful and simple quantum phenomenon that occurs in molecules, transition metal complexes as well as solids containing transition metal or rare earth ions. It states roughly that ‘a localized electronic system that has a symmetry based orbital degeneracy, will tend to lift the degeneracy by a distortion that results in the reduction of the symmetry on which the degeneracy is based.’ In isolated systems such as a molecule or a transition metal complex it is a dynamical or quasi static phenomenon. They are called dynamic and static Jahn-Teller effects. When it occurs co-operatively in crystals it is a spontaneous symmetry breaking phenomenon and a crystal structure change. This is called a co-operative Jahn-Teller effect. Even before Jahn-Teller theorem appeared, Krishnan and collaborators performed a series of pioneering magneto crystalline anisotropy study of families of paramagnetic salts containing transition metal and rare earth ions lending good support to various new quantum mechanical ideas including those of Bethe, Kramers and Van Vleck on crystal field splitting and magneto crystalline anisotropy. In the biographical Memoirs of the Royal Society of London, K. Lonsdale and H.J. Bhaba wrote: ‘The papers published by Krishnan during this period (30’s) in collaboration with B C Guha, S Banerjee and N C Chakravarty were the foundation stones of the modern fields of crystal magnetism and magnetochemistry’. In the paramagnetic salts that Krishnan and collaborators studied the magnetic ions are well separated from each other by the intervening water molecules and also anion groups. As a result any direct or superexchange or dipolar interactions between the magnetic moments are weak; consequently any co-operative magnetic order such as antiferrmomagnetic order is pushed down to very low temperatures below 1 K. This enabled Krishnan and others to study in detail the magnetic properties of essentially isolated paramagnetic ions in various crystal field environments. Jahn-Teller theorem and its experimental consequences has been studied in great detail in chemistry and physics particularly in the context of electron spin resonance experiments. In the simplest of transition metal complexes there is a cubic (octahedral) environment around the transition metal ion such as Cu. The octahedral environment leads to a crystal field splitting of the five fold degenerate d level into an orbital doublet (eg) and a triplet (t2g). In Krishnan and collaborator’s work one notices repeated reference to deviation from cubic electric field at the center of rare earth ions in several cases and transition metal ions in some cases as inferred from their own anisotropic magnetic susceptibility measurements. The most obvious causes for departure from regular cubic symmetry of the coordination clusters are inequivalence of the ligand atoms in the first or second co-ordination shell and forces of crystal packing. No one suspected that electronic orbital degeneracy can lead to an asymmetry such as a distortion of the octahedra with equivalent ligand atoms. It is at this juncture the theoretical papers of Jahn and Teller appeared, which suggested another important cause for molecular asymmetry. And Krishnan readily appreciated the importance of this theorem for crystal field splitting and arrangement of water molecules around paramagnetic ions in aqueous solutions and wrote an interesting short paper in Nature in 1939 that we reproduce here. It is interesting that Van Vleck, who very much admired K S Krishnan, also developed his theory of Jahn-Teller effect for orbital doublets in paramagnetic ions in the same year. In his paper KSK quotes at least four existing experimental results that support the Jahn-Teller theorem: i) x-ray data that is consistant with a small deviation of the perfect H2O octahedra around the paramagnetic ion in hydrated salts ii) magnetic data, mostly from his group, that exhibits strong magnetic anisotropy that is similar in magnitudes in various salts suggesting that the cause for any distortion arises from the electronic state of the paramagnetic ions rather than the surrounding atoms iii) asymmetry inferred from electronic absorption spectra of cation surrounded by water molecules in aqueous solutions studied by Freed et al. iv) magnetic double refraction exhibited by the aqueous solution of these salts,