J. Phys. Chem. 1984, 88, 3701
3701
Mike Sienko and Colloque Weyl
In 1946, a night letter from Professor R. A. Ogg, Jr., Stanford University, to the Physical Review announced a startling discovery to a skeptical scientific community. Superconductivity had been observed at the temperature of liquid air! A ring sample of solution, made by dissolving metallic sodium in liquid ammonia, on being quenched in liquid air in a magnetic field, showed, on interruption of the field, a persistent electric current that did not decay with time. The impact of such a discovery was obvious. At that time M. J. Sienko was a 23-year-old graduate student at Berkeley, working on the Manhattan project for his Ph.D. Thesis under the direction of Professors W. M. Latimer and E. D. Eastman. At Ogg’s invitation, Sienko went to Stanford in July of 1946 to study the strange phenomena observed in metal-ammonia solutions. The result was a failure; neither Sienko, nor anyone else in the world, could reproduce the remarkable superconducting effect, which even today remains unaccounted for. However, Sienko’s interest in metal-ammonia solutions-their ability to change abruptly from nonmetallic to metallic and their remarkable separation into coexisting metal and nonmetal layers-was aroused. Professor Ogg’s famous experiments thus provided the impetus and platform for more than three decades of inspiration and example from Mike Sienko. Toward the end of his postdoctoral period at Stanford, Sienko received a telegraphed invitation from Professor P. J. W. Debye, then Chairman of the Chemistry Department a t Cornell, who offered him the position of instructor in chemistry and the chance to teach freshman chemistry; the challenge was heartily accepted. Thus, Sienko returned to Cornell and entered the academic market place at the bottom of the ladder. Although charged with heavy teaching responsibilities (freshman chemistry, classes of up to 1200 students), Sienko established a research program to investigate the physical properties of metal-ammonia solutions. H e and his small group studied phase separation, conductivity, salt effects, surface tension, electrode processes, and the metal-nonmetal transition. In the late 1950’s, his interests also began to encompass solid-state systems, in particular the transition-metal oxide “bronzes”, where he utilized elegantly all his intuition and scientifically reasoned hunches developed from his metal-ammonia studies. In 1963, while heavily immersed in the transition-metal oxide work, Sienko was invited by Professor Gerard Lepoutre to present a lecture a t the first Colloque Weyl, held at Lille, France. Working with data from some unpublished theses, and under the valuable stimulus of his Cornell colleague Professor Ben Widom, Sienko was led to the startling discovery that the two-layer coexistence curves for Li-NH3, Na-NH,, and K-NH, were al-
most unique in exhibiting parabolic dependencies, whereas all other critical phenomena (and the best theoretical models) corresponded to cubic coexistence curves. For example, in the “lattice gas” model, as the dimensionality of the gas is increased from 1 to 2 to 3, the critical exponent for describing the temperature vs. composition curve changes, respectively, from infinity to 8 to 3.08. To get a critical exponent equal to 2, the dimensionality of the gas needs to be extended to a value of 4 or above. On this basis, Sienko suggested that the unique character of the metal-ammonia coexistence curve was due to extremely long-range electronic interactions. In 1963, he was also to show that the liquid-liquid phase separation in metal-ammonia solutions was intimately related to the metal-nonmetal transition. For this purpose, he invoked Sir Nevi11 Mott’s revolutionary ideas concerning the metal-nonmetal transition in condensed phases and calculated the critical conditions required for the metallization of a lattice of hydrogen-like donor states in liquid ammonia. The quantitative estimate (3.87 mole percent metal, MPM) was in remarkable agreement with the observed consolute concentration (4.12 MPM for Na-NHJ. Subsequently, with this author in 1978 and 1981, he suggested that the Mott description of the metal-nonmetal transition is appropriate also for supercritical alkali metals as well as “matrix-bound” (liquid ammonia) alkali metals, and that the Mott metallization criterion may indeed have universal applicability. In 1969, with P. Chieux, he carried out the classic investigation of the phase coexistence curve in sodium-ammonia solutions. This work was presented to the Colloque Weyl 11, held at Cornell University in June of 1969. Because of the uniqueness of the parabolic behavior for the coexistence curve (and the considerable irritation it was giving the theorists), Chieux and Sienko had undertaken a precision study of the exact shape of the sodium-ammonia phase diagram. There was no doubt. As the theorists insisted, the curve was indeed cubic near the critical temperature, but changed to parabolic some 1.7 OC away from the critical temperature. This was most reassuring to the theorists, but it opened up an intriguing new problem; the idea of large-scale clustering, or inhomogeneities in the metal-ammonia system. Were there physical clusters? Or are they no more than statistical entities? These vexing questions remain as controversial as ever! At the 1963 Lille meeting, Sienko also drew attention to the strange liquid-solid equilibrium behavior in lithium-ammonia solutions, posing the attractive possibility of the existence of a Li(NH3)40compound and the likelihood that the very large spatial extent of the “3s-like” valence electron function would lead to intriguing solid-state properties. By precise, cryogenic structural, electric, and magnetic studies during the period from 1963, Sienko and his collaborators were able to demonstrate that the exotic compounds Li(NH3)4and Ca(NH,)6 represent unique “expanded metals”, just on the metallic side of the metal-nonmetal transition. His most recent discovery, Li(CH,NH&, appears to be the first example of the elusive, paired-electron state right at the metalnonmetal transition. Sienko’s special scientific flair in all areas was always the pragmatic combination of scientifically reasoned hunches, coupled with skillful synthesis and precise characterization and measurement techniques. In essence, his contribution can be recorded in terms of bringing chemical insight to a physical problem; he contributed significantly to unravelling the chemistry and physics at the metal-nonmetal transition. A final, personal recollection. I first met Mike Sienko on August 5, 1975. I had arrived at Cornell from England blearyeyed and jet-lagged. After discussing my views on my research at Cornell, his parting words were “Enjoy yourself and make good notes!” I did. Mitchell J. Sienko died peacefully, after a brief illness, on the 4th of December, 1983. As Roald Hoffman has succinctly put it (Cornell Chemistry Newsletter, Fall 1983, Issue 3) “We miss him”. P. P. Edwards Cambridge, U. K.
