The crystal structure of the inorganic surface films formed on Mg and Li intercalation compounds and the electrode performance

A crystallographic approach was applied to elucidate the influence of the nature of the surface films on the electrochemical behavior of Li and Mg intercalation compounds. This paper presents two examples: (1) protection of graphite electrodes by Li₂CO₃ surface films, and (2) the unique electrochemical behavior of Mg-containing Chevrel phases (MgCP) obtained by different synthetic routes. In the former case, the elucidation of the protection mechanism and the explanation of the high performance of such protected electrodes are based on the analysis of possible Li-ion motion in the carbonate crystal structure. In the latter case, a combination of synthesis, electrochemistry and XRD analysis was used to explain an unusual phenomenon: the difference between the excellent electrochemical behavior of the Chevrel phase (CP) based on Cu-leached Cu₂Mo₆S₈ (CuCP), and the poor electrochemical activity of the high-temperature synthesized MgCP, with the same phase composition. It is shown that this phenomenon is caused by MgO formation on the surface of the latter material. The different surface chemistry of the MgCPs obtained by the two different synthetic routes was substantiated by revealing the correlation between the electrochemical activity and the chemical stability of these materials under ambient atmosphere conditions. Dedicated to Prof. Mikhail A. Vorotyntsev on the occasion of his 60th birthday.     

Multiple magnetic phase transitions,dilute Fe hyperfine fields and europium valence instabilities in RMn2Si2-xGex,R=rare earth

Mössbauer studies of dilute⁵⁷Fe and¹⁵¹Eu in RMn₂Si_2-xGe_x (R=La, Sm, Eu and Gd) at temperatures 4.2 K to 480 K have been performed. The diamagnetic iron and europium reveal the magnetic order of the Mn and rare earth sublattices through transferred hyperfine interactions. The⁵⁷Fe studies show that in LaMn₂Si₂, LaMn₂Ge₂, and SmMn₂Ge₂ the Mn is magnetically ordered above the known Curie temperatures, and the compounds are antiferromagnets up to T_N=470 K, 415 K and 385 K respectively. Studies of¹⁵¹Eu in R_1-xEu_xMn₂Si₂, (R=La, Gd) display Eu subspectra corresponding to Eu²⁺, Eu³⁺ and intermediate valant Eu. All display large magnetic hyperfine fields.     

Quasiperiodic motions in superquadratic time-periodic potentials

It is shown that for a large class of potentials on the line with superquadratic growth at infinity and with the additional time-periodic dependence all possible motions under the influence of such potentials are bounded for all time and that most (in a precise sense) motions are in fact quasiperiodic. The class of potentials includes, as very particular examples, the exponential, polynomial and much more. This extends earlier results and gives an answer to a problem posed by Littlewood in the mid 1960's. Along the way machinery is developed for estimating the action-angle transformation directly in terms of the potential and also some apparently new identities involving singular integrals are derived.