Unraveling the locus of atomic positions in a spinel oxide with two J-T ions

1. Unraveling the locus of atomic positions in a spinel oxide with two J-T ions Ram Seshadri (UC Santa Barbara) DMR 0449354 Research: Oxide compounds with the ions Ti3+, Mn3+, and Cu2+ are richly endowed with Jahn-Teller phenomena. Such oxides are central to a number of important extant problems in condensed matter science. In this light, we have explored the spinel CuMn2O4 whose simple formula belies extremely rich structural behavior. Using reverse Monte Carlo analysis of total neutron scattering [Los Alamos National Laboratory], we obtain a stochastic description of the structure [shown below] that constitutes a significant departure from more usual unit-cell-based ones. The analysis yields evidence for site exchange and partial charge disproportionation, that has not been previously seen or even suggested in this system. The box on the left displays projected positions of atoms in 10x10x7 unit cells of tetragonal CuMn2O4. Red spheres are O, grey are Mn and, and gold are Cu. On the right is the distribution of Cu-O and Mn-O distances on the tetrahedral A site and the octahedral B site. Dashed vertical lines show expected distances for Mn3+ and Cu2+ on the two sites.

2. Unraveling the locus of atomic positions in a spinel oxide with two J-T ions Ram Seshadri (UC Santa Barbara) DMR 0449354 K-12 Education: As a part of their summer research experience in UCSB, RET participants John Gonzalez [Adolfo Camarillo High School] and Keri Santos [Vista de las Cruces Elementary School] have developed YBa2Cu3O7-based vehicles that levitate on a track or approximately 160 0.5 in. x 0.5 in. x 0.5 in. Nd-Fe-B magnets. The summer project involved preparing the ceramic superconductors, testing their properties, and designing vehicles and the track. The experience has been recorded, with pictures and videos, on a blog found at http://gonzalezret.blogspot.com/. The models will now be used in demonstrations at family science nights at local schools. John and Keri demonstrate one of their levitating vehicles, seen also in the inset. To construct the curving track required extensive testing of the arrangements of the polarities of the individual magnets vis-à-vis one-another.