Friday, May 31, 2013

1305.7119 (Simon A. J. Kimber et al.)

Origin of the giant pressure-induced volume collapse in MnS2    [PDF]

Simon A. J. Kimber, Ashkan Salamat, Shaun R. Evans, Harald O. Jeschke, Kaliappan Muthukumar, Milan Tomic, Francesc Salvat-Pujol, Roser Valenti, Maria V. Kaisheva, Ivo Zizak, Tapan Chatterji
Application of high pressure to crystalline materials can drive dramatic collapses in the unit cell volume. This may be accompanied by electronic changes, structural amorphisation or spin-state transitions. An example of the latter was believed to occur in the simple cubic S= 5/2 material MnS2. Using synchrotron x-ray diffraction, we show that pressure actually induces disorder in hydrostatic conditions. We identify a new structure containing edge-sharing chains as the thermodynamic ground state using an evolutionary algorithm, and experimentally confirm it by in-situ laser recrystallisation at 20 GPa. The slow kinetics of the phase transition are the result of a 22 % volume collapse, which dwarfs previously reported effects. The new phase is confirmed to be low-spin S= 1/2, however, the moments are globally quenched by a metal-metal dimerisation instability. The giant volume anomaly in MnS2 thus emerges due to competition between two polymorphs with magnetic properties at polar opposites: a classical three-dimensional magnet, and an ordered valence-bond solid with a non-magnetic ground state.
View original: http://arxiv.org/abs/1305.7119

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