The structural relationship between negative thermal expansion and quartic anharmonicity of cubic ScF3

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Date/Time:Thursday, 19 Jan 2012 - Thursday, 19 Jan 2012
Location:PHYSICS Room 5
Channel:Condensed Matter Physics
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Chen Li
Materials Science
California Institute of Technology

Cubic scandium tri-fluoride (ScF3) has a large negative thermal expansion over a wide range of temperature. Inelastic neutron scattering experiments were performed with the ARCS spectrometer at the SNS to study the temperature dependence of the lattice dynamics of ScF3 from 7 to 750 K. The measured phonon densities of states show a large anharmonic contribution with a thermal stiffening of modes around 25meV. Phonon calculations with first-principles methods identified the individual modes in the DOS, and frozen phonon calculations showed that some of the modes with motions of F atoms transverse to their bond direction behave as quantum quartic oscillators. The quartic potential originates from harmonic interatomic forces in the DO9 structure of ScF3, and accounts for phonon stiffening with temperature and a significant part of the negative thermal expansion.