title: New Insights into the Negative Thermal Expansion: Direct Experimental Evidence for the "Guitar-String" Effect in Cubic ScF3.
abstract: The understanding of the negative thermal expansion (NTE) mechanism remains challenging but critical for the development of NTE materials. This study sheds light on NTE of ScF3, one of the most outstanding materials with NTE. The local dynamics of ScF3 has been investigated by a combined analysis of synchrotron-based X-ray total scattering, extended X-ray absorption fine structure, and neutron powder diffraction. Very interestingly, we observe that (i) the Sc-F nearest-neighbor distance strongly expands with increasing temperature, while the Sc-Sc next-nearest-neighbor distance contracts, (ii) the thermal ellipsoids of relative vibrations between Sc-F nearest-neighbors are highly elongated in the direction perpendicular to the Sc-F bond, indicating that the Sc-F bond is much softer to bend than to stretch, and (iii) there is mainly dynamically transverse motion of fluorine atoms, rather than static shifts. These results are direct experimental evidence for the NTE mechanism, in which the rigid unit is not necessary for the occurrence of NTE, and the key role is played by the transverse thermal vibrations of fluorine atoms through the "guitar-string" effect.
authors.affiliation: Department of Physical Chemistry, University of Science and Technology Beijing , Beijing 100083, China.
authors.forename: Lei
authors.initials: L
authors.lastname: Hu
authors.affiliation: Department of Physical Chemistry, University of Science and Technology Beijing , Beijing 100083, China.
authors.forename: Jun
authors.initials: J
authors.lastname: Chen
authors.affiliation: Department of Physics and Astronomy, University of Padova , Padova I-35131, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Sanson
authors.affiliation: NIST Center for Neutron Research, National Institute of Standards and Technology , Gaithersburg, Maryland 20878, United States.
authors.forename: Hui
authors.initials: H
authors.lastname: Wu
authors.affiliation: Elettra Synchrotron , Basovizza, Triestre I-34149, Italy.
authors.forename: Clara
authors.initials: C
authors.lastname: Guglieri Rodriguez
authors.affiliation: Elettra Synchrotron , Basovizza, Triestre I-34149, Italy.
authors.forename: Luca
authors.initials: L
authors.lastname: Olivi
authors.affiliation: Argonne National Laboratory, X-ray Science Division , Argonne, Illinois 60439, United States.
authors.forename: Yang
authors.initials: Y
authors.lastname: Ren
authors.affiliation: Department of Physical Chemistry, University of Science and Technology Beijing , Beijing 100083, China.
authors.forename: Longlong
authors.initials: L
authors.lastname: Fan
authors.affiliation: Department of Physical Chemistry, University of Science and Technology Beijing , Beijing 100083, China.
authors.forename: Jinxia
authors.initials: J
authors.lastname: Deng
authors.affiliation: Department of Physical Chemistry, University of Science and Technology Beijing , Beijing 100083, China.
authors.forename: Xianran
authors.initials: X
authors.lastname: Xing