Anharmonicity and scissoring modes in the negative thermal expansion materials ScF 3 and CaZrF 6

DOI: 10.1103/PhysRevB.101.064306

Authors: T. A. Bird (University of Warwick) , J. Woodland-Scott (University of Oxford) , L. Hu (University of Science and Technology Beijing) , M. T. Wharmby (Deutsches Elektronen-Synchrotron (DESY)) , J. Chen (University of Science and Technology Beijing) , A. L. Goodwin (University of Oxford) , M. S. Senn (University of Warwick)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Physical Review B , VOL 101

State: Published (Approved)
Published: February 2020
Diamond Proposal Number(s): 18786

Abstract: We use a symmetry-motivated approach to analyzing x-ray pair distribution functions to study the mechanism of negative thermal expansion in two ReO 3 -like compounds: ScF 3 and CaZrF 6 . Both average and local structures suggest that it is the flexibility of M -F- M linkages ( M = Ca, Zr, Sc) due to dynamic rigid and semirigid “scissoring” modes that facilitates the observed negative thermal expansion (NTE) behavior. The amplitudes of these dynamic distortions are greater for CaZrF 6 than for ScF 3 , which corresponds well with the larger magnitude of the thermal expansion reported in the literature for the former. We show that this flexibility is enhanced in CaZrF 6 due to the rocksalt ordering mixing the characters of two of these scissoring modes. Additionally, we show that in ScF 3 anharmonic coupling between the modes responsible for the structural flexibility and the rigid unit modes contributes to the unusually high NTE behavior in this material.

Journal Keywords: Lattice dynamics; Phonons; Structural properties; Thermal expansion

Subject Areas: Materials, Physics


Instruments: I11-High Resolution Powder Diffraction

Other Facilities: P02.1 at PETRA III

Documents:
h4fg5sss.pdf

Discipline Tags:

Material Sciences Metallurgy Perovskites Physics Hard condensed matter - structures

Technical Tags:

Diffraction X-ray Powder Diffraction