Investigating anomalous thermal expansion of copper halides by inelastic neutron scattering and ab initio phonon calculations

Abstract

We investigate detailed lattice dynamics of copper halides CuX (X=Cl, Br, I) using neutron inelastic scattering measurements and ab-initio calculations aimed at a comparative study of their thermal expansion behavior. We identify the low energy phonons which soften with pressure and are responsible for negative thermal expansion. The eigenvector analysis of these modes suggests that softening of the transverse-acoustic modes would lead to NTE in these compounds. The calculations are in very good agreement with our measurements of phonon spectra and thermal expansion behavior as reported in the literature. Our calculations at high pressure further reveal that large difference in negative thermal expansion behavior in these compounds is associated with the difference of the unit cell volume.

Phonons and anomalous thermal expansion behaviour in crystalline solids

“…71. The measured neutron inelastic spectra for CuX (Cl, Br and I) at 373 K, 473 K and 573 K [143]. Fig.…”

“…The Bradley-Cracknell notation is used for the highsymmetry points. Ґ(0 0 0), X (1 0 0), L( 0.5 0.5 0.5) and W(0.5 1 0) in the notation of cubic unit cell [143]. Fig.…”

“…A comparative study [143]of thermal expansion behavior of isostructural [107][108][109][110]copper halides, CuX (X=Cl, Br, I) has been performed. Certain low energy phonons are found to soften with pressure and are responsible for NTE.…”

“…Based on the eigen-vector analysis of low energy mode it is suggested that transverse acoustic phonons would lead to NTE in these compounds. The difference in the NTE behavior in the three compounds is associated to difference in their volume [143].…”

A systematic study of the negative thermal expansion in zinc-blende and diamond-like semiconductors

“…Material undergoing expansion upon heating and contraction upon cooling is a common expectation. However, some materials are discovered to demonstrate a contraction upon heating, known as negative thermal expansion (NTE) [1][2][3][4][5][6][7]. Such NTE materials have attracted considerable attention for use as thermal-expansion compensators by mixing with a 'normal' material which expands on heating to fabricate composites with tailored expansivity, which have a range of potential engineering, mechanical, ferroelectric and chemical applications [8][9][10][11][12][13].…”