Low energy phonons in the NTE compounds [formula omitted] and [formula omitted

Abstract

The compounds Zn ( CN ) 2 and ZnPt ( CN ) 6 both display negative thermal expansion (NTE) properties, that is to say they undergo a volume contraction with increasing temperature. In the case of Zn ( CN ) 2 this volume contraction occurs over a temperature range from 25 to 375 K with a coefficient of thermal expansion α = - 16.9 ( 2 ) × 10 - 6 K - 1 [A.L. Goodwin, C.J. Kepert, Phys. Rev. B 71 (2005) 14030]. This phenomenon is believed to be related to the presence of low energy rigid unit modes (RUMS) in the phonon dispersion relations of Zn ( CN ) 2 [A.L. Goodwin, C.J. Kepert, Phys. Rev. B 71 (2005) 14030]. We have examined the low energy part of the phonon density of states in Zn ( CN ) 2 and ZnPt ( CN ) 6 using time of flight inelastic neutron scattering from powder samples. In Zn ( CN ) 2 there is a strong peak in the density of states at 2 meV whose temperature dependence can be correlated with that of α . There is a similar peak in the density of states of ZnPt ( CN ) 6 at 7.5 meV, which correlates with the smaller NTE effect in this compound.