NEUTRON INVESTIGATION OF THE DYNAMICAL PROPERTIES OF THE MERCURY-CHAIN COMPOUND HG3-DELTA ASF6

We present in this paper the results of a detailed neutron scattering investigation at various temperatures of the dynamical properties of the linear-mercury-chain compound Hg3-AsF6. The main emphasis has been put on the nature of the response function of the Hg chains at higher temperatures (T150 K). The results are analyzed and discussed in terms of new theoretical results obtained by Emery and Axe, and it is found that the behavior of the Hg chains in this temperature region is one-dimensional liquidlike, in accordance with theoretical predictions. Below Tc=120 K where three-dimensional long-range order is established among the Hg ions, the transverse phononlike excitations in the Hg chains with displacement along the chain develop a small ( 0.1 meV) energy gap at finite wave vector. This feature makes it straightforward to demonstrate the absence of elastic scattering in the sheets of scattering from the Hg chains. We have measured at room temperature acoustic AsF6 sublattice phonons in the (00) and the (00) directions. The observed dispersion slope of the transverse branch along polarized along is about 25% smaller than that of the transverse branch along polarized along, in apparent disagreement with elasticity theory which predicts both modes to be governed by C44. We show that this behavior can be qualitatively explained by the anisotropic coupling between Hg chains and the host lattice, leading to a much smaller elastic regime in the direction than in the direction. © 1979 The American Physical Society.