Neutron investigation of the dynamical properties of the mercury-chain compoundHg3−δ AsF6

Abstract

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 Hg,~AsF, . The main emphasis has been put on the nature of the response function of the Hg chains at higher temperatures (T & 150 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 T, = 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 6((00) and the A(00() directions. The observed dispersion slope of the. transverse branch along 5 polarized along A is about 25% smaller than that of the transverse branch along A polarized along 6, 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 A direction than in the 5 direction.

One-dimensional chain melting in incommensurate potassium

“…In the 1D harmonic liquid model [28,29], the widths of the resulting sheets of diffuse scattering are proportional to the square of the order of the sheet [30], and thus if the guest chains in K-III at 345 K are truly a 1D liquid, then the width of the second-order diffuse sheet should be four times that of the first. Several long exposures were taken to see if we could confirm this relationship in the widths, but without success; the scattering in the second sheet was too weak to determine its width, even when summing multiple long-exposure images.…”

Progressive melting in confined one-dimensional C60chains

“…Equation (4) is the well known structure factor expression of a 1D liquid at finite temperature. [2][3][4] In infinite 1D systems, and in particular in the case of C 60 chains, 13 the cumulative thermal fluctuations are expected to prevent the establishment of long-range order for any nonzero temperature. However, to the best of our knowledge, before the present study no experimental evidence was given for this liquidlike character; x-ray and electron diffraction data 14,15 were analyzed assuming that C 60 molecules formed a 1D crystal, whose Bragg peaks are Dirac functions.…”

Incommensurate structures

“…Experiments measuring the width of successive sheets up to n = 7 were subsequently performed (Heilmann et al 1979). The results, shown in Fig.…”

“…TaSe ? and NbSe^ (after Moncton et al (1977)). cessive peak widths in Hg-^AsFg (after Heilmann et al (1979)). …”