One-Dimensional Fluctuations and the Chain-Ordering Transformation in Hg3-δAsF6
The mercury ions in Hg3 — δAsF6 form arrays of interpenetrating infinite chains with a mean separation between ions which is incommensurate with the AsF6 sublattice. This structure leads to a number of quite remarkable properties. According to our present understanding, based mainly upon a recent series of neutron scattering experiments1 and related theoretical work2, the chains are essentially one-dimensional liquids at room temperature, and are free to flow down extremely narrow channels (~7.5 Å in diameter) formed by the AsF6 sublattice. At about 120 K, the interchain coupling induces a cooperative freezing transition, which takes place in a quite unusual manner. The theory2 predicts that the first stage consists in the formation of an ion charge-density wave of wave vector d*. As the order develops, further charge-density waves with wave vectors nd* are generated harmonically, and their amplitudes grow until the whole assembly becomes an ordered lattice at low temperatures. A complete description of the evolution of long-range order in the chains involves a number of distinct physical processes which will be explored in more detail in subsequent sections. At high temperatures, the chains are essentially independent and they offer the possibility of studying the behavior of one-dimensional liquids.
KeywordsBragg Peak Pair Distribution Function Dynamical Structure Factor Individual Chain Freezing Transition
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