Theoretical estimates of the logarithmic phonon spectral moment for monatomic liquids

TL;DR

This paper compares two methods for estimating the logarithmic phonon spectral moment in monatomic liquids, validating the entropy-based approach for normal melters and highlighting discrepancies in anomalous melters.

Contribution

It introduces and validates a method to estimate the phonon spectral moment using entropy data, confirming its accuracy for normal melters and identifying limitations for anomalous melters.

Findings

Entropy-based estimates agree with neutron scattering data within a few percent for normal melters.

Discrepancies up to a factor of three are observed for anomalous melters.

The entropy method is reliable for calculating liquid transport coefficients.

Abstract

We calculate the logarithmic moment of the phonon frequency spectrum at a single density for 29 monatomic liquids using two methods, both suggested by Wallace's theory of liquid dynamics: The first method relies on liquid entropy data, the second on neutron scattering data in the crystal phase. This theory predicts that for a class of elements called ``normal melters,'' including all 29 of these materials, the two estimates should closely match, and we find that they agree to within a few percent. We also perform the same calculations for four ``anomalous melters,'' for which we expect the two estimates to differ markedly; we find that they disagree by factors almost up to three. From our results we conclude that the liquid entropy estimates of the logarithmic moment, applicable both to normal and anomalous melters, are trustworthy to a few percent, which makes them reliable for use in estimates of various liquid transport coefficients.