Local order and orientational correlations in liquid and crystalline phases of carbon tetrabromide from neutron powder diffraction measurements

TL;DR

This study uses neutron powder diffraction and reverse Monte Carlo simulations to analyze local order and orientational correlations in liquid, plastic, and crystalline phases of CBr4, revealing similarities between liquid and plastic phases and distinct behavior in the ordered crystalline phase.

Contribution

It introduces a combined experimental and simulation approach to characterize local and orientational order in different phases of CBr4, including the calculation of single crystal diffraction patterns from powder data.

Findings

Liquid and plastic phases have similar BrBr radial distribution functions.

Long-range order is evident in the crystalline phase.

Molecular orientations differ significantly between ordered and plastic phases.

Abstract

The liquid, plastic crystalline and ordered crystalline phases of CBr$_4$ were studied using neutron powder diffraction. The measured total scattering differential cross-sections were modelled by Reverse Monte Carlo simulation techniques (RMC++ and RMCPOW). Following successful simulations, the single crystal diffraction pattern of the plastic phase, as well as partial radial distribution functions and orientational correlations for all the three phases have been calculated from the atomic coordinates ('particle configurations'). The single crystal pattern, calculated from a configuration that had been obtained from modelling the powder pattern, shows identical behavior to the recent single crystal data of Folmer et al. (Phys. Rev. {\bf B77}, 144205 (2008)). The BrBr partial radial distribution functions of the liquid and plastic crystalline phases are almost the same, while CC correlations clearly display long range ordering in the latter phase. Orientational correlations also suggest strong similarities between liquid and plastic crystalline phases, whereas the monoclinic phase behaves very differently. Orientations of the molecules are distinct in the ordered phase, whereas in the plastic crystal their distribution seems to be isotropic.