PHYSICAL REVIEW B VOLUME 49, NUMBER 6 1 FEBRUARY 1994-II Neutron diffraction from liquid hydrogen bromide: Study of the orientational correlations C. Andreani and F. Menzinger Department of Physics University of Tor Vergata, Via della Ricerca Scientigca 11, 00172 Roma, Italy M. A. Ricci Department of Physics "E Am. aldi" University of Rome III, Via Segre g, 0015$ Roma, Italy A. K. Soper and J. Dreyer ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Ozon, OX11 OQX United Kingdom (Received 25 June 1993; revised manuscript received 13 October 1993) The three partial structure factors of liquid hydrogen bromide along the coexistence curve have been measured by neutron diffraction using the isotopic substitution technique. The structure factors and corresponding pair correlation functions obtained suggest that it is an excellent approximation to regard the bromine atom as the center of mass of the hydrogen bromide molecule. The anisotropic terms in the intermolecular potential manifest themselves primarily by causing a significant degree of correlation between the orientations of neighboring molecules. In order to gauge the size of these orientational correlations an image reconstruction technique is employed. This analysis shows that there is a pronounced correlation between the relative orientations of neighboring molecules, but that there is not a strong directionality in the local coordination of any particular molecule. The results are discussed in relation to a previous experiment on hydrogen bromide which proposed the formation of hydrogen bonds in this liquid. INTRODUCTION Recently the site-site partial structure factors for liq- uid hydrogen iodide have been measured, using neu- tron diffraction with hydrogen isotope substitution. ' This experiment was the first of several planned for the hydrogen-halide series, the purpose being to demonstrate the role of multipolar forces in generating orientational correlations in these liquids. The hydrogen halides are well suited for this study because the electronic over- lap between molecules gives rise to a hard-core repulsion that is almost isotropic throughout the series, while the anisotropic interactions due to multipolar and polariza- tion forces change monotonically down the series. At one extreme of the hydrogen-halide series, hydrogen flu- oride is expected to exhibit the greatest degree of orien- tational correlation, perhaps because the relatively low atomic number of the halide atom gives rise to the weak- est isotropic repulsive forces and the strongest electro- static ordering forces. At the other end of the series, hydrogen iodide is, because of its large atomic number, expected to show the weakest degree of orientational cor- relation. Therefore it is of some interest to look at the intermediate systems, liquid hydrogen chloride and liquid hydrogen bromide, to see the trend in the orientational structure with atomic number of the halide atom. The neutron difFraction technique, with hydrogen iso- tope substitution, permits all three partial structure fac- tors of the hydrogen-halide system to be extracted in- dependently from one another. These partial structure factors are related by Fourier transform to the corre- sponding site-site pair correlation functions. In liquid hydrogen halides it is an excellent approximation to re- gard the halide atom as the center of mass. Consequently, the halide-halide structure factor is derived directly from the center of mass pair correlation function. The shape of the hydrogen-halide structure factor is then deter- mined by the extent of orientational correlations between one molecule and the center of mass of a neighboring molecule, while the hydrogen-hydrogen structure factor is also sensitive to relative orientations between molecules. Previous neutron diffraction work on liquid hydrogen bromide investigated only the deuterated compound. This work yielded valuable information on the molecular geometry, but because of the close similarity in the neu- tron scattering lengths of deuteri. um and bromine, it was diKcult to extract unambiguous information about spe- cific site-site correlations from the composite structure factor and pair correlation function obtained. Previous work on hydrogen chloride and hydrogen iodide has demonstrated that the shape of the individual hydrogen- hydrogen, hydrogen-halide, and halide-halide correla- tions are quite distinct &om one another and can only be properly determined if three isotopically substituted samples are available. The present work describes a new neutron diffraction study of liquid hydrogen bromide with hydrogen isotope substitution. The experiment was run under the same reduced conditions (T' = T/T = 0. 6, where T = 363 K is the critical temperature) as the previous hydrogen 0163-1829/94/49(6)/3811(10)/$06. 00 49 3811 1994 The American Physical Society