STATIC AND DYNAMIC STRUCTURAL-ANALYSIS OF A SATURATED SOLUTION OF ZNBR2 IN WATER - ANOMALOUS X-RAY-DIFFRACTION AND MOLECULAR-DYNAMICS SIMULATIONS

The supposedly very simple system of a saturated solution of ZnBr2 in water exhibits unusually complex and therefore interesting structural behavior. Motivated by this, Mager did a detailed x-ray diffraction st udy (Th. Mager, PhD. thesis, Universitat Wurzburg, 1991), and we perfo rmed a long molecular dynamics (MD) simulation-using potential paramet ers fro the general purpose GROMOS force field-of such a solution, whi ch can be grossly characterized by the formula ZnBr2 .3H2O. We start b y calculating those properties that are directly accessible through th e experiment from the MD simulation, in order to validate the physical relevance of the simulation. Seeing that the simulation delivers resu lts that are compatible with those of the experiment, we proceed by an alyzing the MD simulation in much more detail according to the static and dynamic structure of the system, thereby gaining insight into the structural behavior of ZnBr2 . 3H(2)O that is very difficult, if at al l possible, to get from experimental studies. To this end we use the V oronoi algorithm to define coordination shells around atoms and ions i n ZnBr2 . 3H(2)O. We study the time averaged as well as the time-resol ved geometry and composition of these coordination shells and find tha t octahedral coordination of Zn2+ ions is the dominant geometry in ZnB r2 . 3H(2)O, and that these octahedra are remarkably stable (after 1 n s only 10% decayed). We further find evidence for polymerlike Zn2+ cha ins, where O atoms of water and Br- ions connect the Zn2+ ions. (C) 19 96 American Institute of Physics.