Prediction, determination and stability of the mixed NaBr–KBr crystal structure

Abstract

The binary system NaBr–KBr was studied. Using a 1:1 mixture between the pristine compounds slowly cooled from the melt at 1053 ​K, it was not possible to stabilize the solid solution (Na0.5K0.5)Br at T ​= ​295 ​K, because exsolution occurs within a short time to give the biphasic system NaBr ​+ ​KBr, as observed by powder X-ray diffraction and scanning electron microscopy. Computation of Helmholtz free energies for both mixed and pure bromides using PHONOPY shows that the free energy gap between the solid solution and the biphasic system needs to be higher than 50 ​kJ/mol in order to stabilize the solid solution. Although this threshold is not reached at room temperature, we could predict the structure of (Na0.5K0.5)Br at 300 ​K using an evolutionary structure-prediction algorithm, USPEX, affording a disordered structure in space group Fm3¯m, in which Na+ and K+ cations share the same crystallographic site. The predicted cation-anion distance is 3.176 ​Å. Finally, the experimental crystal structure was determined at T ​= ​328 ​K. At that temperature, a single crystal is stable enough for data collection at high resolution (dmin ​= ​0.61 ​Å), affording a cation-anion distance of 3.1614 (5) Å. A long-range order model for cation distribution shows that the diffraction pattern results from demixed crystallites of pure NaBr and KBr, rather than from a genuine solid solution, as expected at this temperature.