Structure, elastic and dynamical properties of KN3 and RbN3: A van der Waals density functional study

We report a detailed first principles study on the structural, elastic, vibrational and thermodynamic properties of layered structure energetic alkali metal azides KN₃ and RbN₃. All the calculations were carried out by means of plane wave pseudopotential method with and without including van der Waals interactions. The calculated ground state structural properties are improved to a greater extent by the inclusion of dispersion corrections, implies that the van der Waals interactions play a major role on the physical properties of these systems. The elastic constants and the related bulk mechanical properties for the tetragonal KN₃ and RbN₃ have been calculated using both the methods and found that the compounds are mechanically stable systems. The magnitude of the calculated elastic constants increases in the order RbN₃ < KN₃ implying higher elastic stiffness for KN₃, the fact also confirmed by the higher values of bulk, shear and Young's moduli of KN₃ than RbN₃. Moreover, the calculated elastic constants follows the inequality C₃₃ < C₁₁ which indicates the presence of more number of intermolecular interactions along a-axis over c-axis of the azide lattices. A correlation has been proposed to relate the calculated elastic constants to the decomposition phenomena for the metal azides. The experimentally reported vibrational frequencies at the gamma point were exactly reproduced by the present calculations. In addition, we also present the thermodynamic properties such as heat capacity which compares well with the experiment.