Investigation of the Structural, Vibrational, Electronic, and optical properties of energetic Nitrogen-Rich azidotetrazolates XCN7 (X = N2H5, NH4, K, Cs)

In this work, we present a detailed and comparative study of four salts of 5-azido-1H-tetrazole using Density Functional Theory (DFT) based computational methods. The salts, containing the $C{N}_7^{-}$ anion, include hydrazinium azidotetrazolate (N₂H₅CN₇), ammonium azidotetrazolate (NH₄CN₇), potassium azidotetrazolate (KCN₇), and cesium azidotetrazolate (CsCN₇). These compounds are collectively represented as XCN₇, where X  = N₂H₅, NH₄, K, and Cs. We found that the incorporation of van der Waals interactions was crucial in aligning the theoretical ground state structures with experimental data. Mechanical stability of all the compounds within their respective space groups was verified by calculating the elastic constants and bulk modulus. Vibrational frequency analysis revealed that N₂H₅CN₇ and NH₄CN₇, containing NH bonds, exhibited frequencies around 3300 cm⁻¹, while the metal salts KCN₇ and CsCN₇, lacking NH bonds, showed frequencies below 2000 cm⁻¹. Born effective charge calculations indicated strong covalency within the tetrazole ring and between C, N, and H atoms, contrasted by the ionic nature of the metal atoms. Using the TB-mBJ potential, we accurately computed the electronic structure and optical properties, predicting bandgaps and absorption edges for these XCN₇ compounds. The partial density of states analysis highlighted the significant role of C and N p states in the sensitivity of these compounds. Optical property evaluations confirmed that these compounds are optically anisotropic, exhibiting low sensitivity in the visible region but high sensitivity in the UV and far UV regions. These insights are crucial for predicting and controlling their reactivity, stability, and performance in various applications, particularly in the field of energetic materials.