Polymorphism and thermodynamic ground state of Silver fulminate studied from van der Waals density functional calculations

Silver fulminate (AgCNO) is a primary explosive, which exists in two polymorphic phases namely orthorhombic (\emph{Cmcm}) and trigonal (\emph{R$\bar{3}$}) forms at ambient conditions. In the present study, we have investigated the effect of pressure and temperature on relative phase stability of the polymorphs using planewave pseudopotential approaches based on Density Functional Theory (DFT). van der Waals interactions play a significant role in predicting the phase stability and they can be effectively captured by semiempirical dispersion correction methods incontrast to standard DFT functionals. Based on our total energy calculations using DFT-D2 method, the \emph{Cmcm} structure is found to be the preferred thermodynamic equilibrium phase under studied pressure and temperature range. Hitherto \emph{Cmcm} and \emph{R$\bar{3}$} phases denoted as $\alpha$ and $\beta$-forms of AgCNO, respectively. Also a pressure induced polymorphic phase transition is seen using DFT functionals and the same was not observed with DFT-D2 method. The equation of state and compressibility of both polymorphic phases were investigated. Electronic structure and optical properties were calculated using full potential linearized augmented plane wave method within the Tran-Blaha modified Becke-Johnson potential. The calculated electronic structure shows that $\alpha$, $\beta$ phases are indirect band gap insulators with a band gap values of 3.51 and 4.43 eV, respectively. The nature of chemical bonding is analyzed through the charge density plots and partial density of states. Optical anisotropy, electric-dipole transitions and photo sensitivity to light of the polymorphs are analyzed from the calculated optical spectra. Overall, the present study provides an early indication to experimentalists to avoid the formation of unstable $\beta$-form of AgCNO.