High-pressure ionic and molecular phases of ammonia within density functional theory

We have studied ammonia under pressure using density functional theory (DFT) methods. We have used four density functionals; the local density approximation (LDA) and Perdew-Burke-Ernzerhof (PBE) semilocal functionals, the PBE + G06 semilocal functional which includes an empirical dispersion correction, and the PBE0 hybrid functional, finding results in reasonable agreement with the available experimental data in each case. Using a combination of DFT and a random-structure-searching technique, we have found a molecular phase of ammonia of space group symmetry Pa (3) over bar which has not been reported in experimental studies. This phase is calculated to have a region of thermodynamic stability at low pressures with each of the four density functionals. Results with both the PBE and PBE0 functionals indicate that ammonium amide (NH4+ NH2-)proton transfer ionic solids are stable at pressures above about 100 GPa.