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We have obtained detailed structural information for the energetic salt ammonium perchlorate (AP) at pressures up to similar to 8 GPa through a combination of X-ray and neutron diffraction. Under hydrostatic conditions, AP undergoes a first-order phase transition at 3.98(5) GPa, broadly consistent with results from previous studies. We have successfully solved and refined the structure of the new orthorhombic phase (phase II, space group Pnma), which features a more close-packed structure with more extensive hydrogen bonding than the polymorph obtained at ambient pressure (phase I). Equations of state have been obtained for phase I from 0 to 3.5 GPa and for the new phase 4 to 8.1 GPa. To complement these experimental studies, we have also performed density functional theory (DFT) calculations of the hydrostatic compression of AP in the region of 0.0-3.5 GPa. A comparison of the performance of different pseudopotentials and DFT dispersion correction schemes in calculating crystal geometries at high pressure has been performed. The results highlight the fact that care must be taken when choosing pseudopotentials for high-pressure studies and that no significant improvements in the calculation of crystal geometries of AP are obtained by employing DFT-D corrections.
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- 1 Finished
1/10/00 → 31/03/07