Solid ammonia at high pressure: A single-crystal x-ray diffraction study to 123 GPa

The high-pressure phases of solid ammonia have been investigated by x-ray diffraction in a diamond anvil cell at room temperature. Despite the first-order solid phase transition at 4 GPa, quasi-single crystals of NH3 and ND3 could be obtained and compressed to 123 and 62 GPa, respectively. The observed reflections above 4 GPa are compatible with the structure determined by neutron diffraction on ND3. We found strong evidence for an isosymmetric transition at 12 GPa in NH3 and 18 GPa in ND3. This transition is more likely due to rearrangements of the hydrogen atoms, whereas the N atoms remain on their quasi-hcp sites. The experimental equation of state (EOS) is compared to the one derived from first-principles calculations. A stiffening of the EOS above 55 GPa could indicate the onset of a quantum-disordered regime for some of the H bonds.