Solid ammonia at high pressure: A single-crystal x-ray diffraction study to123GPa

F. Datchi, S. Ninet, M. Gauthier, A. M. Saitta, B. Canny, F. Decremps
2006 Physical Review B  
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 NH 3 and ND 3 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 ND 3 . We found strong evidence for an isosymmetric transition at 12 GPa in NH 3 and 18 GPa in ND
more » ... 3 and 18 GPa in ND 3 . 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 ͑EOS͒ 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.
doi:10.1103/physrevb.73.174111 fatcat:ligannwpy5eg3fz6kprcfilcmq