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

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... 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.