Phase D is a potential water carrier in the slab subducted to the uppermost lower mantle (ULM) and its velocity and density characteristics are important for seismological detection of water cycle in the deep Earth. Here we obtained the density and velocities of phase D under the conditions of the ULM using first-principles calculations based on the density functional theory. In contrast to previous results, both hydrogen bond symmetrization and the corresponding abrupt increase in bulk modulus are absent in the optimized structure up to 80 GPa. The velocities of phase D are higher than those of periclase and only slightly lower than those of bridgmanite by 0.5%–3.5% for VP and by 0%–2.0% for VS in the ULM (660–1000 km), thus the accumulation of phase D can hardly produce obvious low-velocity anomaly in the ULM observed by seismological studies, but it may contribute significantly to the seismic anisotropy because of its strong elastic anisotropy.
Phase D will dehydrate into bridgmanite and stishovite at the depth of ~700–1200 km. The velocity jumps are 6.6% for VP and 5.1% for VS at the depth of 660 km but reduce to 2.3% and 0.3% at the depth of 1000 km, respectively. However, phase D is ~15% less dense than bridgmanite, so the dehydration of phase D could still produce large impedance jumps in the ULM, which may account for some discontinuities observed at the depth of ~1000–1200 km in subduction zones.