Elasticity of quasi-bcc ammonia hemihydrate at high pressures

Ammonia hydrates are important components in planetary interiors, among which ammonia hemihydrate (AHH) exhibits remarkable stability under high pressure. In this study, we report for the first time the elastic properties of the recently discovered quasibcc ammonia hemihydrate (AHH-qbcc) at high pressures, using externally heated diamond anvil cells combined with Raman spectroscopy and Brillouin scattering. We synthesized single crystals of AHH-qbcc in the diamond anvil cell (DAC). Subsequently, we measured its full elastic tensor up to 17.6 GPa at room temperature via Brillouin scattering. The results show that the elastic constants increase linearly with increasing pressure. Based on the obtained elastic constants, we calculated the bulk modulus, shear modulus, Poisson’s ratio, and velocity anisotropy. The results reveal significant anisotropy in wave velocities and Poisson’s ratio on the (110) crystallographic plane of AHH-qbcc. Compared to ice VII, AHH-qbcc has a lower bulk modulus and shear modulus, yet exhibits a higher compressional wave velocity and a similar shear wave velocity. Our findings provide important constraints for understanding the internal structure and seismic velocity profiles of icy planets.