Equation of state of ammonia dihydrate up to 112 GPa by static and dynamic compression experiments in diamond anvil cells

Physical review / B 107(22), 224108 (2023). doi:10.1103/PhysRevB.107.224108
Understanding the high-pressure behavior of ammonia hydrates is relevant for modeling the interior of solarand extrasolar icy bodies. We present here the results of high-pressure x-ray diffraction studies on ammonia-dihydrate (ADH) at room temperature (298 K) up to 112 GPa, employing both static and dynamic compressionexperiments performed in diamond anvil cells. The derived pressure-volume (P-V) compression curves arein excellent agreement regardless of the compression technique. In contrast to early theoretical predictions,our results indicate the stability of the disordered molecular alloy (DMA) phase, a body centered cubic (bcc)structure, and the absence of self-ionization in the investigated pressure range. By combining the P-V data fromseven different compression runs, we derive the first equation of state for ADH-DMA based on the third-orderBirch-Murnaghan formalism with best-fit parameters: V$_0$ = 23.96 ± 0.03 (Å$^3$ /molecule), B$_0$ = 9.95 ± 0.14 GPa,and B′$_0$ = 6.59 ± 0.03. The instantaneous bulk modulus directly derived from the quasicontinuous compressioncurves displays a smooth increase upon compression that further supports the absence of structural transitions.
Published by Inst., Woodbury, NY