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

Abstract

Understanding the high-pressure behavior of ammonia hydrates is relevant for modeling the interior of solar and 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 compression experiments performed in diamond anvil cells. The derived pressure-volume (P-V) compression curves are in 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 from seven different compression runs, we derive the first equation of state for ADH-DMA based on the third-order Birch-Murnaghan formalism with best-fit parameters: ${V}_{0}=23.96\ifmmode\pm\else\textpm\fi{}0.03$ (${\AA{}}^{3}$/molecule), ${B}_{0}=9.95\ifmmode\pm\else\textpm\fi{}0.14$ GPa, and ${B}_{0}^{\ensuremath{'}}=6.59\ifmmode\pm\else\textpm\fi{}0.03$. The instantaneous bulk modulus directly derived from the quasicontinuous compression curves displays a smooth increase upon compression that further supports the absence of structural transitions.