Synthesis and stability of hydrogen iodide at high pressures

Abstract

Through high-pressure Raman spectroscopy and x-ray diffraction experiments, we have investigated the formation, stability field, and structure of hydrogen iodide (HI). Hydrogen iodide is synthesized by the reaction of molecular hydrogen and iodine at room temperature and at a pressure of 0.2 GPa. Upon compression, HI solidifies into cubic phase I, and we present evidence for the emergence of a phase I above 3.8 GPa. Across the wide temperature regime presented here, HI is unstable under compression (11 GPa at 300 K, 18 GPa at 77 K), decomposing into its constituent elements, after which no further reaction between hydrogen and iodine was observed up to pressures of 60 GPa. This study provides both the constraints on the phase diagram of HI and its kinetic stability.

Formation ofH2-rich iodine-hydrogen compounds at high pressure

“…A single HI vibron is in agreement with one unique HI molecular environment. The appearance of the vibron is consistent with rotational disorder as in HI phase I [15,32]. As shown in Fig.…”

“…The icosahedral clusters of H 2 molecules isolate the HI molecule, resulting in an extreme stability range, from 9 to at least 130 GPa. By compressing HI-H 2 mixtures, we observe the formation of another molecular compound, H 2 (HI) 2 , which exhibits similar stability to pure HI [15]. We note that previous theoretical structure searches identifying several H 2 -HI stoichiometries have not considered such extreme H 2 -contents as we report here [13,14].…”

“…At low pressures (0.2-0.5 GPa) H 2 and I 2 react spontaneously. Once synthesized, HI is very unstable on compression and decomposes by ∼10 GPa at 300 K [15]. On further compression to 60 GPa at 300 K, there is no further reaction between I 2 and H 2 .…”

“…These pressures were chosen to determine if the molecular dissociation of I 2 , occurring between 23.2 GPa and 30.4 GPa, influenced the reactivity of I 2 . These pressures are also high enough to ensure the full decomposition of HI and (HI) 2 H 2 into their constituent elements [15].…”

“…However, experimental studies of the high-pressure behaviour of hydrogen iodide (HI) have been hindered by its intrinsic instability due to the rapid decrease of HI intermolecular distances with increasing density, and therefore the decomposition of the HI molecules at pressure [15] and to a lesser extent due to the sensitivity of HI to diagnostic techniques such as laser or x-ray beams. Recent work has shown that at room temperature in a matrix of hydrogen, HI still decomposes at 10 GPa, while cooling, further stabilises HI to about 20 GPa at 80 K [15]. Although theoretical predictions also find HI to be unstable with respect to decomposition, they predict the hydrogen-rich compounds HI(H 2 ) 2 , H 2 I and (H 2 ) 2 I to be more stable, but they have not been observed experimen-tally [13,14].…”