Exploring High-Pressure Transformations in Low-Z (H2, Ne) Hydrates at Low Temperatures

Paulo H B Brant Carvalho,Ulrich Häussermann,Ove Andersson,Alexandre A Leitão,Chris A Tulk,Inna Martha Nangoi,Amber Mace,Jamie J Molaison,Alexander P Lyubartsev

doi:10.3390/cryst12010009

Paulo H B Brant Carvalho, Ulrich Häussermann + Show 7 more

Open Access

https://doi.org/10.3390/cryst12010009

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Abstract

The high pressure structural behavior of H2 and Ne clathrate hydrates with approximate composition H2/Ne·~4H2O and featuring cubic structure II (CS-II) was investigated by neutron powder diffraction using the deuterated analogues at ~95 K. CS-II hydrogen hydrate transforms gradually to isocompositional C1 phase (filled ice II) at around 1.1 GPa but may be metastably retained up to 2.2 GPa. Above 3 GPa a gradual decomposition into C2 phase (H2·H2O, filled ice Ic) and ice VIII’ takes place. Upon heating to 200 K the CS-II to C1 transition completes instantly whereas C1 decomposition appears sluggish also at 200 K. C1 was observed metastably up to 8 GPa. At 95 K C1 and C2 hydrogen hydrate can be retained below 1 GPa and yield ice II and ice Ic, respectively, upon complete release of pressure. In contrast, CS-II neon hydrate undergoes pressure-induced amorphization at 1.9 GPa, thus following the general trend for noble gas clathrate hydrates. Upon heating to 200 K amorphous Ne hydrate crystallizes as a mixture of previously unreported C2 hydrate and ice VIII’.

Highlights

Clathrate hydrates are crystalline inclusion compounds in which a guest species occupy the polyhedral cages formed by a hydrogen-bonded water network [1]
A total of three cubic structure II (CS-II) HH sample batches were prepared (A, B, and C) and we report on four HH experiments (a1, a2, a3, b) based on samples A and B (C was only used for structure/composition characterization of CS-II HH)
The Van der Waals and Coulomb interactions were set to a cutoff radius of 1.25 nm and the long-range part of the electrostatic interactions was treated by the Particle-Particle Particle Mesh (PPPM) method [36]

Summary

Introduction

Clathrate hydrates are crystalline inclusion compounds in which a guest species (i.e., small molecules or monoatomic noble gases) occupy the polyhedral cages formed by a hydrogen-bonded water network [1]. Unique for low Z clathrate hydrate is the high mobility of the guest species, based on defect-free diffusion through 6-membered rings [9,10,11]. This is manifested by the discovery of empty CS-II (ice XVI) from pumping Ne and H2 clathrate hydrate between. Since the low Z CS-II hydrates appear distinguished, it was deemed important to establish their high pressure behavior at low temperatures

Experimental Details and Methods of Analysis

CS-II HH and NeH Starting Materials

Synthesis of C2 NeH

Discussion

Methods