Clathrate type 2 hydrate formation in vacuo under astrophysical conditions

The properties of clathrate hydrates were used to explain the complex and poorly understood physical processes taking place within cometary nuclei and other icy solar system bodies. Most of all the experiments previously conducted used starting compositions which would yield clathrate types I hydrates. The main criterion for type I vs. type II clathrate hydrate formation is the size of the guest molecule. The stoichiometry of the two structure types is also quite different. In addition, the larger molecules which would form type II clathrate hydrates typically have lower vapor pressures. The result of these considerations is that at temperatures where we identified clathrate formation (120-130 K), it is more likely that type II clathrate hydrates will form. We also formed clathrate II hydrates of methanol by direct vapor deposition in the temperature range 125-135 K

High Pressure Properties of a Ba-Cu-Zn-P Clathrate-I.

The high pressure properties of the novel tetrel-free clathrate, Ba₈Cu13.1Zn3.3P29.6, were investigated using synchrotron powder X-ray diffraction. The pressure was applied using a diamond anvil cell. No structural transitions or decomposition were detected in the studied pressure range of 0.1-7 GPa. The calculated bulk modulus for Ba₈Cu13.1Zn3.3P29.6 using a third-order Birch-Murnaghan equation of state is 65(6) GPa at 300 K. This bulk modulus is comparable to the bulk moduli of Ge- and Sn-based clathrates, like A₈Ga16Ge30 (A = Sr, Ba) and Sn19.3Cu4.7P22I₈, but lower than those for the transition metal-containing silicon-based clathrates, Ba₈TxSi46-x, T = Ni, Cu; 3 ≤ x ≤ 5

Equilibrium composition between liquid and clathrate reservoirs on Titan

Hundreds of lakes and a few seas of liquid hydrocarbons have been observed by the Cassini spacecraft to cover the polar regions of Titan. A significant fraction of these lakes or seas could possibly be interconnected with subsurface liquid reservoirs of alkanes. In this paper, we investigate the interplay that would happen between a reservoir of liquid hydrocarbons located in Titan's subsurface and a hypothetical clathrate reservoir that progressively forms if the liquid mixture diffuses throughout a preexisting porous icy layer. To do so, we use a statistical-thermodynamic model in order to compute the composition of the clathrate reservoir that forms as a result of the progressive entrapping of the liquid mixture. This study shows that clathrate formation strongly fractionates the molecules between the liquid and the solid phases. Depending on whether the structure I or structure II clathrate forms, the present model predicts that the liquid reservoirs would be mainly composed of either propane or ethane, respectively. The other molecules present in the liquid are trapped in clathrates. Any river or lake emanating from subsurface liquid reservoirs that significantly interacted with clathrate reservoirs should present such composition. On the other hand, lakes and rivers sourced by precipitation should contain higher fractions of methane and nitrogen, as well as minor traces of argon and carbon monoxide.Comment: Accepted for publication in Icaru

Quasiperiodic ordering in thick Sn layer on ii-Al-Pd-Mn: A possible quasicrystalline clathrate

Realization of an elemental solid-state quasicrystal has remained a distant dream so far in spite of extensive work in this direction for almost two decades. Here, we report the discovery of quasiperiodic ordering in a thick layer of elemental Sn grown on icosahedral ($i$)-Al-Pd-Mn. The STM images and the LEED patterns of the Sn layer show specific structural signatures that portray quasiperiodicity but are distinct from the substrate. Photoemission spectroscopy reveals the existence of the pseudogap around the Fermi energy up to the maximal Sn thickness. The structure of the Sn layer is modeled as a novel form of quasicrystalline clathrate on the basis of the following: Firstly, from ab-initio theory, the energy of bulk Sn clathrate quasicrystal is lower than the high temperature metallic $\beta$-Sn phase, but higher than the low temperature $\alpha$-Sn phase. A comparative study of the free slab energetics shows that surface energy favors clathrate over $\alpha$-Sn up to about 4 nm layer thickness, and matches $\beta$-Sn for narrow window of slab thickness of 2-3 nm. Secondly, the bulk clathrate exhibits gap opening near Fermi energy, while the free slab form exhibits a pronouced pseudogap, which explains the pseudogap observed in photoemission. Thirdly, the STM images exhibit good agreement with clathrate model. We establish the adlayer-substrate compatibility based on very similar (within 1%) the cage-cage separation in the Sn clathrate and the pseudo-Mackay cluster-cluster separation on the $i$-Al-Pd-Mn surface. Furthermore, the nucleation centers of the Sn adlayer on the substrate are identified and these are shown to be a valid part of the Sn clathrate structure. Thus, based on both experiment and theory, we propose that 4 nm thick Sn adlayer deposited on 5-fold surface of $i$-Al-Pd-Mn substrate is in fact a metastable realization of elemental, clathrate family quasicrystal.Comment: 10 figures in the Manuscript and the 8 figures in the Supplementary materia

Pressure stability and low compressibility of intercalated cagelike materials: the case of silicon clathrates

We study the behavior under pressure (up to 35 GPa) of intercalated silicon clathrates, combining x-ray diffraction experiments and ab initio calculations. We show that endohedral doping does not introduce a strong modification of the compressibility of the empty clathrate network and that in particular cases can raise it to values equivalent to the one of the silicon diamond phase. Intercalation can also prevent the collapse of the cage structure up to pressures at least 3 times higher than in the empty clathrate. Further we find that the stability of all studied silicon clathrate networks as well as stressed silicon diamond is limited to average Si-Si interatomic distances higher than 2.30 Angstrom

The fate of ethane in Titan's hydrocarbon lakes and seas

Ethane is expected to be the dominant photochemical product on Titan's surface and, in the absence of a process that sequesters it from exposed surface reservoirs, a major constituent of its lakes and seas. Absorption of Cassini's 2.2 cm radar by Ligeia Mare however suggests that this north polar sea is dominated by methane. In order to explain this apparent ethane deficiency, we explore the possibility that Ligeia Mare is the visible part of an alkanofer that interacted with an underlying clathrate layer and investigate the influence of this interaction on an assumed initial ethane-methane mixture in the liquid phase. We find that progressive liquid entrapment in clathrate allows the surface liquid reservoir to become methane-dominated for any initial ethane mole fraction below 0.75. If interactions between alkanofers and clathrates are common on Titan, this should lead to the emergence of many methane-dominated seas or lakes.Comment: Accepted for publication in Icaru