A NEW GAS HYDRATE STRUCTURE

Abstract

Two gas hydrate structures—cubic structures I and II (CS-I and CS-II, respectively)—were discovered by von Stackelberg, Pauling, and Claussen about half a century ago [1‐3]. Recently, a third type of gas hydrate structure, hexagonal structure III (HS-III) found in double hydrates, has been reported [4]. The vast majority of gas hydrates available at moderate pressures (up to several hundreds of atmospheres) belong to these three structural types [5]. Therefore, the structures of gas hydrates (“ideal clathrates” in which the guest molecule does not participate in the formation of the hydrate framework) noticeably rank below the structures of clathrate hydrates in diversity; more than ten such structures can be counted [6]. Studies of phase diagrams of the systems in which gas hydrates exist at high pressures have shown that several hydrate phases can form in the same guest‐water system [6, 7]. Available structural information on high-pressure gas hydrate phases is scarce. To the best of our knowledge, the list of relevant publications include only eight entries; besides our previous paper [8], only one of these papers reports on the discovery of a new type of gas hydrate structure [9]. In the cited studies, these structures were not characterized by crystallographic data; only unit cell parameters were reported. In this paper, we describe a new (fourth) structural type of gas hydrate characterized by us in the course of neutron diffraction studies of high-pressure gas hydrate phases in the argon‐water and tetrahydrofuran‐water systems. Our neutron diffraction study of the first three (in order of increasing pressure) hydrates in the argon‐ heavy water system [8] showed that the third hydrate (a total of four hydrates were found in this system), which is stable at pressures of 7.7‐9.6 kbar (this range corresponds to the positions of quadruple points in the phase diagram of the argon‐H 2 é system), belongs to the tetragonal structural type, previously unknown among gas hydrates. Figure 1 shows the corresponding neutron powder diffraction pattern. The experimental procedure was described in [8]. The radiation wavelength was λ = 2.4236 Å. We failed to solve the structure of this hydrate by direct methods; however, we found an independent model that was suited to the symmetry and unit cell parameters of this hydrate. The water framework of this structure was found by idealizing the structure of pinacol semiclathrate hydrate [10]. The structure was refined with the FullProf program [11]. The refined unit cell parameters of argon hydrate were as follows: a = 6.342(2) Å , æ = 10.610(3) Å ( ê = 9.2 kbar, room temperature), space group P 4 2 / mnm (the tentative space group ê 4 2 2 1 2 reported in [8] is a subgroup of this group). The unit cell of the framework contains 12 wate molecules forming two cavities. The refined isotropic thermal factors appeared to be similar to those found under corresponding P , T -conditions for ices and helium hydrates based on the ice II lattice [12, 13]. In the course of refinement of all atomic coordinates and unit cell parameters, we obtained the calculated neutron diffraction pattern from a theoretical model that satisfactorily fitted the experimental pattern ( R = 7.9%). The refined coordinates of basis atoms are listed in the table.