Single-crystal X-ray diffraction under high pressure
A saturated KCl solution, corresponding to 26.5 wt% at 298 K and atmospheric pressure (Pinho & Macedo, 2005 ▸), was prepared by dissolving an excess amount of reagent-grade KCl (99.5%) purchased from Wako Corporation in Milli-Q water. The solution was loaded into a diamond anvil cell (DAC) with a small amount of crystalline KCl to achieve the desired measurement conditions, whose details are described later. A pair of Boehler-Almax-type diamond anvils (Boehler & De Hantsetters, 2004 ▸) with a culet diameter of 600 µm were used. Stainless steel (SUS301) plates were used as a gasket with a ϕ = 400 µm hole as a sample space. To obtain high-quality X-ray diffraction data, a PFA (Teflon PFA) ring with an inner diameter of 200 µm was introduced as an inner gasket and a modified ‘clover seat’ backing seat was used (Komatsu et al., 2011 ▸). The details of the backing seat are described in Section S1 in the supporting information. A small ruby sphere was introduced in the sample space to estimate the sample pressure from the ruby fluorescent method (Piermarini et al., 1975 ▸). The sample pressure for the diffraction measurements was determined as the average and the deviation between before and after the measurements.
The sealed sample was compressed up to 2.4 GPa at 295 K and heated to ∼350 K. At these high-pressure and high-temperature conditions, single crystals of ice VII formed after cyclic compression and decompression. After the crystal growth of ice VII, the sample pressure decreased to ∼2.3 GPa at ∼320 K. The sample was compressed and heated again until the remaining solution started to freeze. Further compression and decompression were repeated to obtain single crystals of the KCl hydrate, co-existing with ice VII at 2.3 GPa and 295 K (Fig. 1 ▸).
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Ideally, no co-existing crystals are preferred for measurements without interference from extra Bragg spots. However, water ices inevitably crystallize before the formation of KCl hydrate. We initially tested a KCl-saturated solution without KCl crystals as a starting material, but this resulted in its co-existence with ice VI. Ice VI has an orthorhombic structure with lattice parameters a ∼ 6.2 Å and c ∼ 5.7 Å, and their Bragg spots were harmful for indexing and intensity extraction of the Bragg peaks of the hydrate. We then decided to introduce additional crystalline KCl in a saturated solution to suppress the crystallization of water ice. The solubility of KCl increases up to a certain concentration upon compression. In the experiments, the measurement conditions were tuned to establish two requirements: (i) the co-existence with ice VII stable above 2 GPa rather than ice VI and (ii) the complete dissolution of the added KCl crystals into the solution before the formation of the hydrate. Ice VII has a highly symmetric structure with small lattice parameters of a ∼ 3.3 Å and exhibits a smaller number of Bragg spots than ice VI. Furthermore, the increase of KCl concentration is advantageous for the formation of a larger fraction of KCl hydrate in the sample space. In our preliminary experiments, the remaining crystalline KCl hardly transformed into the hydrate even co-existing with water ice.
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The DAC containing the single-crystalline specimens was placed on an X-ray diffractometer (Rigaku, Synergy custom). The sample was irradiated with X-rays (Mo Kα, λ = 0.7107 Å) from a micro-focused X-ray generator (Rigaku, MicroMax-007) and diffraction was detected with a hybrid photon counting X-ray detector (Rigaku, HyPix-6000HE). Experimental details and results are summarized in Table 1 ▸. The collected diffraction patterns were indexed and the diffraction intensities were extracted using CrysAlis PRO (Agilent, 2014 ▸). The diffraction intensities were corrected for attenuation by the diamond anvils using a self-made ad hoc program. Details of this program are described in Section S3 in the supporting information. In this correction procedure, unreasonable diffraction peaks out of the opening angle of the DAC were eliminated from the geometric calculations. Diffraction intensities less than 3σ were also eliminated to exclude diffractions which were accidentally blocked by the metal parts of the DAC or strongly attenuated by the metal gasket.
The initial structure of the hydrate was determined by direct methods using SIR2018 (Burla et al., 2015 ▸). The crystal structure without H atoms was refined using SHELXL2018 (Sheldrick, 2015 ▸) within WinGX (Farrugia, 2012 ▸), without any parameter restrictions. The crystal structure models are described using VESTA (Momma & Izumi, 2011 ▸).
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