Crystal structure and electrical resistance property of Rb0.21(H2O)yWS2

Determination of the crystal structure of Rb0.21(H2O)yWS2 is beneficial to understanding the topotactic reaction process to form metastable WS2. The temperature dependence of the electrical resistance indicates that Rb0.21(H2O)yWS2 is semiconducting at 80-300 K.


Chemical context
Typical two-dimensional structures of MS 2 compounds (M = transition metals of group IVB-VIB) facilitate the intercalation of various atoms, ions or organic molecules (Whittingham et al., 1978). For example, A x MS 2 (A = alkali metal; M = Nb, Ta, Ti, V) compounds can be prepared in hightemperature solid-state reactions (800-1000 K). These compounds can react with water molecules to form ionic hydrates A + x (H 2 O) y [MS 2 ] xÀ (Omloo & Jellinek, 1970;Lobert et al., 1992) that exhibit ion-exchange and solvent-exchange capacities. Some of the A + x (H 2 O) y [MS 2 ] xÀ compounds show unusual superconducting properties (Schö llhorn & Weiss, 1974;Sernetz et al., 1974). Recently, by removing alkali ions from intercalated A + x (H 2 O) y [MS 2 ] xÀ (A = alkali metal) compounds, several metastable MS 2 (M = Mo, W) phases with new crystal structures and novel physical properties were reported (Fang et al., , 2019. In order to identify the formation mechanism of metastable MS 2 from A + x (H 2 O) y [MS 2 ] xÀ , it is necessary to uncover the role of alkali ions intercalated into the interlayers of MS 2 .
In this communication, we report the preparation of Rb 0.21 (H 2 O) y WS 2 , its crystal structure determination by single crystal X-ray diffraction, its thermal behaviour and its electrical resistance property.

Structural commentary
Rb 0.21 (H 2 O) y WS 2 crystallizes in the monoclinic P2 1 /m (No. 11) space group. The structure consists of one independent W site, two independent S sites and two independent Rb sites, all of them located on a mirror plane (Wyckoff position 2e). The crystal structure features ordered WS 2 layers separated by ISSN 2056-9890 disordered Rb + ions, and of water molecules. The latter could not be localized in the current study, hence y in Rb 0.21 (H 2 O) y WS 2 remains undetermined (see Experimental, and discussion below). Compared with [WS 6 ] 8trigonal prisms in 2H-WS 2 (Schutte et al., 1987), the WS 2 layer in Rb 0.21 (H 2 O) y WS 2 is composed of edge-sharing [WS 6 ] 8.21octahedra. The W-S bond lengths range from 2.403 (4) Å to 2.550 (5) Å , and thus the average W-S distance is larger than that in 2H-WS 2 [2.405 (5) Å ; Schutte et al., 1987]. The WS 2 layers extend parallel to (001) (Fig. 1). The shortest W-W bond length of 2.7678 (15) Å is between pairs of W atoms aligned in the [110] direction, much shorter than the WÁ Á ÁW distance of 3.2524 (18) Å along [010]. Similar metal-metal separations also exist in some metastable MS 2 phases prepared by de-intercalating alkali ions from A x (H 2 O) y MS 2 compounds (Yu et al., 2018;Shang et al., 2018). The Rb + cations show a one-sided coordination to the S atoms of the adjacent layer. The Rb-S bonds range from 3.47 (7) Å to 3.64 (5) Å , comparable to the Rb-S bonds [3.344 (7)-3.561 (1) Å ] in RbCr 5 S 8 (Huster, 1978).
Similar to K x (H 2 O) y TaS 2 and K x (H 2 O) y NbS 2 (Graf et al., 1977), it was impossible to determine the light O atoms of water molecules in the title compound from X-ray diffraction data at room temperature, as a result of diffuse electron density in the interlayer space. However, we could localize the positions of disordered Rb + ions with large displacement parameters. Stacking disorder of the layers is common for layered dichalcogenides, which may contribute to the diffuse electron density. Large displacement parameters of exchangeable cations and water molecules were also reported for

Electrical resistance property
The electrical resistance of Rb 0.21 (H 2 O) y WS 2 increases with the decrease of temperature (80-300 K) (Fig. 2), which is characteristic of a semiconductor.

Synthesis and crystallization
A rubidium dithiotungstate Rb x WS 2 was synthesized in a solid-state reaction. The starting Rb 2 S 2 powder was prepared in a reaction of stoichiometric amounts of Rb pieces and S powder in liquid NH 3 . The obtained Rb 2 S 2 powder, W powder and S powder were mixed in the molar ratio of 1:1:1 in a glove box filled with Ar. The mixture was ground carefully and loaded in a carbon-coated fused-silica tube. The tube was sealed under a 10 À4 Torr atmosphere and slowly heated to 1123 K at 5 K min À1 . After three days, the furnace was cooled down naturally to room temperature. Subsequent removal of the extra flux by washing with distilled water led to the isolation of crystals of Rb 0.21 (H 2 O) y WS 2 . The morphology and element composition were investigated by using an EDXSequipped Hitachi S-4800 scanning electronic microscope. In addition, the Rb/W ratio in the Rb x (H 2 O) y WS 2 crystals was determined by ICP-OES. The SEM image and EDX spectrum of Crystal structure of Rb 0.21 (H 2 O) y WS 2 with displacement ellipsoids drawn at the 30% probability level.  Rb/W from the EDXS analysis is close to 0.21, which is consistent with the the diffraction data and results from ICP-OES measurements ( Table 1). The experimental powder X-ray diffraction (PXRD) pattern matches well with the simulated one (Fig. 4) by using the Rietveld refinement method (Rodríguez-Carvajal, 1993; R p = 9.9%, R wp = 12.6% and 2 = 1.3). In the TG-DTA analyses (Fig. 5), one obvious endothermic effect and concomitant mass loss were observed at 343 K, which is associated with water evaporation. In order to judge whether water molecules are surface-adsorbed water or structural water, the Rb 0.21 (H 2 O) y WS 2 crystals were heated up to 373 K for further PXRD measurement. The sample was prepared in an Ar-protected glove box and sealed with vacuum tape. The (002) reflection clearly moved to higher diffraction angles, indicating the shrinkage of the unit cell due to loss of intercalated water (Fig. 6). However, it was impossible to accurately determine the water content by mass loss alone because of the interference of possible surface-adsorbed water.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. The localization of ordered W and S sites of the WS 2 layers was unproblematic. The highest interlayer difference electron density peak was then treated as a single but partially occupied Rb site. No evidence of superstructure reflections in reciprocal space was found for the ordering of the Rb site. Then, the W, S sites and the underoccupied Rb site were refined with anisotropic displa-cement parameters. Because of very large anisotropic displacement parameters (U 11 = 0.59 Å 2 ) of the Rb site, splitting of this site was considered, resulting in a residual R 1 = 0.051. Modelling the O sites as being part of this disorder, or of remaining electron density peaks in the vicinity of the Rb sites was not successful, and therefore we did not include the apparently disordered water molecules in the final structure model. The remaining maximum and minimum electron densities are located 0.87 and 1.14 Å , respectively, from the W1 site.

rubidium hydrate dithiotungstate
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.