Crystal structure of calcium perchlorate anhydrate, Ca(ClO4)2, from laboratory powder X-ray diffraction data

The crystal structure of anhydrous Ca(ClO4)2 crystallizes isotypically with Ca(AlD4)2.


Chemical context
Recently, the alkaline earth metals, in particular magnesium and calcium, have received attention because of their incorporation in multivalent-ion batteries that can replace Li-ion batteries (Wang et al., 2013;Datta et al., 2014;Amatucci et al., 2001). Calcium has several merits, such as low cost and abundance in nature (Padigi et al., 2015;Rogosic et al., 2014). In addition, the standard reduction potential of the calcium electrode is À2.87 V, which is only about 0.18 V higher than that of lithium (Muldoon et al., 2014). Thus, calcium perchlorate is mainly used as a salt next to organic electrolytes in Ca-ion batteries (Hayashi et al., 2003). Nevertheless, the crystal structure of anhydrous calcium perchlorate was PXRD Rietveld refinement profiles for anhydrous Ca(ClO 4 ) 2 measured at ambient temperature. Crosses mark experimental data (black), the solid red line represents the calculated profile (red) and the solid green line is the background. The bottom trace represents the difference curve (blue) and the ticks denote the positions of expected Bragg reflections (magenta).
unknown until now (Pearse & Pflaum, 1959) because of the lack of single crystals. Calcium perchlorate is strongly hygroscopic, and growing single crystals of a size sufficient for X-ray structure analysis has not been successful up to date. On the other hand, the crystal structures of the perchlorates of magnesium, barium and other alkaline earth metals have been determined for both hydrated and anhydrous phases (Gallucci & Gerkin, 1988;Lee et al., 2015;Lim et al., 2011;Robertson & Bish, 2010). However, for calcium perchlorate only the hydrated forms were structurally determined (Hennings et al., 2014).
We present here the crystal structure of calcium perchlorate anhydrate, using laboratory powder X-ray diffraction (PXRD) data (Fig. 1).

Structural commentary
The crystal structure of anhydrous calcium perchlorate, Ca(ClO 4 ) 2 , is isotypic with that of Ca(AlD 4 ) 2 (Sato et al., 2009), but is different from barium or magnesium perchlorates (Lee et al., 2015;Lim et al., 2011). Different viewing directions of the crystal structure of Ca(ClO 4 ) 2 are presented in Fig. 2, using ClO 4 À tetrahedra and Ca 2+ cations. The unit cell contains one Ca (on general positions 8c), two Cl (8c), and eight O (8c) sites. The ClO 4 À tetrahedra are slightly distorted [mean Cl-O distance 1.43 (2) Å , angular range 103.5 (4)-114.6 (4) ] and isolated from each other. The local environment around the Ca 2+ cation is presented in Fig. 3. It is coordinated by eight isolated ClO 4 À tetrahedra with an apex oxygen atom of each tetrahedron bonded to the Ca 2+ cation. The resulting coordination sphere can be considered as a distorted square antiprism. The average Ca-O distance is 2.476 Å (Table 1), which is intermediate between those of comparable Mg-O (2.098 Å ) and Ba-O (2.989 Å ) polyhedra (Lee et al., 2015;Lim et al., 2011), and consistent with the sum of the ionic radii of the alkaline earth metals and oxygen (Shannon, 1976

Synthesis and crystallization
In order to prepare calcium perchlorate anhydrate, Ca(ClO 4 ) 2 ÁxH 2 O (reagent grade, Alfa Aesar) was placed in 75 ml glass vials. The vials were placed into a box furnace, heated at 623 K for 12 h with a heating rate of 3 K min À1 , cooled down to 423 K, and transferred to a glove box under an Ar atmosphere. The exposed time in a normal atmosphere during the transfer was about 10 s. The sample was ground using an agate mortar, and placed in a dome-type PXRD sample holder that was sealed tightly to prevent atmospheric exposure during the data collection.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 2. The powder XRD data of anhydrous calcium perchlorate were collected using a Bragg-Brentano diffractometer (PANalytical Empyrean) with Cu K 1 radiation ( = 1.5406 Å ) at 40 kV and 30 mA, using a graphite monochromator and a Pixcel3D 2Â2 detector. X-ray intensities were measured for 12 h at 0.013 intervals in the angular range of 5 2 140 . X-ray diffraction data were indexed by the TREOR90 algorithm (Werner, 1990) in the CRYSFIRE program suite (Shirley, 2002), with 22 indexed reflections starting from the smallest angle. An orthorhombic unit cell was revealed suggesting Pbca as the most probable space group. Based on these results, the refinement process was performed using the GSAS program (Larson & Von Dreele, 2000) and the CRYSTALS program (Betteridge et al., 2003). The process was started with the assumption that there is one dummy atom at an arbitrary position. Then direct methods were applied to calculate the initial solution of the crystal structure using SHELXS97 (Sheldrick, 2008), which yielded a Ca site as a starting postition. The initial model was then replaced with the partial model, and this data was used for a LeBail fit in GSAS. Then, improved structure factors were calculated, which were used for the refinement in CRYSTALS. These processes were repeated until a complete and sufficient structural model converged. Based on these results, the MCE programme (Rohlíček & Hušá k, 2007) was used to draw the calculated Fourier-density map in three dimensions. For the final Rietveld refinement with GSAS, an overall displacement parameter was used, and Cl-O bond lengths were restrained with a tolerance value of 25% from the distances determined from CRYSTALS, where the distances matched well with Shannon's radii sum. Pseudovoigt profile coefficients as parameterized in Thompson et al. (1987), asymmetry correction of Finger et al. (1994) and microstrain broadening of Stephens (1999).  Computer programs: X'Pert Data Collector and X'Pert HighScore Plus (PANalytical, 2011), GSAS (Larson & Von Dreele, 2000), SHELXS97 (Sheldrick, 2008), CRYSTALS (Betteridge et al., 2003) and ATOMS (Dowty, 2000).  (Sheldrick, 2008) and CRYSTALS (Betteridge et al., 2003); program(s) used to refine structure: GSAS (Larson & Von Dreele, 2000); molecular graphics: ATOMS (Dowty, 2000); software used to prepare material for publication: GSAS (Larson & Von Dreele, 2000).

Crystal data
Ca (