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

The crystal structure of calcium perchlorate anhydrate was determined from laboratory X-ray powder diffraction data. The title compound was obtained by heating hydrated calcium perchlorate [Ca(ClO₄)₂·xH₂O] at 623 K in air for 12 h. It crystallizes in the ortho­rhom­bic space group Pbca and is isotypic with Ca(AlD₄)₂. The asymmetric unit contains one Ca, two Cl and eight O sites, all on general sites (Wyckoff position 8c). The crystal structure consists of isolated ClO₄⁻ tetra­hedra and Ca²⁺ cations. The Ca²⁺ cation is coordinated by eight O atoms of eight symmetry-related ClO₄⁻ tetra­hedra within a distorted square-anti­prismatic environment.

1. 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 −2.87 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 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).

Figure 1 PXRD Rietveld refinement profiles for anhydrous Ca(ClO4)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).

2. Structural commentary

The crystal structure of anhydrous calcium perchlorate, Ca(ClO₄)₂, is isotypic with that of Ca(AlD₄)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(ClO₄)₂ are presented in Fig. 2, using ClO₄⁻ tetra­hedra and Ca²⁺ cations. The unit cell contains one Ca (on general positions 8c), two Cl (8c), and eight O (8c) sites. The ClO₄⁻ 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 Ca²⁺ cation is presented in Fig. 3. It is coordinated by eight isolated ClO₄⁻ tetra­hedra with an apex oxygen atom of each tetra­hedron bonded to the Ca²⁺ 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). The coordination number of the Mg²⁺, Ca²⁺, and Ba²⁺ cations in the anhydrous perchlorates increases from 6, 8, and to 12, respectively.

Table 1
Selected bond lengths (Å)

Ca1—O1^i	2.451 (6)	Cl1—O2	1.411 (6)
Ca1—O2^ii	2.412 (6)	Cl1—O6	1.414 (6)
Ca1—O3	2.448 (6)	Cl1—O7	1.421 (6)
Ca1—O4^iii	2.370 (6)	Cl1—O8	1.423 (6)
Ca1—O5^ii	2.429 (6)	Cl2—O1	1.456 (6)
Ca1—O6^iv	2.512 (6)	Cl2—O3	1.408 (6)
Ca1—O7^i	2.519 (6)	Cl2—O4	1.453 (6)
Ca1—O8	2.413 (6)	Cl2—O5	1.442 (6)
Symmetry codes: (i) -x+1, -y+1, -z+1; (ii) ; (iii) ; (iv) .

Figure 2 The crystal structure of Ca(ClO4)2 with ClO4^− tetra­hedra (yellow) and Ca^2+ cations (purple), showing (a) a view approximately along [001] and (b) approximately along [010].

Figure 3 The local environment of the Ca^2+ cation (purple sphere) surrounded by ClO4^− tetra­hedra (yellow). [Symmetry codes: (i) −x + 1, −y + 1, −z + 1; (ii) x, −y + , z + ; (iii) −x + 1, y − , −z + ; (iv) −x + , −y + 1, z + .]

3. Synthesis and crystallization

In order to prepare calcium perchlorate anhydrate, Ca(ClO₄)₂·xH₂O (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⁻¹, 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.

4. 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 Kα₁ radiation (λ = 1.5406 Å) at 40 kV and 30 mA, using a graphite monochromator and a Pixcel3D 2×2 detector. X-ray intensities were measured for 12 h at 0.013° inter­vals in the angular range of 5° ≤ 2θ ≤ 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 & Hušák, 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).

Table 2
Experimental details

Crystal data
Chemical formula	Ca(ClO4)2
Mr	238.98
Crystal system, space group	Orthorhombic, Pbca
Temperature (K)	295
a, b, c (Å)	13.75102 (8), 9.50887 (5), 9.06168 (5)
V (Å^3)	1184.88 (1)
Z	8
Radiation type	Cu Kα1, λ = 1.5405 Å
Specimen shape, size (mm)	Flat sheet, 24.9 × 24.9
Data collection
Diffractometer	PANalytical Empyrean
Specimen mounting	Packed powder
Data collection mode	Reflection
Scan method	Step
2θ values (°)	2θmin = 5.001 2θmax = 139.993 2θstep = 0.013
Refinement
R factors and goodness of fit	Rp = 0.068, Rwp = 0.104, Rexp = 0.055, R(F^2) = 0.151, χ^2 = 3.610
No. of parameters	44
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).

Supporting information

Computing details

Data collection: X'Pert Data Collector (PANalytical, 2011); cell refinement: GSAS (Larson & Von Dreele, 2000); data reduction: X'Pert HighScore Plus (PANalytical, 2011); program(s) used to solve structure: SHELXS97 (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).

Calcium bis(perchlorate)

Crystal data

Ca(ClO4)2	Z = 8
Mr = 238.98	F(000) = 944.0
Orthorhombic, Pbca	Dx = 2.680 Mg m^−3
Hall symbol: -P_2ac_2ab	Cu Kα1 radiation, λ = 1.5405 Å
a = 13.75102 (8) Å	T = 295 K
b = 9.50887 (5) Å	white
c = 9.06168 (5) Å	flat_sheet, 24.9 × 24.9 mm
V = 1184.88 (1) Å^3	Specimen preparation: Prepared at 295 K

Data collection

PANalytical Empyrean diffractometer	Data collection mode: reflection
Radiation source: sealed X-ray tube, PANalytical Cu Ceramic X-ray tube	Scan method: step
Specimen mounting: packed powder	2θmin = 5.001°, 2θmax = 139.993°, 2θstep = 0.013°

Refinement

Least-squares matrix: full	Excluded region(s): 5 to 12.5 degrees are excluded due to background scattering at low angles, in addition there are no peaks in this region.
Rp = 0.068	Profile function: CW Profile function number 4 with 18 terms Pseudovoigt profile coefficients as parameterized in P. Thompson, D.E. Cox & J.B. Hastings (1987). J. Appl. Cryst.,20,79-83. Asymmetry correction of L.W. Finger, D.E. Cox & A. P. Jephcoat (1994). J. Appl. Cryst.,27,892-900. Microstrain broadening by P.W. Stephens, (1999). J. Appl. Cryst.,32,281-289. #1(GU) = 9.638 #2(GV) = -11.095 #3(GW) = 2.275 #4(GP) = 4.393 #5(LX) = 0.935 #6(ptec) = 0.00 #7(trns) = 0.00 #8(shft) = -4.2154 #9(sfec) = 0.00 #10(S/L) = 0.0005 #11(H/L) = 0.0005 #12(eta) = 0.7500 #13(S400 ) = 0.0E+00 #14(S040 ) = 0.0E+00 #15(S004 ) = 0.0E+00 #16(S220 ) = 0.0E+00 #17(S202 ) = 0.0E+00 #18(S022 ) = 0.0E+00 Peak tails are ignored where the intensity is below 0.0100 times the peak Aniso. broadening axis 0.0 0.0 1.0
Rwp = 0.104	44 parameters
Rexp = 0.055	0 restraints
R(F^2) = 0.15096	(Δ/σ)max = 0.04
10385 data points	Background function: GSAS Background function number 1 with 36 terms. Shifted Chebyshev function of 1st kind 1: 396.859 2: -606.961 3: 459.581 4: -240.760 5: 60.9683 6: 66.1787 7: -127.055 8: 123.403 9: -80.0454 10: 22.9955 11: 31.6319 12: -68.9521 13: 82.3967 14: -74.9306 15: 52.4628 16: -22.9755 17: -7.07207 18: 29.6007 19: -41.2483 20: 39.7866 21: -28.2300 22: 12.3296 23: 2.74056 24: -14.4441 25: 20.2978 26: -20.5325 27: 15.0728 28: -6.57858 29: -1.96745 30: 7.61710 31: -10.5263 32: 10.4139 33: -6.95249 34: 2.74624 35: 0.930279 36: -1.93129

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Ca1	0.39788 (14)	0.5357 (2)	0.7164 (2)	0.0110 (2)*
Cl1	0.34080 (17)	0.6066 (3)	0.3157 (3)	0.0110 (2)*
Cl2	0.55928 (18)	0.7776 (3)	0.4961 (3)	0.0110 (2)*
O1	0.6154 (4)	0.7025 (6)	0.3850 (6)	0.0110 (2)*
O2	0.3176 (4)	0.7464 (6)	0.2773 (6)	0.0110 (2)*
O3	0.5240 (4)	0.6775 (7)	0.5973 (7)	0.0110 (2)*
O4	0.6137 (4)	0.8834 (6)	0.5773 (6)	0.0110 (2)*
O5	0.4842 (4)	0.8546 (7)	0.4199 (6)	0.0110 (2)*
O6	0.2815 (4)	0.5078 (6)	0.2414 (6)	0.0110 (2)*
O7	0.4359 (4)	0.5744 (6)	0.2647 (7)	0.0110 (2)*
O8	0.3387 (4)	0.5833 (6)	0.4708 (7)	0.0110 (2)*

Geometric parameters (Å, º)

Ca1—Cl1	3.776 (3)	Cl2—Ca1	3.769 (3)
Ca1—Cl1^i	3.605 (3)	Cl2—Ca1^v	3.808 (3)
Ca1—Cl1^ii	3.662 (3)	Cl2—Ca1^iii	3.596 (3)
Ca1—Cl1^iii	3.851 (3)	Cl2—Ca1^vii	3.627 (3)
Ca1—Cl2	3.769 (3)	Cl2—O1	1.456 (6)
Ca1—Cl2^i	3.808 (3)	Cl2—O3	1.408 (6)
Ca1—Cl2^iii	3.596 (3)	Cl2—O4	1.453 (6)
Ca1—Cl2^iv	3.627 (3)	Cl2—O5	1.442 (6)
Ca1—O1^iii	2.451 (6)	O1—Ca1^iii	2.451 (6)
Ca1—O2^i	2.412 (6)	O1—Cl2	1.456 (6)
Ca1—O3	2.448 (6)	O2—Ca1^v	2.412 (6)
Ca1—O4^iv	2.370 (6)	O2—Cl1	1.411 (6)
Ca1—O5^i	2.429 (6)	O3—Ca1	2.448 (6)
Ca1—O6^ii	2.512 (6)	O3—Cl2	1.408 (6)
Ca1—O7^iii	2.519 (6)	O4—Ca1^vii	2.370 (6)
Ca1—O8	2.413 (6)	O4—Cl2	1.453 (6)
Cl1—Ca1	3.776 (3)	O5—Ca1^v	2.429 (6)
Cl1—Ca1^v	3.605 (3)	O5—Cl2	1.442 (6)
Cl1—Ca1^vi	3.662 (3)	O6—Ca1^vi	2.512 (6)
Cl1—Ca1^iii	3.851 (3)	O6—Cl1	1.414 (6)
Cl1—O2	1.411 (6)	O7—Ca1^iii	2.519 (6)
Cl1—O6	1.414 (6)	O7—Cl1	1.421 (6)
Cl1—O7	1.421 (6)	O8—Ca1	2.413 (6)
Cl1—O8	1.423 (6)	O8—Cl1	1.423 (6)
O1^iii—Ca1—O2^i	147.7 (2)	O6^ii—Ca1—O8	77.5 (2)
O1^iii—Ca1—O3	113.4 (2)	O7^iii—Ca1—O8	116.53 (2)
O1^iii—Ca1—O5^i	135.9 (2)	O2—Cl1—O6	112.2 (4)
O1^iii—Ca1—O6^ii	79.0 (2)	O2—Cl1—O7	109.4 (4)
O1^iii—Ca1—O7^iii	73.10 (19)	O2—Cl1—O8	112.7 (4)
O1^iii—Ca1—O8	78.62 (18)	O6—Cl1—O7	103.5 (4)
O2^i—Ca1—O3	87.3 (2)	O6—Cl1—O8	110.8 (4)
O2^i—Ca1—O5^i	71.4 (2)	O7—Cl1—O8	107.8 (4)
O2^i—Ca1—O6^ii	70.80 (19)	O1—Cl2—O3	107.6 (4)
O2^i—Ca1—O7^iii	139.2 (2)	O1—Cl2—O4	114.6 (4)
O2^i—Ca1—O8	84.0 (2)	O1—Cl2—O5	107.3 (4)
O3—Ca1—O5^i	75.6 (2)	O3—Cl2—O4	108.4 (4)
O3—Ca1—O6^ii	145.7 (2)	O3—Cl2—O5	114.1 (4)
O3—Ca1—O7^iii	67.4 (2)	Ca1^iii—O1—Cl2	132.3 (4)
O3—Ca1—O8	74.3 (2)	Ca1^v—O2—Cl1	139.7 (4)
O5^i—Ca1—O6^ii	118.8 (2)	Ca1—O3—Cl2	154.6 (4)
O5^i—Ca1—O7^iii	71.6 (2)	Ca1^v—O5—Cl2	158.7 (4)
O5^i—Ca1—O8	141.7 (2)	Ca1^vi—O6—Cl1	135.9 (4)
O6^ii—Ca1—O7^iii	144.7 (2)	Ca1^iii—O7—Cl1	154.5 (4)

Symmetry codes: (i) x, −y+3/2, z+1/2; (ii) −x+1/2, −y+1, z+1/2; (iii) −x+1, −y+1, −z+1; (iv) −x+1, y−1/2, −z+3/2; (v) x, −y+3/2, z−1/2; (vi) −x+1/2, −y+1, z−1/2; (vii) −x+1, y+1/2, −z+3/2.