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

The crystal structure of strontium perchlorate anhydrate, Sr(ClO₄)₂, was determined and refined from laboratory powder X-ray diffraction data. The material was obtained by dehydration of Sr(ClO₄)₂·3H₂O at 523 K for two weeks. It crystallizes in the ortho­rhom­bic space group Pbca and is isotypic with Ca(AlD₄)₂ and Ca(ClO₄)₂. The asymmetric unit contains one Sr, two Cl and eight O sites, all on general positions (Wyckoff position 8c). The crystal structure consists of Sr²⁺ cations and isolated ClO₄⁻ tetra­hedra. The Sr²⁺ cation is coordinated by eight O atoms from eight ClO₄⁻ tetra­hedra. The validity of the crystal structure model for Sr(ClO₄)₂ anhydrate was confirmed by the bond valence method.

1. Chemical context

The alkaline earth metal ions (Mg²⁺, Ca²⁺, Sr²⁺ and Ba²⁺) have received attention as ion carriers for next-generation batteries (Wang et al., 2013), and their perchlorates are used as inorganic salts of conventional nona­queous electrolytes for electrochemical cells in Mg- and Ca-ion batteries (Whittingham et al., 2018; Tchitchekova et al., 2017; Padigi et al., 2015). It is crucial to obtain anhydrous salts to achieve high electrochemical cell performance since hydrated salts can cause unwanted side reactions as a result of increased water content in the nona­queous electrolyte. Strontium perchlorate is highly hygroscopic and exists in several hydrated forms. So far, Sr(ClO₄)₂·3H₂O, Sr(ClO₄)₂·4H₂O and Sr(ClO₄)₂·9H₂O have been identified by single-crystal X-ray diffraction (Hennings et al., 2014). However, the crystal structure of the anhydrous phase has not been reported to date because of the difficulty in growing single crystals. Previously, we have determined the structures of anhydrous magnesium, barium and calcium perchlorate from laboratory powder X-ray diffraction data (Lim et al., 2011; Lee et al., 2015, 2018). Using the same techniques for the Sr salt, we were able to determine and refine the crystal structure of strontium perchlorate anhydrate.

2. Structural commentary

The crystal structure of anhydrous strontium perchlorate, Sr(ClO₄)₂, is isotypic with Ca(AlD₄)₂ (Sato et al., 2009) and Ca(ClO₄)₂ (Lee et al., 2018). Compared with Ca(ClO₄)₂, the unit-cell parameters a, b and c of Sr(ClO₄)₂ are increased by 3.0, 2.9 and 3.4%, respectively, because Sr²⁺ (1.26 Å for eight-coordination) has a larger ionic radius than Ca²⁺ (1.12 Å for eight-coordination; Shannon, 1976).

There are one Sr, two Cl and eight O sites in the asymmetric unit, all on general positions 8c. The crystal structure (Fig. 1) is composed of Sr²⁺ cations and isolated ClO₄⁻ tetra­hedra. The isolated ClO₄⁻ tetra­hedra are slightly distorted and exhibit a range of 105.4 (7)–113.5 (7)° for the O—Cl—O angles. The local environment around the Sr²⁺ cation is presented in Fig. 2. It is coordinated by eight O atoms from eight ClO₄⁻ tetra­hedra, with an average Sr—O distance of 2.582 Å (Table 1). The latter is inter­mediate between those of Ca—O (2.476 Å; Lee et al., 2018) and Ba—O (2.989 Å; Lee et al., 2015) polyhedra, and in good agreement with the sum of the ionic radii of the respective alkaline earth metal and oxygen ions (Shannon, 1976).

Table 1
Selected bond lengths (Å)

Sr1—O1^i	2.512 (5)	Cl1—O5	1.436 (5)
Sr1—O2^ii	2.591 (5)	Cl1—O6	1.436 (5)
Sr1—O3^iii	2.546 (5)	Cl1—O7	1.426 (5)
Sr1—O4^iv	2.622 (5)	Cl1—O8	1.423 (5)
Sr1—O5^v	2.650 (5)	Cl2—O1	1.469 (5)
Sr1—O6^ii	2.540 (5)	Cl2—O2	1.414 (5)
Sr1—O7^vi	2.590 (5)	Cl2—O3	1.425 (5)
Sr1—O8	2.604 (8)	Cl2—O4	1.422 (5)
Symmetry codes: (i) ; (ii) -x+1, -y+1, -z+1; (iii) ; (iv) ; (v) ; (vi) .

Figure 1 The local environment of the Sr^2+ cation (yellow sphere) surrounded by ClO4^− tetra­hedra (purple). Symmetry codes refer to Table 1.

Figure 2 The crystal structure of Sr(ClO4)2 in two different viewing directions, i.e. approximately along (a) [001] and (b) [010]. Sr^2+ cations are yellow and ClO4 tetra­hedra are purple.

Empirical bond valence sums (BVSs) can be used to check structure models (Brown, 2002). In this regard, the BVSs for the ions in the crystal structure of Sr(ClO₄)₂ were calculated with the program Valence (Brown & Altermatt, 1985; Brese & O'Keeffe, 1991; Hormillosa et al., 1993). The expected charges of the ions match well with the obtained BVS values (given in valence units), thus confirming the validity of the crystal structure: Sr 2.18, Cl1 6.99, Cl2 6.96, O1 1.91, O2 2.08, O3 2.06, O4 2.03, O5 1.96, O6 2.02, O7 2.03 and O8 2.04.

3. Synthesis and crystallization

Anhydrous strontium perchlorate was obtained by dehydration of Sr(ClO₄)₂·3H₂O (98%, Alfa Aesar). The hydrated Sr(ClO₄)₂ powder was ground thoroughly in an agate mortar and added to a glass bottle. The bottle was placed in an oven at 523 K for two weeks under atmospheric conditions and then transferred to a glove-box under an argon atmosphere. For the powder X-ray diffraction measurements, anhydrous Sr(ClO₄)₂ was again ground in an agate mortar and placed in a tightly sealed dome-type X-ray sample holder commercially available from Bruker. The dome was double-sealed with vacuum grease to prevent hydration during measurement.

4. Refinement details

Details of the crystal data collection and structure refinement are summarized in Table 2. Powder X-ray diffraction (PXRD) data for anhydrous Sr(ClO₄)₂ were collected from a Bragg–Brentano diffractometer (PANalytical Empyrean) using Cu Kα₁ radiation, a focusing primary Ge(111) monochromator (λ = 1.5406 Å) and a position-sensitive PIXcel 3D 2×2 detector. The angular range was 10 ≤ 2θ ≤ 130°, with a step of 0.0131° and a total measurement time of 8 h at room temperature. The PXRD pattern was indexed using the TREOR90 algorithm (Werner, 1990) run in CRYSFIRE (Shirley, 2002) through the positions of 23 reflections, resulting in an ortho­rhom­bic unit cell. Systematic reflection conditions suggested the space group Pbca. The crystal structure was determined by a combination of the powder profile refinement program GSAS (Larson & Von Dreele, 2000) and the single-crystal structure refinement program CRYSTALS (Betteridge et al., 2003). For a three-dimensional view of the Fourier electron-density maps, MCE was applied (Rohlícek & Husák, 2007). Initially, a structural model with only one dummy atom at an arbitrary position in the unit cell was used. Structure factors were extracted from the powder data and then direct methods were applied to calculate the initial solution of the crystal structure using SHELXS97 (Sheldrick, 2008) run in CRYSTALS, which yielded the Sr site as a starting atomic position. The initial dummy atom model was then replaced with the partial model, and this data was adopted for a Le Bail fit in GSAS. Improved structure factors were then extracted, which were used for the refinement in CRYSTALS. Such processes were iterated until a complete and satisfactory structural model was obtained. Finally, Rietveld refinement in GSAS was employed to complete the structure model, resulting in reasonable isotropic displacement parameters and agreement indices. For the final Rietveld refinement with GSAS, the Sr—O and Cl—O bond lengths were restrained with a tolerance value of 2% with respect to the distances determined from CRYSTALS, which matched reasonably well with the radii sums of Shannon (1976). The final Rietveld plot is displayed in Fig. 3.

Table 2
Experimental details

Crystal data
Chemical formula	Sr(ClO4)2
Mr	286.52
Crystal system, space group	Orthorhombic, Pbca
Temperature (K)	298
a, b, c (Å)	14.18206 (10), 9.78934 (11), 9.37624 (10)
V (Å^3)	1301.73 (2)
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 = 10.009, 2θmax = 129.991, 2θstep = 0.013
Refinement
R factors and goodness of fit	Rp = 0.086, Rwp = 0.125, Rexp = 0.096, R(F^2) = 0.14871, χ^2 = 1.716
No. of parameters	40
No. of restraints	23
Computer programs: X'Pert Data Collector (PANalytical, 2011), GSAS (Larson & Von Dreele, 2000), X'Pert HighScore Plus (PANalytical, 2011), SHELXS97 (Sheldrick, 2008), CRYSTALS (Betteridge et al., 2003) and VESTA (Momma & Izumi, 2011).

Figure 3 PXRD Rietveld refinement profiles for anhydrous Sr(ClO4)2 measured at ambient temperature. Black dots mark experimental data, the solid red line represents the calculated profile and the solid green line is the background. The bottom trace presents the difference curve (blue) and the ticks denote the expected Bragg reflection positions (magenta).

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: VESTA (Momma & Izumi, 2011); software used to prepare material for publication: GSAS (Larson & Von Dreele, 2000).

Strontium perchlorate anhydrate

Crystal data

Sr(ClO4)2	Z = 8
Mr = 286.52	F(000) = 1088.0
Orthorhombic, Pbca	Dx = 2.925 Mg m^−3
Hall symbol: -P_2ac_2ab	Cu Kα1 radiation, λ = 1.5405 Å
a = 14.18206 (10) Å	T = 298 K
b = 9.78934 (11) Å	white
c = 9.37624 (10) Å	flat_sheet, 24.9 × 24.9 mm
V = 1301.73 (2) Å^3	Specimen preparation: Prepared at 298 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 = 10.009°, 2θmax = 129.991°, 2θstep = 0.013°

Refinement

Least-squares matrix: full	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) = 0.000 #2(GV) = 0.000 #3(GW) = 0.000 #4(GP) = 9.252 #5(LX) = 0.900 #6(ptec) = 0.00 #7(trns) = 0.00 #8(shft) = -3.8100 #9(sfec) = 0.00 #10(S/L) = 0.0208 #11(H/L) = 0.0005 #12(eta) = 0.7500 #13(S400 ) = 0.0E+00 #14(S040 ) = 7.8E-04 #15(S004 ) = 1.5E-04 #16(S220 ) = 3.7E-04 #17(S202 ) = 6.1E-04 #18(S022 ) = -1.1E-03 Peak tails are ignored where the intensity is below 0.0010 times the peak Aniso. broadening axis 0.0 0.0 1.0
Rp = 0.086	40 parameters
Rwp = 0.125	23 restraints
Rexp = 0.096	(Δ/σ)max = 0.05
R(F^2) = 0.14871	Background function: GSAS Background function number 1 with 34 terms. Shifted Chebyshev function of 1st kind 1: 118.082 2: -166.900 3: 123.865 4: -59.7925 5: 10.6865 6: 21.3336 7: -31.0975 8: 23.2200 9: -6.96687 10: -9.51726 11: 20.8794 12: -23.8022 13: 18.4347 14: -9.14997 15: -1.10995 16: 9.35323 17: -13.3633 18: 13.2873 19: -9.61569 20: 4.08246 21: 1.61524 22: -5.79316 23: 6.77390 24: -5.01271 25: 2.27833 26: 0.646733 27: -2.78842 28: 3.78393 29: -3.23100 30: 2.18997 31: -0.908158 32: -0.401332 33: 0.778547 34: -0.792308
9139 data points	Preferred orientation correction: March-Dollase AXIS 1 Ratio= 0.79858 h= 1.000 k= 0.000 l= 0.000 Prefered orientation correction range: Min= 0.71363, Max= 1.96360

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

	x	y	z	Uiso*/Ueq
Sr1	0.60125 (9)	0.46444 (17)	0.2136 (2)	0.0206 (3)*
Cl1	0.4370 (3)	0.2794 (4)	0.4905 (4)	0.0254 (3)*
Cl2	0.1592 (2)	0.3965 (4)	0.6866 (4)	0.0275 (3)*
O1	0.1833 (5)	0.2549 (6)	0.7237 (14)	0.0271 (14)*
O2	0.2200 (4)	0.4878 (11)	0.7587 (12)	0.0335 (14)*
O3	0.1636 (7)	0.4175 (13)	0.5363 (6)	0.0364 (14)*
O4	0.0665 (4)	0.4250 (10)	0.7362 (11)	0.0225 (14)*
O5	0.3759 (6)	0.2145 (7)	0.3888 (11)	0.0356 (14)*
O6	0.3829 (6)	0.3692 (11)	0.5799 (10)	0.0361 (14)*
O7	0.4728 (6)	0.1764 (11)	0.5835 (12)	0.0315 (14)*
O8	0.5132 (6)	0.3514 (12)	0.4267 (12)	0.0305 (14)*

Geometric parameters (Å, º)

Sr1—Cl1	3.931 (4)	Cl2—Sr1^ix	3.945 (3)
Sr1—Cl1^i	3.937 (4)	Cl2—Sr1^ii	3.778 (3)
Sr1—Cl1^ii	3.779 (4)	Cl2—Sr1^x	3.746 (4)
Sr1—Cl1^iii	3.669 (4)	Cl2—Sr1^xi	3.898 (4)
Sr1—Cl2^iv	3.945 (3)	Cl2—O1	1.469 (5)
Sr1—Cl2^ii	3.778 (3)	Cl2—O2	1.414 (5)
Sr1—Cl2^v	3.746 (4)	Cl2—O3	1.425 (5)
Sr1—Cl2^vi	3.898 (4)	Cl2—O4	1.422 (5)
Sr1—O1^v	2.512 (5)	O1—Sr1^x	2.512 (5)
Sr1—O2^ii	2.591 (5)	O1—Cl2	1.469 (5)
Sr1—O3^vi	2.546 (5)	O2—Sr1^ii	2.591 (5)
Sr1—O4^iv	2.622 (5)	O2—Cl2	1.414 (5)
Sr1—O5^iii	2.650 (5)	O3—Sr1^xi	2.546 (5)
Sr1—O6^ii	2.540 (5)	O3—Cl2	1.425 (5)
Sr1—O7^i	2.590 (5)	O4—Sr1^ix	2.622 (5)
Sr1—O8	2.604 (8)	O4—Cl2	1.422 (5)
Cl1—Sr1	3.931 (4)	O5—Sr1^viii	2.650 (5)
Cl1—Sr1^vii	3.937 (4)	O5—Cl1	1.436 (5)
Cl1—Sr1^ii	3.779 (4)	O6—Sr1^ii	2.540 (5)
Cl1—Sr1^viii	3.669 (4)	O6—Cl1	1.436 (5)
Cl1—O5	1.436 (5)	O7—Sr1^vii	2.590 (5)
Cl1—O6	1.436 (5)	O7—Cl1	1.426 (5)
Cl1—O7	1.426 (5)	O8—Sr1	2.604 (8)
Cl1—O8	1.423 (5)	O8—Cl1	1.423 (5)
O1^v—Sr1—O2^ii	71.2 (3)	O5^iii—Sr1—O8	136.8 (4)
O1^v—Sr1—O3^vi	84.3 (4)	O6^ii—Sr1—O7^i	139.7 (3)
O1^v—Sr1—O4^iv	138.9 (3)	O6^ii—Sr1—O8	74.3 (4)
O1^v—Sr1—O5^iii	144.9 (3)	O7^i—Sr1—O8	78.3 (4)
O1^v—Sr1—O6^ii	109.2 (4)	O5—Cl1—O6	109.7 (5)
O1^v—Sr1—O7^i	88.9 (4)	O5—Cl1—O7	108.0 (7)
O1^v—Sr1—O8	71.3 (4)	O5—Cl1—O8	113.5 (7)
O2^ii—Sr1—O3^vi	77.6 (3)	O6—Cl1—O7	105.4 (7)
O2^ii—Sr1—O4^iv	143.6 (3)	O6—Cl1—O8	110.3 (7)
O2^ii—Sr1—O5^iii	75.5 (3)	O7—Cl1—O8	109.7 (7)
O2^ii—Sr1—O6^ii	73.8 (3)	O1—Cl2—O2	110.0 (7)
O2^ii—Sr1—O7^i	146.3 (3)	O1—Cl2—O3	111.1 (7)
O2^ii—Sr1—O8	118.0 (4)	O1—Cl2—O4	108.8 (6)
O3^vi—Sr1—O4^iv	117.7 (3)	O2—Cl2—O3	110.7 (8)
O3^vi—Sr1—O5^iii	77.9 (3)	O2—Cl2—O4	106.4 (6)
O3^vi—Sr1—O6^ii	141.9 (4)	O3—Cl2—O4	109.6 (6)
O3^vi—Sr1—O7^i	73.4 (3)	Sr1^x—O1—Cl2	139.0 (5)
O3^vi—Sr1—O8	142.8 (4)	Sr1^ii—O2—Cl2	139.4 (5)
O4^iv—Sr1—O5^iii	76.1 (3)	Sr1^xi—O3—Cl2	157.0 (6)
O4^iv—Sr1—O6^ii	75.8 (3)	Sr1^ix—O4—Cl2	153.3 (6)
O4^iv—Sr1—O7^i	67.6 (3)	Sr1^viii—O5—Cl1	125.1 (4)
O4^iv—Sr1—O8	71.2 (4)	Sr1^ii—O6—Cl1	142.2 (5)
O5^iii—Sr1—O6^ii	70.9 (4)	Sr1^vii—O7—Cl1	156.2 (7)
O5^iii—Sr1—O7^i	114.04 (3)	Sr1—O8—Cl1	153.9 (7)

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