Crystal structure of barium perchlorate anhydrate, Ba(ClO₄)₂, from laboratory X-ray powder data

The previously unknown crystal structure of barium perchlorate anhydrate, determined and refined from laboratory X-ray powder diffraction data, represents a new structure type. The structure can be described as a three-dimensional polyhedral network resulting from the corner- and edge-sharing of BaO₁₂ polyhedra and ClO₄ tetra­hedra.

Abstract

The previously unknown crystal structure of barium perchlorate anhydrate, determined and refined from laboratory X-ray powder diffraction data, represents a new structure type. The title compound was obtained by heating hydrated barium perchlorate [Ba(ClO₄)₂·xH₂O] at 423 K in vacuo for 6 h. It crystallizes in the ortho­rhom­bic space group Fddd. The asymmetric unit contains one Ba (site symmetry 222 on special position 8a), one Cl (site symmetry 2 on special position 16f) and two O sites (on general positions 32h). The structure can be described as a three-dimensional polyhedral network resulting from the corner- and edge-sharing of BaO₁₂ polyhedra and ClO₄ tetra­hedra. Each BaO₁₂ polyhedron shares corners with eight ClO₄ tetra­hedra, and edges with two ClO₄ tetra­hedra. Each ClO₄ tetra­hedron shares corners with four BaO₁₂ polyhedra, and an edge with the other BaO₁₂ polyhedron.

Chemical context  

The alkaline earth metal ions (Mg, Ca, Sr and Ba) have been of increasing inter­est as ion carriers for post Li ion batteries (Wang et al., 2013 ▸), and their perchlorates are often used as conventional organic electrolyte salts for electrochemical cells such as magnesium (Amatucci et al., 2001 ▸; Levi et al., 2010 ▸) and calcium ion batteries (Padigi et al., 2015 ▸). Since such salts adsorb water easily from the atmosphere and the water causes unwanted side reactions in the electrochemical cells, removing water from the salts and its confirmation before use would be very important. However, due to the difficulty in growing a single crystal of such anhydrous perchlorates, no crystal structure had ever been solved before we first identified the magnesium perchlorate structure from powder X-ray diffraction data (Lim et al., 2011 ▸). Barium perchlorate is a very strong oxidizing agent due to the high oxidation state of chlorine VII, and it is commonly stabilized as hydrate forms in the atmos­phere. Several different forms of the hydrates are expected to exist, as observed in the magnesium analogues (Robertson & Bish, 2010 ▸; West, 1935 ▸). The crystal structure of the trihydrate form was determined from single-crystal data (Gallucci & Gerkin, 1988 ▸), but the anhydrous form, Ba(ClO₄)₂, has not been reported to date. We present here its crystal structure, as determined and refined from laboratory powder X-ray diffraction data (Fig. 1 ▸). This is the second crystal structure reported among the anhydrate alkaline earth metal perchlor­ates.

Figure 1.

X-ray Rietveld refinement profiles for Ba(ClO₄)₂ recorded at room temperature. Crosses mark experimental points (red) and the solid line is the calculated profile (green). The bottom trace shows the difference curve (purple) and the ticks denote expected peak positions.

Structural commentary  

Anhydrous Ba(ClO₄)₂ crystallizes in a new structure type in terms of atomic ratios (1:2:8) and its polyhedral network is, to our knowledge, unique. The asymmetric unit contains one Ba (site symmetry 222 on special position 8a), one Cl (site symmetry 2 on special position 16f) and two O sites (on general positions 32h). The crystal structure is illustrated in Fig. 2 ▸, where two different views along [010] and [001] are presented for better visualization. The crystal structure is represented with ClO₄ tetra­hedra and Ba atoms in Fig. 2 ▸ a and 2b. The local environment around the Ba atom is presented in Fig. 3 ▸. It is clearly seen that there are chains of [(ClO₄)–Ba–(ClO₄)]_∞ parallel to the b-axis direction. Along each chain, the barium atom is placed between the two ClO₄ tetra­hedra, bonded to two oxygen atoms at each tetra­hedron. The [010] view in Fig. 2 ▸ a clearly shows the two-dimensional arrangement of the chains. The chains are inter­connected through Ba—O bonds. Each chain is surrounded by six neighboring ones that are shifted parallel to b-axis in such a way that a barium atom of the central chain is connected to the oxygen atoms of eight ClO₄ tetra­hedra of six neighboring chains. Four tetra­hedra are from four chains, one from each. The other four tetra­hedra are from two other chains, two from each. The structure may also be described as a three-dimensional polyhedral network resulting from the corner- and edge-sharing of BaO₁₂ polyhedra and ClO₄ tetra­hedra. Each BaO₁₂ polyhedron shares corners with eight ClO₄ tetra­hedra, and edges with two ClO₄ tetra­hedra. Each ClO₄ tetra­hedron shares corners with four BaO₁₂ polyhedra, and an edge with the other BaO₁₂ polyhedron. The oxygen atoms in a ClO₄ tetra­hedron consist of two O1 and two O2 ones. O1 is bonded to three atoms, one Cl and two Ba atoms, forming an almost planar environment. On the other hand, O2 is bonded to only two atoms, Cl and Ba. Selected bond lengths are given in Table 1 ▸.

Figure 2.

The unit cell structures for Ba(ClO₄)₂ with (ClO₄) tetra­hedra (yellow) and Ba atoms (green), showing (a) the [010] view and (b) the [001] view.

Figure 3.

The local environment of the Ba²⁺ cation (green sphere) surrounded by (ClO₄₎ tetra­hedra (yellow). [Symmetry codes: (i) x + , y − , −z + ; (ii) −x, y − , z − ; (iii) x + , −y + , z − ; (iv) −x, −y + , −z + ; (v) x, −y + , −z + ; (vi) −x + , −y + , z; (vii) −x + , y, −z + ; (viii) x, −y + , −z + ; (ix) x, y − , z − ; (x) −x + , −y + , z − ; (xi) −x + , y − , −z + .]

Table 1. Selected bond lengths ().

Ba1O1	2.901(4)	Ba1O2iii	2.903(4)
Ba1O1i	2.939(4)	Cl1O1	1.441(4)
Ba1O1ii	2.901(4)	Cl1O2	1.437(4)

Symmetry codes: (i) ; (ii) ; (iii) .

It is inter­esting to see the significant difference in crystal structures between Ba(ClO₄)₂ and Mg(ClO₄)₂ due to the difference in the cation radii, 1.61 Å for Ba²⁺ and 0.72 Å for Mg²⁺ (Shannon, 1976 ▸). The much bigger cation, Ba²⁺, is coordinated by eight ClO₄ tetra­hedra, while the magnesium is coordinated by only six. Accordingly, the repulsion between two cations of Ba²⁺–Cl⁷⁺ must be much weaker that that of the magnesium compound since the inter­atomic Ba—Cl distances of 3.55–4.06 Å are much longer than that (3.3 Å) of Mg—Cl for the same charges. This might be a reason why magnesium perchlorate is much more highly reactive with water when exposed to the atmosphere.

The empirical expression for bond valence, which has been widely adopted to estimate valences in inorganic solids (Brown, 2002 ▸), was used to check the Ba(ClO₄)₂ crystal structure. The bond-valence sums (Brown & Altermatt, 1985 ▸; Brese & O’Keeffe, 1991 ▸) calculated with the program Valence (Hormillosa et al., 1993 ▸) [given in v.u. (valence units): Ba 2.20, Cl 6.89, O1 2.04 and O2 1.73] match the expected charges of the ions reasonably well.

Synthesis and crystallization  

The anhydrous form of barium perchlorate was prepared by dehydration from Ba(ClO₄)₂·xH₂O (97%, Aldrich). The powder was thoroughly ground in an agate mortar and put into the bottom of a fused-silica tube with the other end sealed with a rubber septum. The tube was inserted into a box furnace through a hole on top of the furnace so that the bottom of the tube was at the center of the furnace inside, and the other end outside connected to a vacuum pump through a needle stuck into the septum. It was heated at a rate of 4K/min up to 423K for 6 h under continuous vacuum. After furnace cooling, powder sampling for X-ray measurement was processed in an Ar atmosphere glove-box, and a tightly sealed dome-type X-ray sample holder commercially available from Bruker was used to prevent hydration during measurement.

Refinement details  

Crystal data, data collection and structure refinement details are summarized in Table 2 ▸. The powder X-ray diffraction (XRD) data were collected at room temperature on a Bragg–Brentano diffractometer (PANalytical Empyrean) with a Cu Kα1 X-ray tube, a focusing primary Ge (111) monochromator (λ = 1.54059 Å), and a position-sensitive PIXcel 3D 2x2 detector, the angular range of 15 ≤ 2θ ≤ 130°, step 0.0260 and total measurement time of 13 h at room temperature. The structure determination from the powder XRD data was performed using 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 density maps, MCE was used (Rohlíček & Hušák, 2007 ▸). The XRD pattern was indexed using the program TREOR90 (Werner, 1990 ▸) run in CRYSFIRE (Shirley, 2002 ▸) via the positions of 20 diffraction peaks, resulting in an ortho­rhom­bic unit cell. The systematic absences suggested the space group Fddd. The structure determination was performed in the same way as in our previous work (Lee & Hong, 2008 ▸) where the details were described. At the beginning, a structural model with only a dummy atom at an arbitrary position in the unit cell was used. Structure factors were extracted from the powder data, then direct methods were used for the initial solution of the structure using SHELXS97 (Sheldrick, 2008 ▸) run in CRYSTALS, which yielded a couple of atom positions. However, not all the atoms could be identified at once. The partial model at this stage replaced the initial dummy-atom model, and was used for a Le Bail fit in GSAS. Then, improved structure factors were extracted, which were used for the improved data in the refinement in CRYSTALS. These processes were iterated until a complete and satisfactory structural model was obtained. Finally, Rietveld refinement was employed to complete the structure determination, resulting with reasonable temperature factors and an R_wp factor of 0.06.

Table 2. Experimental details.

Crystal data
Chemical formula	Ba(ClO4)2
M r	336.23
Crystal system, space group	Orthorhombic, F d d d
Temperature (K)	298
a, b, c ()	14.304(9), 11.688(7), 7.2857(4)
V (3)	1218.1(11)
Z	8
Radiation type	Cu K 1, = 1.54059
Specimen shape, size (mm)	Flat sheet, 20 20
Data collection
Diffractometer	PANalytical Empyrean
Specimen mounting	Packed powder
Data collection mode	Reflection
Scan method	Step
2 values ()	2min = 14.992 2max = 129.964 2step = 0.026
Refinement
R factors and goodness of fit	R p = 0.041, R wp = 0.060, R exp = 0.045, R(F 2) = 0.05733, 2 = 1.769
No. of data points	4423
No. of parameters	25

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 ▸).

Supplementary Material

CCDC reference: 1063587

Additional supporting information: crystallographic information; 3D view; checkCIF report

supplementary crystallographic information

Crystal data

Ba(ClO4)2	Z = 8
Mr = 336.23	F(000) = 1232.0
Orthorhombic, Fddd	Dx = 3.667 Mg m−3
Hall symbol: -F 2uv 2vw	Cu Kα1 radiation, λ = 1.54059 Å
a = 14.304 (9) Å	T = 298 K
b = 11.688 (7) Å	white
c = 7.2857 (4) Å	flat sheet, 20 × 20 mm
V = 1218.1 (11) Å3

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 = 14.992°, 2θmax = 129.964°, 2θstep = 0.026°

Refinement

Least-squares matrix: full	Profile function: CW Profile function number 3 with 19 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. #1(GU) = 0.000 #2(GV) = 0.000 #3(GW) = 0.000 #4(GP) = 4.256 #5(LX) = 1.263 #6(LY) = 7.277 #7(S/L) = 0.0005 #8(H/L) = 0.0005 #9(trns) = -1.26 #10(shft)= 1.5725 #11(stec)= 4.60 #12(ptec)= 1.24 #13(sfec)= 0.00 #14(L11) = -0.018 #15(L22) = -0.022 #16(L33) = -0.202 #17(L12) = 0.017 #18(L13) = 0.017 #19(L23) = 0.000 Peak tails are ignored where the intensity is below 0.0010 times the peak Aniso. broadening axis 0.0 0.0 1.0
Rp = 0.041	25 parameters
Rwp = 0.060	0 restraints
Rexp = 0.045	(Δ/σ)max = 0.01
R(F2) = 0.05733	Background function: GSAS Background function number 1 with 36 terms. Shifted Chebyshev function of 1st kind 1: 567.648 2: -857.136 3: 662.653 4: -377.264 5: 131.512 6: 59.5938 7: -169.995 8: 206.701 9: -185.404 10: 133.783 11: -68.7256 12: 6.71856 13: 43.4515 14: -72.2732 15: 82.7653 16: -73.3661 17: 50.0183 18: -22.8495 19: -2.57480 20: 20.6662 21: -29.1651 22: 28.9267 23: -24.2542 24: 14.4066 25: -5.32227 26: -4.03875 27: 10.7050 28: -13.4416 29: 11.1646 30: -9.08855 31: 2.53787 32: -0.292410 33: -1.46976 34: 0.544854 35: -1.31862 36: 0.893355
χ2 = 1.769	Preferred orientation correction: March-Dollase AXIS 1 Ratio= 0.97385 h= 1.000 k= 0.000 l= 0.000 Prefered orientation correction range: Min= 0.96103, Max= 1.08275
4423 data points

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

	x	y	z	Uiso*/Ueq
Ba1	0.125	0.125	0.125	0.0139 (2)*
Cl1	0.125	0.42875 (18)	0.125	0.0160 (7)*
O1	0.0471 (3)	0.3533 (3)	0.1575 (5)	0.0162 (11)*
O2	0.1412 (3)	0.5016 (4)	0.2807 (4)	0.0170 (12)*

Geometric parameters (Å, º)

Ba1—O1	2.901 (4)	Ba1—O2viii	2.903 (4)
Ba1—O1i	2.939 (4)	Ba1—O2ix	2.903 (4)
Ba1—O1ii	2.939 (4)	Ba1—O2x	2.903 (4)
Ba1—O1iii	2.939 (4)	Ba1—O2xi	2.903 (4)
Ba1—O1iv	2.939 (4)	Cl1—O1	1.441 (4)
Ba1—O1v	2.901 (4)	Cl1—O1vii	1.441 (4)
Ba1—O1vi	2.901 (4)	Cl1—O2	1.437 (4)
Ba1—O1vii	2.901 (4)	Cl1—O2vii	1.437 (4)
O1—Ba1—O1i	110.93 (10)	O1i—Ba1—O2ix	110.87 (10)
O1—Ba1—O1ii	78.56 (7)	O1i—Ba1—O2x	125.15 (10)
O1—Ba1—O1iii	106.64 (7)	O1i—Ba1—O2xi	63.76 (10)
O1—Ba1—O1iv	63.56 (12)	O1vi—Ba1—O1vii	134.82 (15)
O1—Ba1—O1v	134.82 (15)	O2xii—Ba1—O2ix	170.85 (15)
O1—Ba1—O1vi	170.64 (14)	O2viii—Ba1—O2x	120.42 (15)
O1—Ba1—O1vii	46.25 (15)	O2viii—Ba1—O2xi	60.43 (15)
O1—Ba1—O2viii	60.21 (10)	O1—Cl1—O1vii	104.5 (3)
O1—Ba1—O2ix	123.92 (10)	O1—Cl1—O2	110.96 (19)
O1—Ba1—O2x	65.35 (10)	O1—Cl1—O2vii	111.6 (2)
O1—Ba1—O2xi	110.84 (10)	O2—Cl1—O2vii	107.3 (3)
O1i—Ba1—O1ii	170.09 (14)	Ba1—O1—Ba1xiii	116.44 (12)
O1i—Ba1—O1iii	66.21 (14)	Ba1—O1—Cl1	104.6 (2)
O1i—Ba1—O1iv	114.73 (14)	Ba1xiii—O1—Cl1	133.1 (2)
O1i—Ba1—O2viii	60.66 (10)	Ba1viii—O2—Cl1	164.5 (2)

Symmetry codes: (i) x+1/4, y−1/4, −z+1/2; (ii) −x, y−1/4, z−1/4; (iii) x+1/4, −y+1/2, z−1/4; (iv) −x, −y+1/2, −z+1/2; (v) x, −y+1/4, −z+1/4; (vi) −x+1/4, −y+1/4, z; (vii) −x+1/4, y, −z+1/4; (viii) x, −y+3/4, −z+3/4; (ix) x, y−1/2, z−1/2; (x) −x+1/4, −y+3/4, z−1/2; (xi) −x+1/4, y−1/2, −z+3/4; (xii) x, −y+7/4, −z+11/4; (xiii) −x, y+1/4, z+1/4.