Structure and thermal behavior of the new superprotonic conductor Cs₂(HSO₄)(H₂PO₄)

Ongoing studies of the CsHSO₄–CsH₂PO₄ system, aimed at developing novel proton conducting solids, resulted in the new compound Cs₂(HSO₄)(H₂PO₄) (dicesium hydrogensulfate dihydrogenphosphate). Single-crystal X-ray diffraction (performed at room temperature) revealed Cs₂(HSO₄)(H₂PO₄) to crystallize in space group P2₁/n with lattice parameters a = 7.856 (8), b = 7.732 (7), c = 7.827 (7) Å, and β = 99.92 (4)°. The compound has a unit-cell volume of 468.3 (8) ų and two formula units per cell, giving a calculated density of 3.261 Mg m⁻³. Six non-H atoms and two H atoms were located in the asymmetric unit, with SO₄ and PO₄ groups randomly arranged on the single tetrahedral anion site. Refinement using all observed reflections yielded weighted residuals of 0.0890 and 0.0399 based on F² and F values, respectively. Anisotropic temperature factors were employed for all six non-H atoms and fixed isotropic temperature factors for the two H atoms. The structure contains zigzag chains of hydrogen-bonded anion tetrahedra that extend in the [010] direction. Each tetrahedron is additionally linked to a tetrahedron in a neighboring chain to give a planar structure with hydrogen-bonded sheets lying parallel to (101). Thermal analysis of the superprotonic transition in Cs₂(HSO₄)(H₂PO₄) showed that the transformation to the high-temperature phase occurs by a two-step process. The first is a sharp transition at 334 K and the second a gradual transition from 342 to 378 K. The heat of transformation for the entire process (∼330–382 K) is 44 ± 2 J g⁻¹. Thermal decomposition of Cs₂(HSO₄)(H₂PO₄) takes place at much higher temperatures, with an onset of approximately 460 K.

Computing details

Program(s) used to solve structure: SHELXS86 (Sheldrick, 1985); program(s) used to refine structure: SHELXL93 (Sheldrick, 1994); molecular graphics: ATOMS (Dowty, 1999).

(bs0007)

Crystal data

Cs2H3O8PS	F(000) = 416
Mr = 458.85	Dx = 3.261 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71070 Å
a = 7.856 (8) Å	Cell parameters from 20 reflections
b = 7.732 (7) Å	θ = 4–14.3°
c = 7.827 (7) Å	µ = 7.69 mm^−1
β = 99.92 (4)°	T = 296 K
V = 468.3 (8) Å^3	, colourless
Z = 2	0.2 × 0.15 × 0.05 mm

Data collection

Syntex four-circle diffractometer	Rint = 0.047
θ–2θ scans	θmax = 30.0°, θmin = 3.4°
Absorption correction: empirical (using intensity measurements) ψ scan	h = −11→10
Tmin = 0.36, Tmax = 1.00	k = −10→10
2629 measured reflections	l = 0→11
1364 independent reflections	3 standard reflections
1210 reflections with I ≥ 2σ(I)	intensity decay: 0.7%

Refinement

Refinement on F^2	49 parameters
Least-squares matrix: full	H-atom parameters constrained
R[F^2 > 2σ(F^2)] = 0.035	w = [σ^2(Fo^2) + (0.0502P)^2 + 0.23049P]^-1 where P = 1/3[max(Fo^2, 0) + 2Fc^2]
wR(F^2) = 0.086	(Δ/σ)max = 0.516
S = 1.04	Δρmax = 1.43 e Å^−3
1361 reflections	Δρmin = −1.32 e Å^−3

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Cs(1)	0.7280 (1)	0.1588 (1)	0.1180 (1)	0.031 (1)
S	0.7565 (1)	0.1621 (1)	−0.3887 (1)	0.022 (1)	0.5
P	0.7565 (1)	0.1621 (1)	−0.3887 (1)	0.022 (1)	0.5
O(1)	0.6584 (4)	0.0269 (3)	0.4937 (4)	0.032 (1)
O(2)	0.8377 (2)	0.2882 (2)	−0.4882 (3)	0.034 (1)
O(3)	0.6287 (2)	0.2447 (2)	−0.2880 (2)	0.033 (1)
O(4)	0.8930 (2)	0.0690 (3)	−0.2607 (2)	0.039 (1)
H(1)	0.5387 (4)	0.0329 (6)	0.5012 (6)	0.080 (1)	0.5
H(2)	0.8558 (2)	−0.0469 (3)	−0.2449 (2)	0.080 (1)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cs	0.032 (1)	0.028 (1)	0.031 (1)	0.000 (1)	0.000 (1)	−0.004 (1)
S	0.020 (1)	0.019 (1)	0.026 (1)	−0.001 (1)	0.004 (1)	−0.003 (1)
P	0.020 (1)	0.019 (1)	0.026 (1)	−0.001 (1)	0.004 (1)	−0.003 (1)
O(1)	0.030 (2)	0.029 (2)	0.039 (2)	−0.013 (1)	0.008 (1)	−0.006 (1)
O(2)	0.033 (2)	0.031 (2)	0.037 (2)	0.004 (1)	0.010 (1)	−0.006 (1)
O(3)	0.032 (2)	0.031 (2)	0.039 (2)	−0.006 (1)	0.012 (1)	−0.003 (1)
O(4)	0.032 (2)	0.030 (2)	0.050 (2)	0.007 (1)	−0.008 (1)	−0.002 (1)

Bond lengths (Å)

Cs(1)—O(2)^i	3.055 (4)	Cs(1)—O(2)^v	3.214 (4)
Cs(1)—O(2)^ii	3.063 (4)	Cs(1)—O(1)	3.247 (4)
Cs(1)—O(1)^iii	3.151 (3)	Cs(1)—O(4)^vi	3.475 (4)
Cs(1)—O(3)^iv	3.196 (4)	Cs(1)—O(4)	3.502 (3)
Cs(1)—O(3)	3.208 (3)	Cs(1)—O(4)^ii	3.622 (4)

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