Structures and phase transitions of the A₇PSe₆ (A = Ag, Cu) argyrodite-type ionic conductors. III. α-Cu₇PSe₆

The crystal structure of the third polymorph of the Cu₇PSe₆ argyrodite compound, α-Cu₇PSe₆, heptacopper phosphorus hexaselenide, is determined by means of single-crystal diffraction from twinned crystals and X-ray powder diffraction, with the help of extensive NMR measurements. In the low-temperature form, i.e. below the last phase transition, α-Cu₇PSe₆ crystallizes in orthorhombic symmetry, space group Pna2₁, with a = 14.3179 (4), b = 7.1112 (2), c = 10.1023 (3) Å, V = 1028.590 (9) ų (deduced from powder data, T = 173 K) and Z  = 4. Taking into account a twinning by reticular merohedry, the refinement of the α-Cu₇PSe₆ structure leads to the residual factors R = 0.0466 and wR = 0.0486 for 127 parameters and 3714 observed, independent reflections (single-crystal data, T = 173 K). A full localization of the Cu⁺d¹⁰ element is reached with one twofold-, one threefold- and five fourfold-coordinated Cu atoms. The observation of two phase transitions for Cu₇PSe₆, to be compared with only one for Ag₇PSe₆, is attributed to the d¹⁰ element stability in a low coordination environment, copper being less prone to lower coordination sites than silver, especially at low temperature.

Computing details

(alpha_Cu7PSe6)

Crystal data

Cu7PSe6	V = 1028.59 (5) Å^3
Mr = 949.6	Z = 4
Orthorhombic, Pna21	F(000) = 1688
Hall symbol: P 2c -2n	Dx = 6.117 Mg m^−3
a = 14.3179 (4) Å	Mo Kα radiation, λ = 0.71073 Å
b = 7.1112 (2) Å	µ = 352.9 mm^−1
c = 10.1023 (3) Å	× × mm

Data collection

STOE IPDS diffractometer	3714 reflections with 3
Absorption correction: spherical ?	h = ?→?
Tmin = 0.053, Tmax = 0.145	k = ?→?
29369 independent reflections	l = ?→?

Refinement

Refinement on F	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.0004F^2)
R[F^2 > 2σ(F^2)] = 0.047	(Δ/σ)max = 0.004
wR(F^2) = 0.080	Δρmax = 2.30 e Å^−3
S = 1.06	Δρmin = −2.02 e Å^−3
29369 reflections	Extinction correction: B-C type 1 Lorentzian isotropic
127 parameters	Extinction coefficient: 0.018565

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

	x	y	z	Uiso*/Ueq
P	0.1239 (3)	0.7531 (6)	0.2500 (5)	0.0027 (4)
Se1a	0.0023 (2)	0.7567 (3)	0.1179 (5)	0.0059 (6)
Se1b	0.2511 (2)	0.2514 (2)	0.6242 (5)	0.0060 (6)
Se1c	0.3787 (1)	0.5038 (3)	0.8764 (5)	0.0067 (6)
Se1d	−0.1194 (1)	0.5047 (3)	0.8734 (5)	0.0058 (6)
Se2	0.1253 (1)	0.7180 (3)	0.7351 (5)	0.0075 (6)
Se3	0.3847 (1)	0.7398 (3)	0.5057 (5)	0.0089 (7)
Cu1a	0.2375 (2)	0.4640 (4)	0.9976 (6)	0.0224 (9)
Cu1b	0.7709 (2)	0.4158 (4)	0.0472 (5)	0.0173 (8)
Cu2	0.2936 (2)	0.7675 (4)	0.7451 (6)	0.0237 (8)
Cu3a	0.9671 (2)	0.0921 (4)	0.0879 (6)	0.0240 (9)
Cu3b	0.5513 (2)	0.9364 (4)	0.9281 (5)	0.0191 (9)
Cu4a	0.8575 (3)	0.0214 (6)	0.3652 (7)	0.0523 (14)
Cu4b	0.3765 (2)	0.8298 (5)	0.2702 (5)	0.0308 (10)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Se1a	0.0065 (9)	0.007 (1)	0.004 (1)	0.0010 (7)	0.0010 (7)	−0.0001 (8)
Se1b	0.0061 (9)	0.007 (1)	0.005 (1)	−0.0010 (8)	0.0041 (7)	0.0009 (9)
Se1c	0.0085 (9)	0.006 (1)	0.006 (1)	0.0014 (9)	0.0015 (9)	0.0024 (7)
Se1d	0.0069 (9)	0.006 (1)	0.005 (1)	0.0022 (9)	−0.0002 (9)	0.0030 (7)
Se2	0.0074 (9)	0.0057 (9)	0.009 (1)	−0.0009 (8)	−0.001 (1)	−0.0016 (7)
Se3	0.0061	0.012 (1)	0.009 (1)	0.0008 (8)	0.0012 (8)	0.0000 (9)
Cu1a	0.017 (1)	0.016 (1)	0.034 (2)	−0.008 (1)	−0.012 (1)	0.010 (1)
Cu1b	0.016 (1)	0.020 (1)	0.016 (1)	−0.003 (1)	−0.008 (1)	0.004 (1)
Cu2	0.013 (1)	0.024 (2)	0.034 (2)	−0.006 (1)	0.010 (1)	0.004 (2)
Cu3a	0.025 (2)	0.019 (2)	0.028 (2)	0.003 (1)	0.000 (1)	−0.004 (1)
Cu3b	0.021 (1)	0.024 (2)	0.013 (1)	0.000 (1)	0.004 (1)	0.001 (1)
Cu4a	0.048 (2)	0.044 (2)	0.065 (3)	0.004 (2)	−0.004 (2)	−0.040 (2)
Cu4b	0.007 (1)	0.072 (2)	0.013 (1)	0.002 (2)	0.002 (1)	0.011 (1)

Bond lengths (Å)

P—Se1a	2.194 (5)	Cu2—Se3	2.755 (7)
P—Se1c^i	2.195 (5)	Cu3a—Se2^vi	2.405 (6)
P—Se1b^i	2.193 (5)	Cu3a—Se1a^viii	2.457 (4)
P—Se1d^ii	2.218 (5)	Cu3a—Se3^ix	2.509 (4)
Cu1a—Se3^iii	2.369 (3)	Cu3a—Se1c^x	2.575 (7)
Cu1a—Se1c	2.380 (5)	Cu3b—Se1a^xi	2.461 (6)
Cu1a—Se1b^iv	2.417 (5)	Cu3b—Se2^vii	2.476 (6)
Cu1b—Se1d^v	2.439 (6)	Cu3b—Se1d^vii	2.541 (4)
Cu1b—Se1b^vi	2.511 (4)	Cu3b—Se3^xii	2.599 (4)
Cu1b—Se3^vi	2.523 (3)	Cu4a—Se2^vi	2.285 (6)
Cu1b—Se2^vi	2.592 (6)	Cu4a—Se3^xiii	2.370 (7)
Cu2—Se1d^vii	2.420 (5)	Cu4b—Se1b^i	2.413 (5)
Cu2—Se2	2.437 (3)	Cu4b—Se1a^vii	2.447 (6)
Cu2—Se1c	2.600 (5)	Cu4b—Se3	2.467 (7)

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