Reinvestigation of the low-temperature form of Ag₂Se (naumannite) based on single-crystal data

Abstract

The crystal structure of the low-temperature form of synthetic naumannite [disilver(I) selenide], Ag₂Se, has been reinvestigated based on single-crystal data. In comparison with previous powder diffraction studies, anisotropic displacement parameters are additionally reported. The structure is composed of Se layers and two crystallographically independent Ag atoms. One Ag atom lies close to the Se layer and is surrounded by four Se atoms in a distorted tetra­hedral coordination, while the second Ag atom lies between the Se layers and exhibits a [3 + 1] coordination defined by three close Se atoms, forming a trigonal plane, and one remote Se atom.

Related literature

The crystal structure of the low-temperature form of Ag₂Se has been previously refined by using X-ray (Wiegers, 1971 ▶) and synchrotron (Billetter & Ruschewitz, 2008 ▶) powder diffraction. For the structure of the cubic high-temperature form of Ag₂Se, see: Oliveria et al. (1988 ▶). For general background, see: Frueh (1958 ▶). For ionic radii, see: Shannon (1976 ▶).

Experimental

Crystal data

• Ag₂Se

• M _r = 294.7

• Orthorhombic,

• a = 4.3359 (8) Å

• b = 7.070 (1) Å

• c = 7.774 (1) Å

• V = 238.34 (7) ų

• Z = 4

• Mo Kα radiation

• μ = 31.27 mm⁻¹

• T = 290 K

• 0.30 × 0.04 × 0.02 mm

Data collection

• Rigaku R-AXIS RAPID diffractometer

• Absorption correction: multi-scan (NUMABS; Higashi, 2000 ▶) T _min = 0.053, T _max = 0.278

• 1981 measured reflections

• 464 independent reflections

• 447 reflections with I > 2σ(I)

• R _int = 0.057

Refinement

• R[F ² > 2σ(F ²)] = 0.031

• wR(F ²) = 0.080

• S = 1.14

• 464 reflections

• 29 parameters

• Δρ_max = 1.19 e Å⁻³

• Δρ_min = −1.07 e Å⁻³

• Absolute structure: Flack (1983 ▶), 167 Friedel pairs

• Flack parameter: 0.34 (4)

Data collection: RAPID-AUTO (Rigaku, 2006 ▶); cell refinement: RAPID-AUTO; data reduction: RAPID-AUTO; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008 ▶); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008 ▶); molecular graphics: locally modified version of ORTEP (Johnson, 1965 ▶); software used to prepare material for publication: WinGX (Farrugia, 1999 ▶).

Supplementary Material

Additional supplementary materials: crystallographic information; 3D view; checkCIF report

Table 1. Selected bond lengths (Å).

Ag1—Sei	2.6800 (14)
Ag1—Seii	2.7058 (16)
Ag1—Seiii	2.8282 (14)
Ag1—Seiv	2.9076 (16)
Ag2—Seiii	2.6538 (14)
Ag2—Se	2.7560 (15)
Ag2—Sev	2.8036 (16)
Ag2—Sevi	3.2112 (16)

Symmetry codes: (i) ; (ii) ; (iii) ; (iv) ; (v) ; (vi) .

supplementary crystallographic information

Comment

Structural studies of the low-temperature form (transition point is 406 K) of the mineral naumannite, Ag₂Se, based on powder diffraction data have been reported previously by Wiegers (1971; X-ray data) and Billetter & Ruschewitz (2008; synchrotron data). In case of the related phase Ag₂S, single crystals of the high-temperature form (space group Im3m) are known to convert to polycrystalline powder of the low-temperature form on cooling (Frueh, 1958), and the same conversion has been assumed for the Se analogue (Wiegers, 1971). Consequently, structure determinations of the low-temperature form of Ag₂Se have been carried out only by using powder diffraction methods. In an attempt to prepare new mixed-metal selenides using AgCl as a flux, we were able to isolate single crystals of the low-temperature form of Ag₂Se and report here the results of the structure analysis. In comparison with the previous powder diffraction studies, anisotropic displacement parameters are additionally reported.

The general structural features of AgSe₂ are the same as reported previously (Wiegers, 1971; Billetter & Ruschewitz, 2008). The structure of the low-temperature form of Ag₂Se is closely related to the cubic high-temperature phase, where the Se atoms form a body-centered cubic packing, while the silver atoms are statistically distributed over interstitial sites (Oliveria et al., 1988). As a result, layers composed of Se atoms perpendicular to the b axis are retained in the low-temperature structure (Fig. 1). There are two crystallographically independent Ag atoms. Ag1 atoms lie close to this layer and are surrounded by four Se atoms in a distorted tetrahedral fashion (Se—Ag1—Se, 91.55 (3)–136.30 (5) °). The Ag1—Se distances range from 2.6800 (14) Å to 2.9076 (16) Å. Ag2 atoms are located between the layers and the coordination can be described as [3+1]. Three Se atoms built up a triangle that is bound to the Ag2 atom (Se—Ag2—Se, 94.00 (3)–141.46 (5) °), the coordination of which is augmented by a forth Se atom at a considerably longer distance of 3.2112 (16) Å. The observed Ag—Se distances are in agreement with the sum of the ionic radii of each element (Shannon, 1976) except for the very long Ag—Se bond. The distances and angles as calculated from single-crystal diffraction data differ slightly from those calculated previously from powder diffraction data. For example, the reported Ag1—Se distances are 2.62, 2.71, 2.79, 2.86 Å (Wiegers, 1971) and 2.658 (4), 2.668 (5), 2.861 (5), 2.937 (5) Å (Billetter & Ruschewitz, 2008) Å, and the Ag2—Se distances are 2.72, 2.74, 2.81, 3.28 Å (Wiegers, 1971) and 2.686 (5), 2.764 (5), 2.797 (4), 3.182 (5) Å (Billetter & Ruschewitz, 2008), respectively, with lattice parameters of a = 4.333 (Wiegers, 1971); 4.3373 (2) Å (Billetter & Ruschewitz, 2008), b = 7.062; 7.0702 (3) Å; c = 7.764; 7.7730 (4) Å.

Experimental

Single crystals of the low-temperature form of Ag₂Se were isolated during attemts to prepare new mixed-metal phases of Hf/Zr selenides. A combination of the pure elements, Zr powder, Hf powder, Se powder were mixed in a fused silica tube in a molar ratio of Zr: Hf: Se = 1:1:3 with AgCl. The mass ratio of the reactants and the halide flux was 1:2. The tube was evacuated to 0.133 Pa, sealed and heated gradually (20 K/h) to 600 K, where it was kept for 72 h. The tube was cooled to 200 K at 3 K/h and then was quenched to room temperature. The excess halide flux was removed with distilled water and black needle shaped crystals were obtained. The crystals are stable in air and water. A qualitative XRF analysis of the crystals showed only the presence of Ag and Se.

Refinement

Refinement with TWIN and BASF instruction for the final positional parameters gave a value of 0.34 (4) for the Flack parameter (Flack, 1983). The highest peak and the deepest hole in the final Fourier map are located 1.71 Å and 0.99 Å, respectively, from atom Ag1.

Figures

Fig. 1.

View of Ag2Se (50% probability displacement ellipsoids)

Crystal data

Ag2Se	F(000) = 512
Mr = 294.7	Dx = 8.213 Mg m−3
Orthorhombic, P212121	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2ab	Cell parameters from 1738 reflections
a = 4.3359 (8) Å	θ = 3.4–27.5°
b = 7.070 (1) Å	µ = 31.27 mm−1
c = 7.774 (1) Å	T = 290 K
V = 238.34 (7) Å3	Needle, black
Z = 4	0.30 × 0.04 × 0.02 mm

Data collection

Rigaku R-AXIS RAPID diffractometer	447 reflections with I > 2σ(I)
ω scans	Rint = 0.057
Absorption correction: multi-scan (NUMABS; Higashi, 2000)	θmax = 26.0°, θmin = 3.9°
Tmin = 0.053, Tmax = 0.278	h = −5→5
1981 measured reflections	k = −8→8
464 independent reflections	l = −9→9

Refinement

Refinement on F2	0 restraints
Least-squares matrix: full	w = 1/[σ2(Fo2) + (0.0329P)2 + 1.0072P] where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.031	(Δ/σ)max < 0.001
wR(F2) = 0.080	Δρmax = 1.19 e Å−3
S = 1.14	Δρmin = −1.07 e Å−3
464 reflections	Absolute structure: Flack (1983), 167 Friedel pairs
29 parameters	Flack parameter: 0.34 (4)

Special details

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

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

	x	y	z	Uiso*/Ueq
Ag1	0.8537 (2)	0.11503 (14)	0.45100 (14)	0.0398 (3)
Ag2	0.4745 (3)	0.77441 (14)	0.36152 (14)	0.0447 (4)
Se	0.1124 (2)	0.99787 (14)	0.15274 (14)	0.0242 (3)

Atomic displacement parameters (Ų)

	U11	U22	U33	U12	U13	U23
Ag1	0.0381 (6)	0.0378 (6)	0.0436 (6)	−0.0010 (5)	0.0080 (5)	0.0060 (4)
Ag2	0.0462 (7)	0.0338 (6)	0.0540 (7)	0.0131 (4)	−0.0100 (5)	−0.0057 (5)
Se	0.0249 (5)	0.0190 (6)	0.0286 (6)	0.0001 (4)	−0.0011 (4)	−0.0006 (4)

Geometric parameters (Å, °)

Ag1—Sei	2.6800 (14)	Ag2—Seix	3.2112 (16)
Ag1—Seii	2.7058 (16)	Ag2—Ag1x	2.9979 (16)
Ag1—Seiii	2.8282 (14)	Ag2—Ag1xi	3.0330 (14)
Ag1—Seiv	2.9076 (16)	Ag2—Ag2xii	3.0749 (16)
Ag1—Ag1v	2.9872 (15)	Ag2—Ag2xiii	3.0749 (16)
Ag1—Ag1vi	2.9872 (15)	Ag2—Ag1vi	3.1591 (16)
Ag1—Ag2vii	2.9979 (15)	Ag2—Ag1xiv	3.3692 (17)
Ag1—Ag2iii	3.0330 (14)	Se—Ag2xi	2.6538 (14)
Ag1—Ag2v	3.1591 (16)	Se—Ag1xv	2.6800 (14)
Ag1—Ag2iv	3.3692 (17)	Se—Ag1xvi	2.7058 (16)
Ag2—Seiii	2.6538 (14)	Se—Ag2xvii	2.8036 (16)
Ag2—Se	2.7560 (15)	Se—Ag1xi	2.8282 (14)
Ag2—Seviii	2.8036 (16)	Se—Ag1xiv	2.9076 (16)
Sei—Ag1—Seii	136.30 (5)	Seviii—Ag2—Ag1x	54.90 (4)
Sei—Ag1—Seiii	119.41 (5)	Seiii—Ag2—Ag1xi	90.54 (4)
Seii—Ag1—Seiii	91.55 (3)	Se—Ag2—Ag1xi	58.26 (4)
Sei—Ag1—Seiv	101.71 (5)	Seviii—Ag2—Ag1xi	142.86 (5)
Seii—Ag1—Seiv	92.76 (3)	Ag1x—Ag2—Ag1xi	137.88 (5)
Seiii—Ag1—Seiv	112.04 (5)	Seiii—Ag2—Ag2xii	58.04 (3)
Seiii—Ag2—Seix	94.87 (4)	Se—Ag2—Ag2xii	151.45 (6)
Se—Ag2—Seix	82.95 (3)	Seviii—Ag2—Ag2xii	66.03 (5)
Seviii—Ag2—Seix	105.72 (5)	Ag1x—Ag2—Ag2xii	62.68 (4)
Sei—Ag1—Ag1v	126.05 (6)	Ag1xi—Ag2—Ag2xii	148.22 (5)
Seii—Ag1—Ag1v	96.50 (4)	Seiii—Ag2—Ag2xiii	114.98 (6)
Seiii—Ag1—Ag1v	54.80 (4)	Se—Ag2—Ag2xiii	94.31 (4)
Seiv—Ag1—Ag1v	57.32 (3)	Seviii—Ag2—Ag2xiii	53.43 (4)
Sei—Ag1—Ag1vi	59.58 (3)	Ag1x—Ag2—Ag2xiii	108.32 (6)
Seii—Ag1—Ag1vi	135.65 (6)	Ag1xi—Ag2—Ag2xiii	100.86 (4)
Seiii—Ag1—Ag1vi	59.92 (5)	Ag2xii—Ag2—Ag2xiii	89.67 (6)
Seiv—Ag1—Ag1vi	127.99 (6)	Seiii—Ag2—Ag1vi	59.27 (4)
Ag1v—Ag1—Ag1vi	93.06 (6)	Se—Ag2—Ag1vi	132.61 (5)
Sei—Ag1—Ag2vii	58.86 (4)	Seviii—Ag2—Ag1vi	96.07 (5)
Seii—Ag1—Ag2vii	77.44 (4)	Ag1x—Ag2—Ag1vi	133.16 (4)
Seiii—Ag1—Ag2vii	136.98 (5)	Ag1xi—Ag2—Ag1vi	88.17 (3)
Seiv—Ag1—Ag2vii	109.95 (4)	Ag2xii—Ag2—Ag1vi	72.30 (4)
Ag1v—Ag1—Ag2vii	166.05 (5)	Ag2xiii—Ag2—Ag1vi	57.47 (4)
Ag1vi—Ag1—Ag2vii	100.04 (3)	Seiii—Ag2—Ag1xiv	88.98 (4)
Sei—Ag1—Ag2iii	103.04 (4)	Se—Ag2—Ag1xiv	55.60 (4)
Seii—Ag1—Ag2iii	67.77 (4)	Seviii—Ag2—Ag1xiv	131.41 (5)
Seiii—Ag1—Ag2iii	55.96 (3)	Ag1x—Ag2—Ag1xiv	84.96 (4)
Seiv—Ag1—Ag2iii	155.18 (5)	Ag1xi—Ag2—Ag1xiv	55.32 (3)
Ag1v—Ag1—Ag2iii	107.91 (5)	Ag2xii—Ag2—Ag1xiv	122.24 (4)
Ag1vi—Ag1—Ag2iii	68.06 (4)	Ag2xiii—Ag2—Ag1xiv	147.69 (4)
Ag2vii—Ag1—Ag2iii	81.67 (3)	Ag1vi—Ag2—Ag1xiv	132.52 (4)
Sei—Ag1—Ag2v	66.13 (4)	Ag2xi—Se—Ag1xv	134.75 (5)
Seii—Ag1—Ag2v	93.43 (4)	Ag2xi—Se—Ag1xvi	95.17 (4)
Seiii—Ag1—Ag2v	163.17 (5)	Ag1xv—Se—Ag1xvi	106.26 (4)
Seiv—Ag1—Ag2v	51.68 (3)	Ag2xi—Se—Ag2	93.59 (4)
Ag1v—Ag1—Ag2v	108.60 (5)	Ag1xv—Se—Ag2	127.12 (5)
Ag1vi—Ag1—Ag2v	124.02 (5)	Ag1xvi—Se—Ag2	84.66 (5)
Ag2vii—Ag1—Ag2v	59.85 (3)	Ag2xi—Se—Ag2xvii	68.53 (4)
Ag2iii—Ag1—Ag2v	140.46 (4)	Ag1xv—Se—Ag2xvii	66.24 (4)
Sei—Ag1—Ag2iv	71.43 (4)	Ag1xvi—Se—Ag2xvii	117.42 (5)
Seii—Ag1—Ag2iv	142.30 (5)	Ag2—Se—Ag2xvii	151.82 (5)
Seiii—Ag1—Ag2iv	92.10 (4)	Ag2xi—Se—Ag1xi	131.16 (5)
Seiv—Ag1—Ag2iv	51.45 (3)	Ag1xv—Se—Ag1xi	65.62 (4)
Ag1v—Ag1—Ag2iv	56.62 (4)	Ag1xvi—Se—Ag1xi	123.98 (5)
Ag1vi—Ag1—Ag2iv	76.74 (5)	Ag2—Se—Ag1xi	65.78 (4)
Ag2vii—Ag1—Ag2iv	121.83 (4)	Ag2xvii—Se—Ag1xi	109.05 (5)
Ag2iii—Ag1—Ag2iv	140.93 (4)	Ag2xi—Se—Ag1xiv	69.05 (4)
Ag2v—Ag1—Ag2iv	74.25 (3)	Ag1xv—Se—Ag1xiv	101.71 (5)
Seiii—Ag2—Se	141.46 (5)	Ag1xvi—Se—Ag1xiv	151.20 (5)
Seiii—Ag2—Seviii	123.20 (5)	Ag2—Se—Ag1xiv	72.95 (4)
Se—Ag2—Seviii	94.00 (3)	Ag2xvii—Se—Ag1xiv	80.16 (4)
Seiii—Ag2—Ag1x	103.38 (5)	Ag1xi—Se—Ag1xiv	62.75 (4)
Se—Ag2—Ag1x	89.34 (4)

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