Pirquitasite, Ag₂ZnSnS₄

Pirquitasite, ideally Ag₂ZnSnS₄ (disilver zinc tin tetra­sulfide), exhibits tetra­gonal symmetry and is a member of the stannite group that has the general formula A₂BCX₄, with A = Ag, Cu; B = Zn, Cd, Fe, Cu, Hg; C = Sn, Ge, Sb, As; and X = S, Se. In this study, single-crystal X-ray diffraction data are used to determine the structure of pirquitasite from a twinned crystal from the type locality, the Pirquitas deposit, Jujuy Province, Argentina, with anisotropic displacement parameters for all atoms, and a measured composition of (Ag_1.87Cu_0.13)(Zn_0.61Fe_0.36Cd_0.03)SnS₄. One Ag atom is located on Wyckoff site Wyckoff 2a (symmetry -4..), the other Ag atom is statistically disordered with minor amounts of Cu and is located on 2c (-4..), the (Zn, Fe, Cd) site on 2d (-4..), Sn on 2b (-4..), and S on general site 8g. This is the first determination of the crystal structure of pirquitasite, and our data indicate that the space group of pirquitasite is I-4, rather than I-42m as previously suggested. The structure was refined under consideration of twinning by inversion [twin ratio of the components 0.91 (6):0.09 (6)].

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](Sn-S) = 0.002 Å
• Disorder in main residue
• R factor = 0.027
• wR factor = 0.070
• Data-to-parameter ratio = 24.0

checkCIF/PLATON results

No syntax errors found



Alert level C
STRVA01_ALERT_2_C           Chirality of atom sites is inverted?
           From the CIF: _refine_ls_abs_structure_Flack    0.910
           From the CIF: _refine_ls_abs_structure_Flack_su    0.060
PLAT041_ALERT_1_C Calc. and Reported SumFormula    Strings  Differ          ?
PLAT068_ALERT_1_C Reported F000 Differs from Calcd (or Missing)...          ?
PLAT077_ALERT_4_C Unitcell contains non-integer number of atoms ..          ?
PLAT915_ALERT_3_C Low Friedel Pair Coverage ......................         81 Perc.

Alert level G
PLAT004_ALERT_5_G Info: Polymeric Structure Found with Dimension .          2
PLAT005_ALERT_5_G No _iucr_refine_instructions_details  in the CIF          ?
PLAT045_ALERT_1_G Calculated and Reported Z Differ by ............       0.50 Ratio
PLAT152_ALERT_1_G The Supplied and Calc. Volume s.1. Differ by ...          3 Units
PLAT199_ALERT_1_G Check the Reported _cell_measurement_temperature        293 K
PLAT200_ALERT_1_G Check the Reported   _diffrn_ambient_temperature        293 K
PLAT301_ALERT_3_G Note: Main Residue  Disorder ...................         23 Perc.
PLAT794_ALERT_5_G Note: Tentative Bond Valency for Sn      (IV)          3.91
PLAT794_ALERT_5_G Note: Tentative Bond Valency for Ag1     (I)           1.27
PLAT811_ALERT_5_G No ADDSYM Analysis: Too Many Excluded Atoms ....          !
PLAT860_ALERT_3_G Note: Number of Least-Squares Restraints .......          4
PLAT961_ALERT_5_G Dataset Contains no Negative Intensities .......          !

   0 ALERT level A = Most likely a serious problem - resolve or explain
   0 ALERT level B = A potentially serious problem, consider carefully
   5 ALERT level C = Check. Ensure it is not caused by an omission or oversight
  12 ALERT level G = General information/check it is not something unexpected

   6 ALERT type 1 CIF construction/syntax error, inconsistent or missing data
   1 ALERT type 2 Indicator that the structure model may be wrong or deficient
   3 ALERT type 3 Indicator that the structure quality may be low
   1 ALERT type 4 Improvement, methodology, query or suggestion
   6 ALERT type 5 Informative message, check

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S1. Comment

Pirquitasite is a member of the stannite group of tetragonal sulfides, which exhibit space group I42m or I4, and is an ordered derivative of the sphalerite structure (Johan and Picot, 1982). The stannite group currently contains thirteen species (Table 1), of which only kësterite, ferrokësterite, and pirquitasite are known to display space group I4. Synthetic sulfides with stannite type structures are utilized as the light absorber layer in photovoltaic cells (e.g. Salomé et al. 2012, Sasamura et al. 2012, Tsuji et al. 2010).

Pirquitasite was first described by Johan and Picot (1982), from the Pirquitas deposit, Argentina, as a silver zinc tin sulfide with ideal chemical formula Ag₂ZnSnS₄ and a stannite-like structure. An extensive solid solution between hocartite (Ag₂FeSnS₄) and pirquitasite was described by Johan and Picot (1982). Because of the solid solution and the I42m symmetry attributed to hocartite, Johan and Picot (1982) proposed that pirquitasite also exhibits I42m symmetry.

The structure was refined using both I42m and I4, with the R factor for I4 (R = 0.027) significantly lower than for I42m (R = 0.051). The structure of pirquitasite is a derivative of the cubic sphalerite structure that displays cubic closest packed (CCP) layers of S stacked along [111]. Because pirquitasite has a doubled c cell dimension, its stacking direction is [221]. Half of the tetrahedral sites are occupied by Ag, (Zn,Fe), and Sn cations, forming metal layers described by Hall et al. (1978), and it is the arrangement of Ag, (Zn,Fe), and Sn within these layers that differentiates the I4 kësterite structure from the I42m stannite structure.

Stannite and kësterite were originally recognized as distinct species because of different Fe—Zn compositional ratios and different optical properties (Orlova, 1956; Hall et al. 1978). Structural and chemical analyses by Hall et al. (1978) and Kissin and Owens (1979) not only showed a miscibility gap between the pure Fe end-member stannite and the pure Zn end-member kësterite, but found the two minerals differed in symmetry from I42m (stannite) to I4 (kësterite). In I42m, Cu atoms are ordered to the Wyckoff 4d site, (Fe,Zn) atoms are ordered to Wyckoff 2a, Sn is ordered to 2b (Hall et al. 1978). For comparison, the I4 symmetry has Cu atoms ordered to two sites: 2a and 2c, (Zn,Fe) ordered to 2d, Sn ordered to 2b (Hall et al. 1978). As pointed out by Hall et al. (1978), two distinct metal layers perpendicular to [001] result from this ordering in each mineral. Stannite exhibits one layer of Cu atoms only, with the other layer consisting of ordered Fe and Sn atoms, while kësterite exhibits one layer of ordered Cu and Sn atoms and one layer of ordered Zn and Cu atoms (Hall et al. 1978). This is illustrated for pirquitasite versus stannite in Fig. 1, which shows the pirquitasite structure (Fig. 1a) with one layer containing ordered Ag and Sn, the second containing ordered Zn and Ag. For comparison, the two stannite metal layers consist of one layer of Fe and Sn atoms and a second layer containing only Cu atoms (Fig. 1 b). The Ag—Sn layers in pirquitasite and Fe—Sn layers in stannite are ordered identically: Ag—Sn—Ag—Sn and Fe—Sn—Fe—Sn respectively when viewed along (100).

The mineral hocartite (tetragonal Ag₂FeSnS₄) is reported to exhibit space group I42m (Johan and Picot, 1982), but its structure is as yet unreported. It is likely that the hocartite-pirquitasite series follows the same systematics as the stannite-kësterite series.

An interesting feature is the distortion displayed by the AgS₄ tetrahedra, with tetrahedral angle variance of 8.86° displayed by Ag1S₄ and 25.40° displayed by Ag2S₄. M-S bond lengths are 2.539 Å and 2.497 Å for the Ag1S₄ and Ag2S₄ tetrahedra, respectively. As our sample contains approximately 13% apfu Cu, this Cu appears to be located in the Ag2 site because the bond lengths are smaller and the tetrahedron can accomodate the distortion. Bond valence calculations gave sums of 1.28 valence units (VU) and 1.35 VU for Ag1 and Ag2, respectively, corroborating that Cu is ordered to the Ag2 site. In a study of the mechanism of incorporation of Cu, Fe, and Zn in the stannite-kësterite series, Bonazzi et al. (2003) studied synthetic crystals, quenched from 1023 Kelvin, of composition Cu₂Fe_1-XZn_XS₄ (X = 0, 1/5, 1/2, 0.7, 0.8, 1), which showed decreasing tetrahedral angle distortion with increasing Zn content across the stannite-kësterite compositions.

Related literature

For related structures in the stannite–kësterite series, see: Orlova (1956); Hall et al. (1978); Kissin & Owens (1979); Bonazzi et al. (2003). For previous work on hocartite and pirquitasite, see: Johan & Picot (1982). For details on synthetic stannite group phases, see: Salomé et al. (2012); Sasamura et al. (2012); Tsuji et al. 2010). For other stannite group minerals, see: Chen et al. (1998); Frenzel (1959); Garin & Parthé (1972); Johan et al. (1971); Kaplunnik et al. (1977); Kissin & Owens (1989); Marumo & Nowaki (1967); Murciego et al. (1999); Szymański (1978); Wintenberger (1979).

S2. Experimental

The pirquitasite specimen used in this study comes from the type locality, the Pirquitas deposit, Jujuy Province, Argentina and is in the collection of the RRUFF project (http://rruff.info/R061016). The chemical composition, (Ag_1.87Cu_0.13)(Zn_0.61Fe_0.36Cd_0.03)SnS₄, was determined with a CAMECA SX100 electron microprobe. The composition was normalized to four cations.

S3. Refinement

The structure was refined with the inversion twin (-1 0 0/0 - 1 0/0 0 - 1) to a ratio of 0.91 (6). During refinement, the chemistry was constrained to the empirical formula of (Ag_1.87Cu_0.13)(Zn_0.61Fe_0.36Cd_0.03)SnS₄. The maximum residual electron density in the difference Fourier maps was located at (0.0434, 0.0434, 0.2204), 0.56 Å from Ag2 and the minimum at (0, 0, 0.0693) 0.75 Å from Ag1.

Computing details

Data collection: APEX2 (Bruker, 2004); cell refinement: SAINT (Bruker, 2004); data reduction: SAINT (Bruker, 2004); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: XtalDraw (Downs & Hall-Wallace, 2003); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

Fig. 1. Diagrams of displacement ellipsoids drawn at the 99.999% level for (a) pirquitasite and (b) stannite viewed along (100), with [001] vertical. The two types of metal layers are stacked along [001].

Disilver zinc tin tetrasulfide

Crystal data

(Ag1.87Cu0.13)(Zn0.61Fe0.36Cd0.03)SnS4	Dx = 4.765 Mg m^−3
Mr = 520.26	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I4	Cell parameters from 527 reflections
Hall symbol: I -4	θ = 6.3–32.3°
a = 5.7757 (12) Å	µ = 12.58 mm^−1
c = 10.870 (2) Å	T = 293 K
V = 362.60 (13) Å^3	Cuboid, grey
Z = 2	0.05 × 0.05 × 0.04 mm
F(000) = 470

Data collection

Bruker APEXII CCD area-detector diffractometer	575 independent reflections
Radiation source: fine-focus sealed tube	570 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.013
ϕ and ω scan	θmax = 32.0°, θmin = 3.8°
Absorption correction: multi-scan (SADABS; Sheldrick, 2005)	h = −4→8
Tmin = 0.572, Tmax = 0.633	k = −8→7
1312 measured reflections	l = −16→12

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0272P)^2 + 2.1498P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.027	(Δ/σ)max < 0.001
wR(F^2) = 0.070	Δρmax = 1.05 e Å^−3
S = 1.17	Δρmin = −0.87 e Å^−3
575 reflections	Extinction correction: SHELXL, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
24 parameters	Extinction coefficient: 0.0061 (6)
4 restraints	Absolute structure: Flack (1983)
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.91 (6)

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.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Ag1	0.0000	0.0000	0.0000	0.0364 (4)
Ag2	0.0000	0.5000	0.2500	0.0301 (6)	0.87
Cu	0.0000	0.5000	0.2500	0.0301 (6)	0.13
Zn	0.5000	0.0000	0.2500	0.0220 (6)	0.61
Fe	0.5000	0.0000	0.2500	0.0220 (6)	0.36
Cd	0.5000	0.0000	0.2500	0.0220 (6)	0.03
Sn	0.5000	0.5000	0.0000	0.01176 (18)
S	0.7325 (3)	0.2526 (4)	0.12847 (11)	0.0214 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ag1	0.0360 (5)	0.0360 (5)	0.0372 (4)	0.000	0.000	0.000
Ag2	0.0274 (7)	0.0274 (7)	0.0355 (9)	0.000	0.000	0.000
Cu	0.0274 (7)	0.0274 (7)	0.0355 (9)	0.000	0.000	0.000
Zn	0.0248 (8)	0.0248 (8)	0.0163 (9)	0.000	0.000	0.000
Fe	0.0248 (8)	0.0248 (8)	0.0163 (9)	0.000	0.000	0.000
Cd	0.0248 (8)	0.0248 (8)	0.0163 (9)	0.000	0.000	0.000
Sn	0.0114 (2)	0.0114 (2)	0.0125 (3)	0.000	0.000	0.000
S	0.0250 (6)	0.0212 (6)	0.0181 (6)	0.0053 (5)	−0.0023 (5)	0.0044 (5)

Geometric parameters (Å, º)

Ag1—S^i	2.5430 (17)	Zn—S^iv	2.383 (2)
Ag1—S^ii	2.5430 (17)	Zn—S^viii	2.383 (2)
Ag1—S^iii	2.5430 (17)	Zn—S	2.383 (2)
Ag1—S^iv	2.5430 (17)	Zn—S^vii	2.383 (2)
Ag2—S^iii	2.485 (2)	Sn—S^ii	2.4070 (16)
Ag2—S^v	2.485 (2)	Sn—S	2.4070 (16)
Ag2—S^vi	2.485 (2)	Sn—S^v	2.4070 (16)
Ag2—S^vii	2.485 (2)	Sn—S^ix	2.4070 (16)
S^i—Ag1—S^ii	113.39 (6)	S^iv—Zn—S^viii	107.90 (4)
S^i—Ag1—S^iii	107.55 (3)	S^iv—Zn—S	112.66 (8)
S^ii—Ag1—S^iii	107.55 (3)	S^viii—Zn—S	107.90 (4)
S^i—Ag1—S^iv	107.55 (3)	S^iv—Zn—S^vii	107.90 (4)
S^ii—Ag1—S^iv	107.55 (3)	S^viii—Zn—S^vii	112.66 (8)
S^iii—Ag1—S^iv	113.39 (6)	S—Zn—S^vii	107.90 (4)
S^iii—Ag2—S^v	115.77 (7)	S^ii—Sn—S	109.67 (4)
S^iii—Ag2—S^vi	106.42 (3)	S^ii—Sn—S^v	109.67 (4)
S^v—Ag2—S^vi	106.42 (3)	S—Sn—S^v	109.08 (7)
S^iii—Ag2—S^vii	106.42 (3)	S^ii—Sn—S^ix	109.08 (7)
S^v—Ag2—S^vii	106.42 (3)	S—Sn—S^ix	109.67 (4)
S^vi—Ag2—S^vii	115.77 (7)	S^v—Sn—S^ix	109.67 (4)

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