supplementary materials


Acta Cryst. (2013). E69, i8-i9    [ doi:10.1107/S1600536813001013 ]

Pirquitasite, Ag2ZnSnS4

B. N. Schumer, R. T. Downs, K. J. Domanik, M. B. Andrade and M. J. Origlieri

Abstract top

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 (Ag1.87Cu0.13)(Zn0.61Fe0.36Cd0.03)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)].

Comment top

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 Cu2Fe1-XZnXS4 (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 top

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).

Experimental top

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, (Ag1.87Cu0.13)(Zn0.61Fe0.36Cd0.03)SnS4, was determined with a CAMECA SX100 electron microprobe. The composition was normalized to four cations.

Refinement top

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 (Ag1.87Cu0.13)(Zn0.61Fe0.36Cd0.03)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 top

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 top
[Figure 1] 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 top
Crystal data top
(Ag1.87Cu0.13)(Zn0.61Fe0.36Cd0.03)SnS4Dx = 4.765 Mg m3
Mr = 520.26Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I4Cell parameters from 527 reflections
Hall symbol: I -4θ = 6.3–32.3°
a = 5.7757 (12) ŵ = 12.58 mm1
c = 10.870 (2) ÅT = 293 K
V = 362.60 (13) Å3Cuboid, grey
Z = 20.05 × 0.05 × 0.04 mm
F(000) = 470
Data collection top
Bruker APEXII CCD area-detector
diffractometer
575 independent reflections
Radiation source: fine-focus sealed tube570 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.013
φ and ω scanθmax = 32.0°, θmin = 3.8°
Absorption correction: multi-scan
(SADABS; Sheldrick, 2005)
h = 48
Tmin = 0.572, Tmax = 0.633k = 87
1312 measured reflectionsl = 1612
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: full w = 1/[σ2(Fo2) + (0.0272P)2 + 2.1498P]
where P = (Fo2 + 2Fc2)/3
R[F2 > 2σ(F2)] = 0.027(Δ/σ)max < 0.001
wR(F2) = 0.070Δρmax = 1.05 e Å3
S = 1.17Δρmin = 0.87 e Å3
575 reflectionsExtinction correction: SHELXL, Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4
24 parametersExtinction coefficient: 0.0061 (6)
4 restraintsAbsolute structure: Flack (1983)
Primary atom site location: structure-invariant direct methodsFlack parameter: 0.91 (6)
Crystal data top
(Ag1.87Cu0.13)(Zn0.61Fe0.36Cd0.03)SnS4Z = 2
Mr = 520.26Mo Kα radiation
Tetragonal, I4µ = 12.58 mm1
a = 5.7757 (12) ÅT = 293 K
c = 10.870 (2) Å0.05 × 0.05 × 0.04 mm
V = 362.60 (13) Å3
Data collection top
Bruker APEXII CCD area-detector
diffractometer
575 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 2005)
570 reflections with I > 2σ(I)
Tmin = 0.572, Tmax = 0.633Rint = 0.013
1312 measured reflectionsθmax = 32.0°
Refinement top
R[F2 > 2σ(F2)] = 0.0274 restraints
wR(F2) = 0.070Δρmax = 1.05 e Å3
S = 1.17Δρmin = 0.87 e Å3
575 reflectionsAbsolute structure: Flack (1983)
24 parametersFlack parameter: 0.91 (6)
Special details top

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 (Å2) top
xyzUiso*/UeqOcc. (<1)
Ag10.00000.00000.00000.0364 (4)
Ag20.00000.50000.25000.0301 (6)0.87
Cu0.00000.50000.25000.0301 (6)0.13
Zn0.50000.00000.25000.0220 (6)0.61
Fe0.50000.00000.25000.0220 (6)0.36
Cd0.50000.00000.25000.0220 (6)0.03
Sn0.50000.50000.00000.01176 (18)
S0.7325 (3)0.2526 (4)0.12847 (11)0.0214 (3)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Ag10.0360 (5)0.0360 (5)0.0372 (4)0.0000.0000.000
Ag20.0274 (7)0.0274 (7)0.0355 (9)0.0000.0000.000
Cu0.0274 (7)0.0274 (7)0.0355 (9)0.0000.0000.000
Zn0.0248 (8)0.0248 (8)0.0163 (9)0.0000.0000.000
Fe0.0248 (8)0.0248 (8)0.0163 (9)0.0000.0000.000
Cd0.0248 (8)0.0248 (8)0.0163 (9)0.0000.0000.000
Sn0.0114 (2)0.0114 (2)0.0125 (3)0.0000.0000.000
S0.0250 (6)0.0212 (6)0.0181 (6)0.0053 (5)0.0023 (5)0.0044 (5)
Geometric parameters (Å, º) top
Ag1—Si2.5430 (17)Zn—Siv2.383 (2)
Ag1—Sii2.5430 (17)Zn—Sviii2.383 (2)
Ag1—Siii2.5430 (17)Zn—S2.383 (2)
Ag1—Siv2.5430 (17)Zn—Svii2.383 (2)
Ag2—Siii2.485 (2)Sn—Sii2.4070 (16)
Ag2—Sv2.485 (2)Sn—S2.4070 (16)
Ag2—Svi2.485 (2)Sn—Sv2.4070 (16)
Ag2—Svii2.485 (2)Sn—Six2.4070 (16)
Si—Ag1—Sii113.39 (6)Siv—Zn—Sviii107.90 (4)
Si—Ag1—Siii107.55 (3)Siv—Zn—S112.66 (8)
Sii—Ag1—Siii107.55 (3)Sviii—Zn—S107.90 (4)
Si—Ag1—Siv107.55 (3)Siv—Zn—Svii107.90 (4)
Sii—Ag1—Siv107.55 (3)Sviii—Zn—Svii112.66 (8)
Siii—Ag1—Siv113.39 (6)S—Zn—Svii107.90 (4)
Siii—Ag2—Sv115.77 (7)Sii—Sn—S109.67 (4)
Siii—Ag2—Svi106.42 (3)Sii—Sn—Sv109.67 (4)
Sv—Ag2—Svi106.42 (3)S—Sn—Sv109.08 (7)
Siii—Ag2—Svii106.42 (3)Sii—Sn—Six109.08 (7)
Sv—Ag2—Svii106.42 (3)S—Sn—Six109.67 (4)
Svi—Ag2—Svii115.77 (7)Sv—Sn—Six109.67 (4)
Symmetry codes: (i) y, x1, z; (ii) y, x+1, z; (iii) x1, y, z; (iv) x+1, y, z; (v) x+1, y+1, z; (vi) y1/2, x+3/2, z+1/2; (vii) y+1/2, x1/2, z+1/2; (viii) y+1/2, x+1/2, z+1/2; (ix) y+1, x, z.
Table 1. Minerals of the Stannite Group top
MineralFormulaSpace GroupReference
StanniteCu2FeSnS4I42mHall et al. (1978)
HocartiteAg2FeSnS4I42mJohan & Picot (1982)
KuramiteCu21+Cu2+SnS4I42mChen et al. (1998)
ČernyiteCu2CdSnS4I42mSzymański (1978)
VelikiteCu2HgSnS4I42mKaplunnik et al. (1977)
FamatiniteCu21+Cu2+SbS4I42mGarin & Parthé (1972)
LuzoniteCu21+Cu2+AsS4I42mMarumo & Nowaki (1967)
BarquilliteCu2(Cd,Fe2+)GeS4I42mMurciego et al. (1999)
BriartiteCu2FeGeS4I42mWintenberger (1979)
PermingeatiteCu21+Cu2+SbSe4I42mJohan et al. (1971)
KesteriteCu2ZnSnS4I4Kissin & Owens (1979)
FerrokesteriteCu2(Fe,Zn)SnS4I4Kissin & Owens (1989)
PirquitasiteAg2ZnSnS4I4This study
IdaiteCu2+Cu2+FeS4UnknownFrenzel (1959)
Acknowledgements top

We gratefully acknowledge the support of the Arizona Science Foundation and CNPq 202469/2011–5 from the Brazilian Government for MBA. Special thanks go to Dr David Brown for pointing out that bond-valence calculations corroborate the ordering of Cu to the Ag2 site.

references
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