Mn_1-xFe_xS, x 0.05, an example of an anti-wurtzite structure

The mineral rambergite, hexagonal manganese sulfide, is found to crystallize in the inverse wurtzite structure, i.e. the wurtzite type structure but with the opposite absolute configuration, which can also be named anti-wurtzite. Rambergite was first found in the Garpenberg area, Dalarna, Sweden. The sample used in this investigation contained approximately 5% iron, as determined by microprobe analysis, giving the overall composition Mn_1-x Fe_x S, x 0.05.

Key indicators

• Single-crystal X-ray study
• T = 293 K
• Mean [sigma](Mn-S) = 0.001 Å
• Disorder in main residue
• R factor = 0.010
• wR factor = 0.027
• Data-to-parameter ratio = 13.0

checkCIF results

No syntax errors found


 Alert Level B:



PLAT_111  Alert B ADDSYM detects (pseudo) centre of symmetry ...        100 Perc Fit

PLAT_113  Alert B ADDSYM suggests Pseudo/New Spacegroup ........       P63/mmc

PLAT_301  Alert B Main Residue Disorder ........................      45.00 Perc.

General Notes



CELLZ_01
         From the CIF: _cell_formula_units_Z    2
         From the CIF: _chemical_formula_sum  Fe0.05 Mn0.95 S
         TEST: Compare cell contents of formula and atom_site data

         atom    Z*formula  cif sites diff
         Fe         2.00      0.10    1.90
         Mn         1.90      1.90    0.00
         S          2.00      2.00    0.00
          Difference between formula and atom_site contents detected.
          ALERT: Large difference may be due to a
          symmetry error - see SYMMG tests

REFLT_03
         From the CIF: _diffrn_reflns_theta_max           24.81
         From the CIF: _reflns_number_total                 78
         Count of symmetry unique reflns           43
         Completeness (_total/calc)            181.40%
         TEST3: Check Friedels for noncentro structure
         Estimate of Friedel pairs measured        35
         Fraction of Friedel pairs measured     0.814
         Are heavy atom types Z>Si present          yes
          Please check that the estimate of the number of Friedel pairs is
          correct. If it is not, please give the correct count in the
          _publ_section_exptl_refinement section of the submitted CIF.


 0 Alert Level A = Potentially serious problem

 3 Alert Level B = Potential problem

 0 Alert Level C = Please check

---

S1. Comment

The mineral rambergite, hexagonal MnS was first found in the Garpenberg area, Dalarna, Sweden (Kalinowski, 1996). The name rambergite has been approved by the IMA Commission on new Minerals and Mineral names. The manganese position in the Garpenberg sample is slightly substituted by other metals, mainly iron (5% in this very sample) but also minor substituents of Sb, Zn and Ag were found by microprobe analysis on powder samples. The Fe content varied between 0.1% and 6% for different samples. No superstructure reflections could be detected, thus we assume negligible ordering of iron. The occurrence of rambergite has also been reported from Ronneburg, Thuringia, Germany (Witzke, 1999) without any indications of Fe substitution of the Mn position. In contrast to the wurtzite type structure, the present sample of rambergite crystallizes with the inverse absolute configuration. The inverted absolute structure of wurtzite is equal to the anti-wurtzite model where the Mn and S atoms are swapped in the respect that the calculated structure factor amplitudes are similar for the inverse wurtzite model and the anti-wurtzite model. This equality of the inverse wurtzite and the anti-wurtzite can be derived from traditional structure factor formalism including anomalous dispersion corrections. Without the anomalous dispersion effects the wurtzite and anti-wurtzite are equal. It is not known if rambergite exists with the normal wurtzite absolute configuration. The c/a ≈ 1.619 for rambergite and the value of the z(S)=0.6224 (2) enables the u-parameter of normal wurtzite to be calculated as u=(1 - z)=0.3776 (2). These values fit very well in the (u, c/a) correlation scheme discussed for other wurtzite type structures, ZnS and ZnO (Erich & Elcombe, 1989). The effect, concerning the parameter shifts, of changing the absolute configuration is rather small. It is of the order of 2σ when using the derived s.u.'s from the normal wurtzite model but increases when using the derived s.u.'s of the inverse wurtzite model. The derived s.u.'s of the parameters and derived quantities of the inverse wurtzite are generally smaller than the corresponding quantities for the normal wurtzite model. It is clear from comparison of R-values and specially the Flack parameter for both the normal wurtzite model and the inverse wurtzite model, that the rambergite crystal should be of the inverse wurtzite structure type. Even though wurtzite is regarded as a simple structure it is a definitive, however small, difference of the structure factor expression for the wurtzite and anti-wurtzite due to anomalous dispersion corrections. Further investigations on several rambergite samples could be of interest to deduce the possibility for both absolute configurations to exist. The authors do not know of any multiple investigation of wurtzites with single-crystal methods in order to deduce the possibility of both absolute configurations. Many investigations on wurtzites have been done with powder diffraction data but these are necessarily insufficient for determining absolute configuration. The physical factors that determines the absolute configuration of certain crystals is at present unclear. Some materials may be found with both absolute configurations, zinc hydroxide is one of these (Eriksson, 2001). Some important semiconductors, (Al,Ga,In)-nitrides crystallize in the wurtzite structure. The effect of the polarity of the corresponding structures on electronic properties are however unknown to the authors.

S2. Experimental

The sample is a naturally occurring mineral from Garpenberg, Dalarna, Sweden (Kalinowski, 1996) and has been given the name Rambergite. No signs of superstructure reflections could be detected but a slight broadening of the reflections could be detected, perhaps due to the occurrence of concentration gradients.

S3. Refinement

The refinements with the wurtzite model gave clear indications that the inverse model was the correct one. The wurtzite model gave wr2=0.0841, R1(all data)=0.0316 while the inverse wurtzite gave large improvements, R1(all data)=0.0128. The calculated Flack parameter was 1.0 (1) for the wurtzite model, thus clearly indicating the wrong absolute configuration. The z-coordinate of the mixed metal position was arbitrarily locked at 1.0 in order for the model to be easily compared to other wurtzites type structures. At present it is unknown if both absolute configurations exists for rambergite as only two single crystals of the mineral were available and unfortunately one of them was lost in the initial diffraction experiments. The iron content (5%) was determined by microprobe analyses. No attempt was made to deduce the iron content from refinements with the diffraction data. Iron and manganese are much too similar for this to be successful. The ADP's for Fe and Mn were constrained to equal each other in the least-squares calculations.

Computing details

Data collection: DIF4 (STOE, 1988); cell refinement: DIF4 (STOE, 1988); data reduction: X-RED (STOE, 1997); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: Diamond (Bergerhoff, 1996).

Figures

Fig. 1. Stereoview of the rambergite structure with slightly more than the unit-cell content. Mn is grey and S yellow. Displacement ellipsoids are shown at the 90% level.

(I)

Crystal data

Fe0.05Mn0.95S	Dx = 3.267 Mg m^−3
Mr = 87.05	Mo Kα radiation, λ = 0.71073 Å
Hexagonal, P63mc	Cell parameters from 24 reflections
a = 3.982 (2) Å	θ = 20.3–26.4°
c = 6.445 (3) Å	µ = 8.08 mm^−1
V = 88.49 (8) Å^3	T = 293 K
Z = 2	Prism, dark brown
F(000) = 82	0.22 × 0.17 × 0.13 mm

Data collection

STOE AED4 diffractometer	72 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.037
Graphite monochromator	θmax = 24.8°, θmin = 5.9°
ω/2θ scans	h = −4→4
Absorption correction: numerical X-RED (STOE, 1997)	k = −4→4
Tmin = 0.165, Tmax = 0.364	l = −7→7
546 measured reflections	3 standard reflections every 120 min
78 independent reflections	intensity decay: 2%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: none
R[F^2 > 2σ(F^2)] = 0.010	w = 1/[σ^2(Fo^2) + (0.020P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.027	(Δ/σ)max < 0.001
S = 1.05	Δρmax = 0.35 e Å^−3
78 reflections	Δρmin = −0.34 e Å^−3
6 parameters	Absolute structure: (Flack, 1983)
0 restraints	Absolute structure parameter: 0.02 (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.

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)
Mn	0.6667	0.3333	1.0000	0.0154 (2)	0.95
Fe	0.6667	0.3333	1.0000	0.0154 (2)	0.05
S	0.6667	0.3333	0.62235 (17)	0.0144 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mn	0.0156 (2)	0.0156 (2)	0.0149 (3)	0.00782 (11)	0.000	0.000
Fe	0.0156 (2)	0.0156 (2)	0.0149 (3)	0.00782 (11)	0.000	0.000
S	0.0135 (4)	0.0135 (4)	0.0161 (5)	0.0067 (2)	0.000	0.000

Geometric parameters (Å, º)

Mn—S	2.4338 (16)	S—Mn^iv	2.4303 (12)
Mn—S^i	2.4303 (12)	S—Fe^v	2.4303 (12)
Mn—S^ii	2.4303 (12)	S—Mn^v	2.4303 (12)
Mn—S^iii	2.4303 (12)	S—Mn^vi	2.4303 (12)
S—Fe^iv	2.4303 (12)	S—Fe^vi	2.4303 (12)
S—Mn—S^i	108.93 (3)	Mn^iv—S—Mn^v	110.00 (3)
S—Mn—S^ii	108.93 (3)	Fe^v—S—Mn^v	0.0
S^i—Mn—S^ii	110.00 (3)	Mn—S—Mn^vi	108.93 (3)
S—Mn—S^iii	108.93 (3)	Fe^iv—S—Mn^vi	110.00 (3)
S^i—Mn—S^iii	110.00 (3)	Mn^iv—S—Mn^vi	110.00 (3)
S^ii—Mn—S^iii	110.00 (3)	Fe^v—S—Mn^vi	110.00 (3)
Mn—S—Fe^iv	108.93 (3)	Mn^v—S—Mn^vi	110.00 (3)
Mn—S—Mn^iv	108.93 (3)	Mn—S—Fe^vi	108.93 (3)
Fe^iv—S—Mn^iv	0.0	Fe^iv—S—Fe^vi	110.00 (3)
Mn—S—Fe^v	108.93 (3)	Mn^iv—S—Fe^vi	110.00 (3)
Fe^iv—S—Fe^v	110.00 (3)	Fe^v—S—Fe^vi	110.00 (3)
Mn^iv—S—Fe^v	110.00 (3)	Mn^v—S—Fe^vi	110.00 (3)
Mn—S—Mn^v	108.93 (3)	Mn^vi—S—Fe^vi	0.0
Fe^iv—S—Mn^v	110.00 (3)

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