Commensurate monoclinic form of Ca₂Zn_0.97Co_0.03Ge₂O₇

Cobalt-doped dicalcium zinc germanate, synthesized by slow cooling from the melt, is monoclinic and has a layered structure, which is different from the modulated melilite-type structure of Ca₂ZnGe₂O₇. The monoclinic form has two different Ca, one Zn and two Ge sites, and seven independent O-atom positions; all are in general position 4e of the space group P2₁/n. The topology of the structure is described and compared with that of Ca₂ZnGe_1.25Si_0.75O₇.

S1. Comment

Minerals/compounds with general formula A₂BC₂X₇ (A is a large cation such as Ba, Sr and Ca; B is a small four-coordinated metal cation such as Mg, Zn, Co and Ni; C = Si and Ge; X = ??) most frequently belong to the group of melilite-type structures. They show tetragonal symmetry and several of them are known to be incommensurably modulated, e.g. Ca₂MgSi₂O₇ and Ca₂CoSi₂O₇ (Seifert et al., 1987; Hemingway et al., 1986, Riester & Böhm, 1997). It has been suggested that this temperature-dependent modulation may be due to the structural misfit between the large A cations and the framework of five-membered rings of B and C tetrahedra (Van Heureck et al., 1992; Hemingway et al., 1986; Seifert et al., 1987). For Ca₂ZnGe₂O₆, Armbruster et al. (1990) report two polymorphs. High-Ca₂ZnGe₂O₇ possesses an incommensurately modulated structure at room temperature; the average structure can be described in space group P42₁m, which is the typical symmetry of melilite-type compounds. Van Heureck et al. (1992) confirmed the modulated structure of high-Ca₂ZnGe₂O₇ using high-resolution transmission electron microscopy. Low-Ca₂ZnGe₂O₇, grown at much lower temperature from a Na₂WO₄ flux by Armbruster et al. (1990), also displays a modulated fine structure; however the average structure was described in the monoclinic system, space group P2₁. The latter polymorph represents a stacking variant of melilite-like layers with an ABA'ABA' stacking sequence (Armbruster et al., 1990). These authors also report the synthesis of a new tetrahedral sheet structure with composition Ca₂ZnGe_1.25Si_0.75O₇ at low temperatures of 723 K using hydrothermal techniques. This structure is monoclinic, space group P2₁/n, and well ordered. In this paper, the structure refinement of weakly Co-doped, unmodulated Ca₂ZnGe₂O₆ is presented and compared with Ca₂ZnGe_1.25Si_0.75O₇ described previously by Armbruster et al. (1990).

The structure of the title compound consists of layers of corner-sharing tetrahedra running parallel to (101). The layers consist of Ge₂O₇ groups linked by ZnO₄ tetrahedra and are separated by Ca atoms. Fig. 1 shows a displacement ellipsoid plot of the title compound including atomic nomenclature, while Figs. 2 and 3 display polyhedral representations in different orientations. While in the melilite-type structure only five-membered rings occur, the tetrahedral layer in the title compound consists of four-, five and strongly deformed six-membered rings (shaded areas in Fig. 2). As described by Armbruster et al. (1990), the layers are stacked in an ABAB sequence, where the A and B layers are symmetrically equivalent but shifted relative to each other along [101]/2. The average Ge–O bond lengths for the Ge1 and Ge2 tetrahedra are identical; however, the polyhedral volume is smaller and the bond length distortion (BLD; Renner & Lehmann, 1986), tetrahedral angle variance and mean quadratic elongation (TAV and TQE; Robinson et al., 1971) are larger for the Ge2 site, revealing this site to be more distorted (Table 1). The larger angular distortion of the Ge2 site is mainly due to a large O6—Ge2—O5 bonding angle of 122.3 (1)°, opposite the O6—O5 tetrahedral edge length of 3.036 (2) Å, which is the longest among all the tetrahedral edges. It can be concluded that within the monoclinic non-modulated structure of Ca₂ZnGe₂O₇ (this study) the Ge tetrahedra are much more regular than those in the tetragonal and monoclinic modulated forms described by Armbruster et al. (1990). Owing to the exclusive occupation of Ge1 and Ge2 sites by germanium, individual and average bond lengths are distinctly larger (Table 1) in the title compound than in Ca₂ZnGe_1.25Si_0.75O₇ of Armbruster et al. (1990). For both sites, the replacement of Ge by Si causes a reduction of tetrahedral distortion, which is more evident for the Ge2 site (Table 1). As the Ge2 site is smaller even in Ca₂ZnGe₂O₇, it may be argued that this site is more accessible for Si incorporation, as has been observed by Armbruster et al. (1990). The Ge1—O3—Ge2 angle is remarkably small [119.7 (1)°]; Si substitution causes an increase of this angle to 122° (Armbruster et al., 1990), while in Ca₂ZnSi₂O₇ (hardystonite) the Si—O1—Si angle is 141.47 (8)° (Louisnathan, 1969). Bond valence sums S (Brese & O'Keeffe, 1991) for the two Ge sites in the title compound are close to the ideal values of 4.0 valence units (v.u.); the valence sums for the coordinating O atoms are between 1.9 and 2.10 v.u., except for O7 with S = 1.58 v.u. and O4 with S = 1.84 v.u. The former is bonded to atoms Ge2, Ca1 and Ca2, while the latter O atom has bonds to Ge1, two Ca1 and one Ca2 sites. The highest bond valence sum is found for atom O3, which is bonded to atoms Ge1, Ge2 and Ca2 (S = 2.10 v.u.).

Individual and average bond and edge lengths of the ZnO₄ tetrahedra are of comparable size in the title compound and in Ca₂ZnGe_1.25Si_0.75O₇, while the TAV and TQE are lower for the Ge end member. Generally this site appears to be distinctly distorted (Table 1). The valence sum of the Zn atom is 2.09 v.u., which is close to the ideal value.

The Ca atoms reside within two different types of channels running along [101]. The Ca1 site is hosted in a channel arising from the stacking of the five-membered rings of tetrahedra, which are rotated relative to each other. Here Ca1 is in a distorted sixfold oxygen coordination. The volume, angular variance and quadratic elongation of this octahedron are large (Table 1). Individual and average Ca1—O bond lengths and distortion parameters are almost identical in the pure Ge and the Si-substituted compound (Table 1), suggesting that tetrahedral substitution has a minor influence on the bonding characteristics of the Ca1 site. The Ca2 site is located in a channel that is formed by the alternate stacking of four- and six-membered rings, and it is eightfold coordinated. The average Ca2—O bond is larger than the average Ca1—O bond but is of similar size to the equivalent bond in the Armbruster et al. (1990) compound (Table 1). Both Ca sites are distinctly under-bonded, with bond valence sums of 1.82 and 1.74 v.u.

S2. Experimental

The title compound was obtained by chance during synthesis of germanate clinopyroxenes in the system CaZnGe₂O₆–CaNiGe₂O₆. Pressed tablets of a homogeneous finely ground mixture of CaCO₃, NiO, ZnO and GeO₂ in the stoichiometry of CaZn_0.5Ni_0.5Ge₂O₆ were placed into an open platinum crucible, fired to 1598 K at a rate of 100 K h⁻¹, held at this temperature for 12 h to ensure complete melting and cooled slowly to 1373 K at a rate of 6 K h⁻¹ afterwards. The experiment yielded large yellow clinopyroxene crystals of up to 2 mm in size. In addition, a small amount (< 1 wt%) of emerald-green cubic spinel NiGe₂O₄ and some pale-pink crystals of the title compound were formed. The colour of the title compound results from traces of cobalt. It should be noted that at the same time an experiment to grow the CaCoGe₂O₆ clinopyroxene compound was performed in the same high-temperature furnace. The presence of cobalt in the title compound was validated by electron microprobe chemical analysis, yielding the following chemical composition (in wt%): 27.86 CaO, 19.71 ZnO, 0.56 CoO, 51.71 GeO₂. This corresponds to a structural formula of Ca_2.01 (2)Zn_0.98 (2)Co_0.03 (1)Ge_2.00 (1)O₇. No nickel was found in this compound.

S3. Refinement

Experimental image-plate data could be indexed on the basis of a monoclinic unit cell; no superstructure or satellite reflections were found in reconstructed precession images. Attempts to index the data on the basis of a tetragonal or monoclinic cell similar to the those used by Armbruster et al. (1990) failed. Analysis of systematic extinction rules and E statistics confirmed space group P2₁/n. After full anisotropic refinement of all parameters on F² an occupation of Ca1, Ca2 and Zn sites with Co was tested. Only for the Zn site was a weak significant positive occupation found, and thus Co was put on the tetragonally coordinated Zn site. We are aware that the scattering power is very similar for Zn and Co. However, as the amount of Co indicated from structure refinenent is quite close to that determined from electron microprobe analysis, the procedure used seems to be justified. A second data set, collected on a Bruker SMART APEX diffractometer at the University of Salzburg, yielded identical results, including the same amount of Co on the Zn site. As the data set from the image plate extends to larger diffraction angles, results from the Stoe IPDS measurement are presented here.

Computing details

Data collection: X-AREA (Stoe & Cie, 2002); cell refinement: X-AREA; data reduction: X-AREA; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: WinGX (Version 1.70.01; Farrugia, 1999).

Figures

	Fig. 1. A view of (I) (90% probability displacement ellipsoids). [Symmetry codes: (i) −x + 1, −y + 1, −z + 1; (ii) −x + 1/2, y + 1/2, −z + 3/2; (iii) x, y + 1, z; (iv) −x + 3/2, y + 1/2, −z + 3/2; (v) −x + 1, −y + 1, −z + 1; (vi) x + 1/2, −y + 3/2, z + 1/2. i and v are the same
	Fig. 2. A polyhdral representation of a tetrahedral layer of (I), viewed parallel to [101]. Ca atoms above and beyond the tetrahedral layer are shown, and the four-, five- and six-membered rings of tetrahedra are outlined by shading.
	Fig. 3. A polyhdral representation of (I), viewed along [010], displaying the layered character of the structure.

dicalcium zinc cobalt digermanate

Crystal data

Ca2Zn0.97Co0.03Ge2O7	F(000) = 759.6
Mr = 402.53	Dx = 4.227 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 9.1608 (8) Å	Cell parameters from 5650 reflections
b = 7.9863 (7) Å	θ = 2.6–28.1°
c = 9.4590 (8) Å	µ = 14.78 mm^−1
β = 113.9084 (12)°	T = 298 K
V = 632.65 (9) Å^3	Cuboid, pink
Z = 4	0.15 × 0.12 × 0.10 mm

Data collection

Stoe IPDS-II diffractometer	1386 reflections with I > 2σ(I)
Radiation source: sealed X-ray tube, Simens	Rint = 0.044
ω scans	θmax = 28.1°, θmin = 2.6°
Absorption correction: numerical via equivalents using X-SHAPE (Stoe & Cie 1996)	h = −11→12
Tmin = 0.13, Tmax = 0.24	k = −10→10
5971 measured reflections	l = −12→12
1517 independent reflections

Refinement

Refinement on F^2	1 restraint
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0323P)^2 + 0.4636P] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.022	(Δ/σ)max = 0.001
wR(F^2) = 0.053	Δρmax = 0.69 e Å^−3
S = 1.07	Δρmin = −0.82 e Å^−3
1517 reflections	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
112 parameters	Extinction coefficient: 0.0198 (8)

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	Occ. (<1)
Ca1	0.45903 (7)	0.78514 (7)	1.03006 (6)	0.00821 (14)
Ca2	0.70927 (7)	0.93272 (9)	0.79986 (7)	0.01310 (15)
Zn	0.32741 (4)	0.90361 (4)	0.61685 (4)	0.00667 (13)	0.970 (15)
Co	0.32741 (4)	0.90361 (4)	0.61685 (4)	0.00667 (13)	0.030 (16)
Ge1	0.37237 (3)	0.54508 (4)	0.70941 (3)	0.00489 (10)
Ge2	0.46826 (3)	0.23160 (4)	0.57441 (3)	0.00484 (10)
O1	0.4539 (3)	0.7406 (3)	0.7791 (2)	0.0093 (4)
O2	0.3694 (2)	0.4340 (3)	0.8656 (2)	0.0119 (4)
O3	0.5137 (2)	0.4371 (3)	0.6560 (2)	0.0079 (4)
O4	0.1895 (2)	0.5436 (3)	0.5488 (2)	0.0077 (4)
O5	0.4668 (2)	0.0960 (3)	0.7198 (2)	0.0078 (4)
O6	0.6487 (2)	0.1778 (3)	0.5641 (2)	0.0103 (4)
O7	0.2899 (3)	0.2309 (3)	0.4148 (2)	0.0124 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ca1	0.0083 (3)	0.0102 (3)	0.0048 (2)	0.0030 (2)	0.0012 (2)	0.00075 (19)
Ca2	0.0089 (3)	0.0225 (3)	0.0066 (3)	0.0078 (2)	0.0018 (2)	−0.0005 (2)
Zn	0.00598 (19)	0.0070 (2)	0.00728 (19)	−0.00012 (12)	0.00294 (13)	−0.00064 (11)
Co	0.00598 (19)	0.0070 (2)	0.00728 (19)	−0.00012 (12)	0.00294 (13)	−0.00064 (11)
Ge1	0.00525 (16)	0.00471 (16)	0.00358 (15)	0.00002 (10)	0.00062 (11)	−0.00011 (9)
Ge2	0.00423 (15)	0.00573 (16)	0.00379 (16)	−0.00022 (10)	0.00081 (11)	−0.00063 (9)
O1	0.0131 (10)	0.0047 (9)	0.0064 (9)	0.0015 (8)	0.0002 (8)	−0.0007 (7)
O2	0.0074 (10)	0.0189 (11)	0.0102 (10)	0.0032 (8)	0.0044 (8)	0.0092 (8)
O3	0.0062 (9)	0.0077 (9)	0.0105 (9)	−0.0004 (8)	0.0042 (8)	−0.0030 (7)
O4	0.0037 (9)	0.0109 (10)	0.0055 (9)	0.0005 (7)	−0.0013 (7)	−0.0005 (7)
O5	0.0096 (9)	0.0078 (10)	0.0055 (9)	−0.0042 (8)	0.0024 (7)	−0.0011 (7)
O6	0.0067 (9)	0.0157 (11)	0.0076 (9)	0.0013 (8)	0.0020 (8)	−0.0039 (8)
O7	0.0069 (9)	0.0183 (11)	0.0079 (10)	0.0015 (8)	−0.0012 (8)	−0.0043 (8)

Geometric parameters (Å, º)

Ca1—O2^i	2.291 (2)	Ca2—O7^iii	2.880 (2)
Ca1—O5^i	2.379 (2)	Zn—O2^ii	1.892 (2)
Ca1—O1	2.382 (2)	Zn—O6^v	1.924 (2)
Ca1—O4^ii	2.418 (2)	Zn—O5^vi	1.983 (2)
Ca1—O4^iii	2.461 (2)	Zn—O1	1.988 (2)
Ca1—O7^ii	2.573 (2)	Ge1—O2	1.733 (2)
Ca2—O3^iv	2.399 (2)	Ge1—O1	1.741 (2)
Ca2—O7^v	2.418 (2)	Ge1—O4	1.7470 (19)
Ca2—O5^vi	2.418 (2)	Ge1—O3	1.791 (2)
Ca2—O4^iii	2.441 (2)	Ge2—O7	1.717 (2)
Ca2—O6^iv	2.477 (2)	Ge2—O6	1.749 (2)
Ca2—O1	2.737 (2)	Ge2—O5	1.755 (2)
Ca2—O6^vi	2.848 (2)	Ge2—O3	1.789 (2)
O2^i—Ca1—O5^i	90.39 (8)	O7^v—Ca2—O6^vi	77.26 (7)
O2^i—Ca1—O1	92.76 (7)	O5^vi—Ca2—O6^vi	61.40 (6)
O5^i—Ca1—O1	158.94 (8)	O4^iii—Ca2—O6^vi	129.44 (7)
O2^i—Ca1—O4^ii	170.75 (8)	O6^iv—Ca2—O6^vi	150.03 (4)
O5^i—Ca1—O4^ii	82.21 (7)	O1—Ca2—O6^vi	116.07 (6)
O1—Ca1—O4^ii	92.22 (7)	O3^iv—Ca2—O7^iii	80.19 (7)
O2^i—Ca1—O4^iii	88.55 (7)	O7^v—Ca2—O7^iii	137.20 (6)
O5^i—Ca1—O4^iii	79.38 (7)	O5^vi—Ca2—O7^iii	71.19 (7)
O1—Ca1—O4^iii	79.89 (7)	O4^iii—Ca2—O7^iii	70.82 (7)
O4^ii—Ca1—O4^iii	84.65 (7)	O6^iv—Ca2—O7^iii	124.76 (7)
O2^i—Ca1—O7^ii	107.07 (8)	O1—Ca2—O7^iii	129.11 (7)
O5^i—Ca1—O7^ii	77.59 (7)	O6^vi—Ca2—O7^iii	66.01 (6)
O1—Ca1—O7^ii	121.04 (7)	O2^ii—Zn—O6^v	125.25 (9)
O4^ii—Ca1—O7^ii	76.83 (7)	O2^ii—Zn—O5^vi	107.72 (9)
O4^iii—Ca1—O7^ii	152.10 (7)	O6^v—Zn—O5^vi	114.57 (9)
O3^iv—Ca2—O7^v	77.92 (7)	O2^ii—Zn—O1	107.13 (10)
O3^iv—Ca2—O5^vi	145.03 (8)	O6^v—Zn—O1	102.98 (9)
O7^v—Ca2—O5^vi	110.10 (7)	O5^vi—Zn—O1	94.04 (8)
O3^iv—Ca2—O4^iii	108.53 (7)	O2—Ge1—O1	106.80 (10)
O7^v—Ca2—O4^iii	151.51 (8)	O2—Ge1—O4	113.40 (10)
O5^vi—Ca2—O4^iii	80.94 (7)	O1—Ge1—O4	116.63 (10)
O3^iv—Ca2—O6^iv	68.03 (7)	O2—Ge1—O3	104.88 (10)
O7^v—Ca2—O6^iv	79.19 (7)	O1—Ge1—O3	106.88 (10)
O5^vi—Ca2—O6^iv	145.94 (7)	O4—Ge1—O3	107.45 (9)
O4^iii—Ca2—O6^iv	77.94 (7)	O7—Ge2—O6	122.31 (10)
O3^iv—Ca2—O1	146.46 (7)	O7—Ge2—O5	112.33 (10)
O7^v—Ca2—O1	86.01 (7)	O6—Ge2—O5	101.53 (10)
O5^vi—Ca2—O1	68.27 (7)	O7—Ge2—O3	110.76 (10)
O4^iii—Ca2—O1	73.57 (7)	O6—Ge2—O3	100.92 (10)
O6^iv—Ca2—O1	80.23 (7)	O5—Ge2—O3	107.59 (9)
O3^iv—Ca2—O6^vi	89.02 (7)

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