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Single crystals of a new germanate, Na2.54Ca1.73Ge3O9, have been synthesized. The structure has a six-membered ring of GeO4 tetrahedra, which is similar to the rings of the silicate analogue Na2Ca2Si3O9, and both structures contain pseudo-cubic subcells with an edge length of 3.8 Å. The details of the two compounds are slightly different, however. For example, two O atoms are statistically distributed about twofold axes in the title compound, while the silicate analogue has no such O-atom distributions. In addition, the title germanate has an extra partially populated metal site containing 54 (4)% Na, with no equivalent site in the silicate analogue.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270101012963/iz1014sup1.cif
Contains datablocks global, I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270101012963/iz1014Isup2.hkl
Contains datablock I

Comment top

In the ternary system Na2O-CaO-SiO2, Koppen & Padurow (1958) identified the compound Na2Ca2Si3O9 as having a rhombohedral lattice (a = 7.41 Å, α = 89.7°), by means of Debye-Scherrer photography. Glasser & Mileson (1968) studied the same compound by means of a Weissenberg method and found that its structure had a trigonal space group, P3121 or P3221. Maki & Sugimura (1968) studied solid solutions of the Na2O-CaO-SiO2 system, and found a clear phase boundary between the compounds Na2Ca2Si3O9 (low-temperature modification) and Na2CaSi2O6 (high-temperature modification). Their results were confirmed by Moir & Glasser (1974). The crystal structures of two minerals, both having a composition close to Na2Ca2Si3O9, were studied by Fischer & Tillmanns (1983). One was combeite from Zaire, with the space group R3 m, and the other was an unknown mineral from Eifel, with the space group P31 or P32. They thought that this compound had two modifications, with the former probably corresponding to the quenched high-temperature modification, while the latter corresponded to the low-temperature phase of Maki & Sugimura (1968). Single crystals of Na2CaSi2O6 (high-temperature modification) and Na2Ca2Si3O9 (low-temperature modification) were synthesized and studied in detail by Ohsato et al. in 1985 and 1986, respectively. These authors reported some interesting structural characteristics, such as the disorder of the Na/Ca atoms and the splitting of the O and metal atoms.

The silicates and their germanate analogues are being investigated here in order to find the common rules and principles among these compounds. The Na2O-CaO-GeO2 system has been studied and three different types of single-crystal have been synthesized. The first was Na3.70Ca1.15Ge3O9, which has a 12-membered ring (Nishi & Takeuchi, 1988), while the second was Na2CaGe2O6, which has a six-membered ring (Nishi & Takeuchi, 1990), analogous to Na2CaSi2O6 (high-temperature modification; Ohsato et al., 1985). The crystal structure of the third compound, Na2.54Ca1.73Ge3O9, corresponding to Na2Ca2Si3O9 (low-temperature modification; Ohsato et al., 1986), is the subject of the present paper.

After the least-squares refinement two O atoms (O1 and O2), which were located on twofold axes, showed abnormal atomic displacement parameters. In addition, the bond lengths between Ge and these O atoms were abnormal. Consequently, atoms O1 and O2 were assigned to general positions and split into two components about their twofold axes, with each component separated by 0.87 (2) and 0.93 (2) Å for O1 and O2, respectively.

The trigonal unit cell basically includes pseudo-cubic sub-cells with an edge length of 3.8 Å, similar to related compounds such as Na3.70Ca1.15Ge3O9 (Nishi & Takeuchi, 1988), Na2CaGe2O6 (Nishi & Takeuchi, 1990), the silicate analogue Na2CaSi2O6 (Ohsato et al., 1985) and Na2Ca2Si3O9 (Ohsato et al., 1986).

The six-membered ring of the title germanate, which is very similar to that of the silicate analogue Na2Ca2Si3O9 (Ohsato et al., 1986), is a little deformed in comparison with the regular six-membered ring of Na2CaGe2O6 (Nishi & Takeuchi, 1990) and the silicate analogue Na2CaSi2O6 (Ohsato et al., 1985). The arrangement of the six-membered rings is shown in Fig. 1.

The refined site occupancies of the disordered Na and Ca atoms over the sites M1—M6 are as follows: M1 = 0.56 (2) Na + 0.44 (2) Ca, M2 = 0.93 (2) Na + 0.07 (2) Ca, M3 = 0.54 (4) Na, M4 = 0.72 (2) Na + 0.28 (2) Ca, M5 = 0.12 (4) Na + 0.88 (4) Ca and M6 = Ca. Thus it can be seen that the site occupancies of the Na atoms range from 0% (site M6) to 93 (2)% (site M2). Judging from the average M—O distances for each M polyhedron, we can conclude that the greater the Na atom population of a given M site, the larger is the average value of the M—O distances. The average value of the M3—O distances is abnormally large, on account of the M3 site vacancy [46 (4)%]. The M3 site itself constitutes the fundamental difference between this germanate and the silicate analogue Na2Ca2Si3O9 (Ohsato et al., 1986). The former has Na atoms with an occupancy of 54 (4)% in this site, but there is no equivalent populated site in the latter.

The total numbers of Na and Ca atoms for the title germanate and Na2Ca2Si3O9 (Ohsato et al., 1986) are 4.27 (= 2.54 + 1.73) and 4 (= 2 + 2). Accordingly, this germanate requires extra sites to accommodate the excess Na atoms. The new M3 site, located at one of the vertices of the pseudo-cubic sub-cells, plays precisely this role. In Fig. 2, the M1—M6 polyhedra are shown.

Related literature top

For related literature, see: Fischer & Tillmanns (1983); Glasser & Mileson (1968); Koppen & Padurow (1958); Maki & Sugimura (1968); Moir & Glasser (1974); Nishi & Takeuchi (1988, 1990); Ohsato et al. (1985, 1986).

Experimental top

Single crystals of sodium calcium germanate were synthesized from a mixture of Na2CO3, CaCO3 and GeO2 at 1500 K. The electron microprobe analyses of the crystals showed that the chemical formula is Na2.54Ca1.73Ge3O9. The intensities of the reflections were measured at the X-ray laboratory of Nihon University, Japan.

Computing details top

Data collection: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1992); cell refinement: MSC/AFC Diffractometer Control Software; data reduction: TEXSAN (Molecular Structure Corporation, 1992); program(s) used to solve structure: ORFFE (Busing et al., 1964); program(s) used to refine structure: LINUS (Coppens & Hamilton, 1970); molecular graphics: Please provide details.

Figures top
[Figure 1] Fig. 1. The arrangement of six-membered rings in the unit cell. The hatched tetrahedra are at z 7/6, the dotted ones at z 5/6 and those without patterns at z 3/6. Atoms O1 and O2 are drawn at the average positions of each split half.
[Figure 2] Fig. 2. The arrangement of polyhedra about the cation sites. M1 polyhedra are at z 0.09, M2 at z 0.0, M3 at z = 0, M4 at z 0.16, M5 at z = 1/6 and M6 at z = 0. Atoms O1 and O2 are drawn as in Fig. 1. The refined site occupancies are given in the text.
(I) top
Crystal data top
Ca1.73Ge3Na2.54O9Dx = 3.60 Mg m3
Mr = 489.5Mo Kα radiation, λ = 0.71073 Å
Trigonal, P3121Cell parameters from 20 reflections
a = 10.780 (2) Åθ = 10.5–20.5°
c = 13.449 (2) ŵ = 11.07 mm1
V = 1353.5 (3) Å3T = 293 K
Z = 6Sphere, colourless
F(000) = 2937.00.14 mm (radius)
Data collection top
Rigaku AFC-5
diffractometer
Rint = 0.063
ω/2θ scansθmax = 40.0°
Absorption correction: for a sphere
International Tables, Vol. 2, Table 5.3.6B
h = 1616
Tmin = 0.114, Tmax = 0.158k = 019
4377 measured reflectionsl = 024
3108 independent reflections3 standard reflections every 150 reflections
2142 reflections with I > 3σ(I) intensity decay: 1.1%
Refinement top
Refinement on F w = 1/(σ2(F) + 0.0001F2)
Least-squares matrix: full(Δ/σ)max = 0.04
R[F2 > 2σ(F2)] = 0.062Δρmax = 1.1 e Å3
wR(F2) = 0.068Δρmin = 0.1 e Å3
S = 1.34Extinction correction: isotropic (Zachariasen, 1963)
2142 reflectionsExtinction coefficient: 0.000050 (2)
117 parameters
Crystal data top
Ca1.73Ge3Na2.54O9Z = 6
Mr = 489.5Mo Kα radiation
Trigonal, P3121µ = 11.07 mm1
a = 10.780 (2) ÅT = 293 K
c = 13.449 (2) Å0.14 mm (radius)
V = 1353.5 (3) Å3
Data collection top
Rigaku AFC-5
diffractometer
2142 reflections with I > 3σ(I)
Absorption correction: for a sphere
International Tables, Vol. 2, Table 5.3.6B
Rint = 0.063
Tmin = 0.114, Tmax = 0.1583 standard reflections every 150 reflections
4377 measured reflections intensity decay: 1.1%
3108 independent reflections
Refinement top
R[F2 > 2σ(F2)] = 0.062117 parameters
wR(F2) = 0.068Δρmax = 1.1 e Å3
S = 1.34Δρmin = 0.1 e Å3
2142 reflections
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/UeqOcc. (<1)
Ge10.1968 (1)0.1495 (1)0.77072 (8)0.012 (1)
Ge20.4984 (1)0.3237 (1)0.89852 (7)0.011 (1)
Ge30.6278 (1)0.1484 (1)0.76144 (7)0.011 (1)
M1(Na)0.3154 (4)0.0136 (4)0.5848 (3)0.027 (2)0.56 (2)
M1(Ca)0.3154 (4)0.0136 (4)0.5848 (3)0.027 (2)0.44 (2)
M2(Na)0.5057 (9)0.3419 (8)0.6642 (4)0.035 (2)0.93 (2)
M2(Ca)0.5057 (9)0.3419 (8)0.6642 (4)0.035 (2)0.07 (2)
M3(Na)0.815 (1)0.00.333330.032 (2)0.54 (4)
M4(Na)0.5198 (6)0.3589 (6)0.1596 (3)0.037 (2)0.72 (2)
M4(Ca)0.5198 (6)0.3589 (6)0.1596 (3)0.037 (2)0.28 (2)
M5(Na)0.8241 (3)0.00.833330.020 (1)0.12 (4)
M5(Ca)0.8241 (3)0.00.833330.020 (1)0.88 (4)
M6(Ca)0.3074 (3)0.00.333330.019 (1)
O10.183 (2)0.018 (2)0.863 (1)0.023 (3)*0.5
O20.568 (2)0.023 (2)0.864 (1)0.022 (3)*0.5
O30.349 (2)0.283 (2)0.827 (1)0.051 (3)*
O40.599 (1)0.274 (1)0.822 (1)0.048 (4)*
O50.258 (1)0.156 (1)0.6511 (8)0.032 (3)*
O60.433 (1)0.238 (1)0.0095 (8)0.039 (3)*
O70.549 (1)0.079 (1)0.6546 (8)0.033 (3)*
O80.060 (1)0.185 (1)0.7912 (7)0.027 (3)*
O90.596 (1)0.508 (1)0.8871 (8)0.031 (4)*
O100.810 (1)0.214 (1)0.7724 (7)0.029 (3)*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Ge10.0185 (9)0.0242 (9)0.0331 (7)0.0100 (7)0.0037 (7)0.0082 (6)
Ge20.0281 (9)0.0269 (9)0.0195 (7)0.0196 (7)0.0036 (7)0.0043 (5)
Ge30.0185 (9)0.0183 (8)0.0261 (7)0.0122 (7)0.0028 (5)0.0004 (6)
M10.067 (5)0.045 (5)0.058 (3)0.037 (4)0.021 (12)0.016 (5)
M20.070 (4)0.106 (7)0.047 (4)0.054 (5)0.006 (4)0.028 (5)
M30.052 (7)0.136 (9)0.030 (3)0.068 (4)0.022 (6)0.044 (11)
M40.062 (6)0.134 (7)0.055 (4)0.073 (5)0.023 (3)0.042 (7)
M50.039 (4)0.038 (4)0.040 (3)0.019 (2)0.0035 (7)0.0069 (14)
M60.038 (4)0.028 (4)0.040 (3)0.014 (2)0.0001 (6)0.0002 (11)
Geometric parameters (Å, º) top
M1—O6i2.31 (1)M4—O9ix2.900 (9)
M1—O52.38 (1)M5—O5vii2.27 (1)
M1—O72.39 (1)M5—O5ii2.27 (1)
M1—O9ii2.41 (1)M5—O8xi2.386 (8)
M1—O8iii2.520 (8)M5—O8xii2.386 (8)
M1—O10iv2.73 (1)M5—O102.52 (1)
M2—O52.41 (1)M5—O10xiii2.52 (1)
M2—O7v2.39 (1)M5—O22.92 (2)
M2—O42.60 (2)M5—O2xiii2.92 (2)
M2—O32.64 (2)M6—O10iv2.274 (9)
M2—O2iv2.24 (1)M6—O10xiv2.274 (9)
M2—O2v3.16 (2)M6—O9ix2.30 (1)
M2—O6vi2.915 (8)M6—O9ii2.30 (1)
M2—O4iv2.85 (1)M6—O8iii2.412 (8)
M2—O73.09 (2)M6—O8x2.412 (8)
M3—O6vii2.44 (1)Ge1—O81.721 (13)
M3—O6viii2.44 (1)Ge1—O51.727 (11)
M3—O1ix2.57 (1)Ge1—O31.725 (15)
M3—O1ii2.57 (1)Ge1—O11.833 (16)
M3—O3ix2.79 (2)Ge1—O1xiii1.719 (18)
M3—O3ii2.79 (2)Ge2—O91.728 (10)
M3—O8ix3.115 (9)Ge2—O6xv1.711 (10)
M3—O8ii3.115 (9)Ge2—O31.734 (19)
M4—O62.33 (1)Ge2—O41.762 (13)
M4—O7iv2.37 (1)Ge3—O71.646 (10)
M4—O4ix2.43 (1)Ge3—O21.809 (15)
M4—O3x2.51 (2)Ge3—O2xiii1.694 (17)
M4—O9x2.70 (1)Ge3—O101.729 (10)
M4—O10ix2.821 (8)Ge3—O41.736 (13)
O1—Ge1—O1xiii28.0 (14)O6xv—Ge2—O3105.5 (6)
O8—Ge1—O5119.5 (6)O6xv—Ge2—O4119.7 (6)
O8—Ge1—O3107.9 (9)O3—Ge2—O4103.5 (8)
O8—Ge1—O1108.4 (8)O2—Ge3—O2xiii30.6 (12)
O8—Ge1—O1xiii116.7 (5)O7—Ge3—O2114.6 (6)
O5—Ge1—O3100.9 (6)O7—Ge3—O2xiii85.4 (5)
O5—Ge1—O1122.9 (8)O7—Ge3—O10118.8 (5)
O5—Ge1—O1xiii98.9 (6)O7—Ge3—O4120.7 (7)
O3—Ge1—O191.4 (8)O2—Ge3—O1098.5 (7)
O3—Ge1—O1xiii111.7 (8)O2xiii—Ge3—O10106.8 (5)
O9—Ge2—O6xv123.2 (6)O2—Ge3—O495.1 (8)
O9—Ge2—O3101.3 (7)O2xiii—Ge3—O4119.7 (6)
O9—Ge2—O4100.6 (5)O4—Ge3—O10104.5 (5)
Symmetry codes: (i) xy, y, z+2/3; (ii) x+1, yx, z+4/3; (iii) yx, x, z1/3; (iv) yx+1, x+1, z1/3; (v) x+1, yx+1, z+4/3; (vi) yx+1, x+1, z+2/3; (vii) y+1, xy, z+1/3; (viii) x+1, yx, z+1/3; (ix) y+1, xy, z2/3; (x) y, x, z+1; (xi) x+1, y, z; (xii) xy+1, y, z+5/3; (xiii) xy, y, z+5/3; (xiv) y, x1, z+1; (xv) x, y, z+1.

Experimental details

Crystal data
Chemical formulaCa1.73Ge3Na2.54O9
Mr489.5
Crystal system, space groupTrigonal, P3121
Temperature (K)293
a, c (Å)10.780 (2), 13.449 (2)
V3)1353.5 (3)
Z6
Radiation typeMo Kα
µ (mm1)11.07
Crystal size (mm)0.14 (radius)
Data collection
DiffractometerRigaku AFC-5
diffractometer
Absorption correctionFor a sphere
International Tables, Vol. 2, Table 5.3.6B
Tmin, Tmax0.114, 0.158
No. of measured, independent and
observed [I > 3σ(I)] reflections
4377, 3108, 2142
Rint0.063
(sin θ/λ)max1)0.904
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.062, 0.068, 1.34
No. of reflections2142
No. of parameters117
No. of restraints?
Δρmax, Δρmin (e Å3)1.1, 0.1

Computer programs: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1992), MSC/AFC Diffractometer Control Software, TEXSAN (Molecular Structure Corporation, 1992), ORFFE (Busing et al., 1964), LINUS (Coppens & Hamilton, 1970), Please provide details.

Selected geometric parameters (Å, º) top
M1—O6i2.31 (1)M4—O9ix2.900 (9)
M1—O52.38 (1)M5—O5vii2.27 (1)
M1—O72.39 (1)M5—O5ii2.27 (1)
M1—O9ii2.41 (1)M5—O8xi2.386 (8)
M1—O8iii2.520 (8)M5—O8xii2.386 (8)
M1—O10iv2.73 (1)M5—O102.52 (1)
M2—O52.41 (1)M5—O10xiii2.52 (1)
M2—O7v2.39 (1)M5—O22.92 (2)
M2—O42.60 (2)M5—O2xiii2.92 (2)
M2—O32.64 (2)M6—O10iv2.274 (9)
M2—O2iv2.24 (1)M6—O10xiv2.274 (9)
M2—O2v3.16 (2)M6—O9ix2.30 (1)
M2—O6vi2.915 (8)M6—O9ii2.30 (1)
M2—O4iv2.85 (1)M6—O8iii2.412 (8)
M2—O73.09 (2)M6—O8x2.412 (8)
M3—O6vii2.44 (1)Ge1—O81.721 (13)
M3—O6viii2.44 (1)Ge1—O51.727 (11)
M3—O1ix2.57 (1)Ge1—O31.725 (15)
M3—O1ii2.57 (1)Ge1—O11.833 (16)
M3—O3ix2.79 (2)Ge1—O1xiii1.719 (18)
M3—O3ii2.79 (2)Ge2—O91.728 (10)
M3—O8ix3.115 (9)Ge2—O6xv1.711 (10)
M3—O8ii3.115 (9)Ge2—O31.734 (19)
M4—O62.33 (1)Ge2—O41.762 (13)
M4—O7iv2.37 (1)Ge3—O71.646 (10)
M4—O4ix2.43 (1)Ge3—O21.809 (15)
M4—O3x2.51 (2)Ge3—O2xiii1.694 (17)
M4—O9x2.70 (1)Ge3—O101.729 (10)
M4—O10ix2.821 (8)Ge3—O41.736 (13)
O1—Ge1—O1xiii28.0 (14)O6xv—Ge2—O3105.5 (6)
O8—Ge1—O5119.5 (6)O6xv—Ge2—O4119.7 (6)
O8—Ge1—O3107.9 (9)O3—Ge2—O4103.5 (8)
O8—Ge1—O1108.4 (8)O2—Ge3—O2xiii30.6 (12)
O8—Ge1—O1xiii116.7 (5)O7—Ge3—O2114.6 (6)
O5—Ge1—O3100.9 (6)O7—Ge3—O2xiii85.4 (5)
O5—Ge1—O1122.9 (8)O7—Ge3—O10118.8 (5)
O5—Ge1—O1xiii98.9 (6)O7—Ge3—O4120.7 (7)
O3—Ge1—O191.4 (8)O2—Ge3—O1098.5 (7)
O3—Ge1—O1xiii111.7 (8)O2xiii—Ge3—O10106.8 (5)
O9—Ge2—O6xv123.2 (6)O2—Ge3—O495.1 (8)
O9—Ge2—O3101.3 (7)O2xiii—Ge3—O4119.7 (6)
O9—Ge2—O4100.6 (5)O4—Ge3—O10104.5 (5)
Symmetry codes: (i) xy, y, z+2/3; (ii) x+1, yx, z+4/3; (iii) yx, x, z1/3; (iv) yx+1, x+1, z1/3; (v) x+1, yx+1, z+4/3; (vi) yx+1, x+1, z+2/3; (vii) y+1, xy, z+1/3; (viii) x+1, yx, z+1/3; (ix) y+1, xy, z2/3; (x) y, x, z+1; (xi) x+1, y, z; (xii) xy+1, y, z+5/3; (xiii) xy, y, z+5/3; (xiv) y, x1, z+1; (xv) x, y, z+1.
 

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