Atomic and domain structure of the low-temperature phase of barium metagermanate (BaGeO₃)

The crystal structure of the low-temperature form of barium metagermanate (BaGeO₃) has been determined from laboratory X-ray powder diffraction data collected at 298.5 (5) K. The structure was found to consist of alternating layers of Ba cations and [GeO₃]₃ rings, and is closely related to pseudo-wollastonite. The rings show a twofold positional disorder owing to stacking faults. The stacking is not random, but can be rationalized by a twinning mechanism mapping the two non-congruent enantiomorphic polytypes of the structure onto each other. This model also explains the diffuse scattering and twinning observed in SAED and HRTEM, as well as the size and strain-like broadening effects found in the XRPD pattern.

Computing details

Data collection: STOE WinXPow; program(s) used to solve structure: intuitive; program(s) used to refine structure: FULLPROF.

Figures

(BaGeO3)

Crystal data

Ge3O9·3(Ba)	V = 1089.76 (2) Å^3
Mr = 257.94	Z = 12
Monoclinic, C2/c	? radiation, λ = 1.540560 Å
Hall symbol: -C 2yc	µ = 5.8 mm^−1
a = 13.18949 (15) Å	T = 299 K
b = 7.62045 (5) Å	white
c = 11.71698 (9) Å	flat_sheet, 8 × 8 mm
β = 112.2783 (7)°	Specimen preparation: Prepared at 1323 K

Data collection

STOE STADI MP diffractometer	Absorption correction: empirical (using intensity measurements) from beam loss ?
Radiation source: X-ray tube	Tmin = ?, Tmax = ?
Asymmetric Ge(111) monochromator	2θmin = 2.053°, 2θmax = 129.983°, 2θstep = 0.010°
Data collection mode: transmission

Refinement

Rp = 3.303	12794 data points
Rwp = 4.268	62 parameters
Rexp = 3.159	12 restraints
RBragg = 4.542
χ^2 = 3.349

Special details

Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates.

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

	x	y	z	Uiso*/Ueq	Occ. (<1)
Ba1	0.0838 (3)	0.2521 (4)	0.5001 (6)	0.0093 (2)*
Ba2	0.25000	0.25000	0.00000	0.0093 (2)*
Ge1	0.6592 (5)	0.8371 (7)	0.2505 (5)	0.0093 (2)*	0.50000
Ge2	0.7953 (5)	0.4653 (8)	0.2508 (5)	0.0093 (2)*	0.50000
Ge3	0.5429 (4)	0.4497 (7)	0.2508 (5)	0.0093 (2)*	0.50000
O1	0.5593 (15)	0.6811 (11)	0.249 (3)	0.0093 (2)*	0.50000
O2	0.7634 (16)	0.6927 (12)	0.247 (3)	0.0093 (2)*	0.50000
O3	0.670 (2)	0.3739 (17)	0.251 (6)	0.0093 (2)*	0.50000
O4	0.6051 (13)	0.934 (2)	0.1082 (7)	0.0093 (2)*
O5	0.6999 (16)	0.937 (2)	0.3920 (7)	0.0093 (2)*
O6	0.4572 (12)	0.404 (3)	0.1055 (7)	0.0093 (2)*

Geometric parameters (Å, º)

Ge1—O1	1.7685	Ge2—O4^i	1.6936
Ge1—O2	1.7734	Ge2—O5^i	1.7118
Ge1—O4	1.7110	Ge3—O1	1.7778
Ge1—O5	1.7166	Ge3—O3	1.7714
Ge2—O2	1.7800	Ge3—O6	1.6849
Ge2—O3	1.7950	Ge3—O6^ii	1.7197
O1—Ge1—O2	99.38	O1—Ge3—O3	101.76
O1—Ge1—O4	104.10	O1—Ge3—O6	103.22
O1—Ge1—O5	105.66	O1—Ge3—O6^ii	105.11
O2—Ge1—O4	106.56	O3—Ge3—O6	101.98
O2—Ge1—O5	109.66	O3—Ge3—O6^ii	106.44
O4—Ge1—O5	127.76	O6—Ge3—O6^ii	134.25
O2—Ge2—O3	99.73	Ge1—O1—Ge3	139.51
O2—Ge2—O4^i	104.54	Ge1—O2—Ge2	141.40
O2—Ge2—O5^i	101.16	Ge2—O3—Ge3	138.11
O3—Ge2—O4^i	108.38	Ge1—O4—Ge2^iii	39.47
O3—Ge2—O5^i	109.31	Ge1—O5—Ge2^iii	39.19
O4^i—Ge2—O5^i	129.31	Ge3—O6—Ge3^ii	38.59
O2—Ge1—O1—Ge3	3.32	O5^i—Ge2—O3—Ge3	−106.33
O4—Ge1—O1—Ge3	113.15	O2—Ge2—O4^i—Ge1^i	−160.88
O5—Ge1—O1—Ge3	−110.28	O3—Ge2—O4^i—Ge1^i	93.45
O1—Ge1—O2—Ge2	−4.44	O2—Ge2—O5^i—Ge1^i	162.43
O4—Ge1—O2—Ge2	−112.30	O3—Ge2—O5^i—Ge1^i	−93.00
O5—Ge1—O2—Ge2	106.02	O3—Ge3—O1—Ge1	−1.94
O1—Ge1—O4—Ge2^iii	164.67	O6—Ge3—O1—Ge1	−107.40
O2—Ge1—O4—Ge2^iii	−90.85	O6^ii—Ge3—O1—Ge1	108.90
O5—Ge1—O4—Ge2^iii	41.54	O1—Ge3—O3—Ge2	0.26
O1—Ge1—O5—Ge2^iii	−163.79	O6—Ge3—O3—Ge2	106.70
O2—Ge1—O5—Ge2^iii	89.96	O6^ii—Ge3—O3—Ge2	−109.55
O4—Ge1—O5—Ge2^iii	−41.30	O1—Ge3—O6—Ge3^ii	−93.54
O3—Ge2—O2—Ge1	3.54	O3—Ge3—O6—Ge3^ii	161.16
O4^i—Ge2—O2—Ge1	−108.48	O6^ii—Ge3—O6—Ge3^ii	33.53
O5^i—Ge2—O2—Ge1	115.61	O1—Ge3—O6^ii—Ge3^ii	92.11
O2—Ge2—O3—Ge3	−0.78	O3—Ge3—O6^ii—Ge3^ii	−160.44
O4^i—Ge2—O3—Ge3	108.21	O6—Ge3—O6^ii—Ge3^ii	−34.31

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