Bi₄Ge₃O₁₂ at the onset of pressure-induced amorphization

The crystal structure of tetra­bismuth tris­(germanate), Bi₄Ge₃O₁₂ (I3d, Z = 4), is stable to at least 7.30 GPa, as demonstrated by hydro­static single-crystal X-ray diffraction measurements in a diamond anvil cell at room temperature. The highest pressure reached in this study is close to the onset of amorphization at about 8 GPa. The Bi and Ge atoms are located at the 16c (3) and 12a () Wyckoff positions, respectively. The compression mainly affects the distorted BiO₆ octa­hedra, while the GeO₄ tetra­hedra are relatively rigid. When compared with the values obtained under ambient conditions, the long Bi-O distances decrease with increasing pressure, while the short Bi-O distances do not change.

S1. Comment

Tetrabismuth tris(germanate) Bi₄Ge₃O₁₂ (BGO) is an important material for scintillation detectors (Milenov et al., 2007) and holographic applications (Marinova et al., 2009). At atmospheric pressure, its crystal structure (I43d, Z = 4) is built up of isolated GeO₄ tetrahedra and strongly deformed BiO₆ octahedra (Durif & Averbuch-Pouchot, 1982; Fischer & Waldner, 1982; Milenov et al., 2007). The asymmetric coordination sphere around the Bi³⁺ cations, with three Bi—O distances of 2.16 Å and three others of 2.60 Å (Milenov et al., 2007), is due to the stereoactivity of its non-bonded lone electron pair (Pushkin et al., 2000).

The onset of irreversible pressure-induced amorphization of Bi₄Ge₃O₁₂ occurs at about 8.0 GPa at room temperature, as evidenced by the decrease of the intensities of the powder X-ray diffraction peaks (Arora et al., 2004), with the process being complete above 12 GPa (Meng et al., 1998; Arora et al., 2004). The compound has a bulk modulus of 48 (2) GPa with a first pressure derivative of 9(1).

At atmospheric pressure, the coordination numbers (CN) of the Bi³⁺ cations in BiO_n polyhedra of crystalline solids vary from 3 to 10 (Pushkin et al., 2000). The volume and activity of the lone pair of electrons (E) diminish when the number of O atoms bonded to the cation increases. The effect of pressure, associated with the increase of CN and with the coordination polyhedra becoming more regular, is similar, and the stereochemical activity of the lone pair is suppressed on compression (Grzechnik, 2007). For instance, α-Bi₂O₃ (P2₁/c, Z = 4) has a structure with two non-equivalent Bi atoms in distorted BiO₅E and BiO₆ octahedral coordinations at ambient conditions (Ivanov et al., 2001). A new polymorph of Bi₂O₃ synthesized at 6 GPa and 1153 K (Atou et al., 1998) has the structure of the A-type rare earth sesquioxides (P3m1, Z = 1), with the Bi atom coordinated by seven O atoms in a capped octahedron. In this new phase, the stereoactivity of the E pair on the Bi³⁺ cation is completely suppressed.

The purpose of this study was to determine the crystal structure of Bi₄Ge₃O₁₂ close to the onset of its pressure-induced amorphization at room temperature. Of special interest was the pressure dependence of the coordination environment around the Bi³⁺ cation. Intuitively, one could argue that BGO reaches a limit of its stability at about 8.0 GPa because either the CN of the Bi³⁺ cation has increased or the octahedron around the Bi³⁺ cation has become regular. To clarify this issue, hydrostatic single-crystal X-ray diffraction measurements were performed in a diamond anvil cell at room temperature.

The indexing of the single-crystal X-ray diffraction data and analysis of the reconstructed reciprocal space at 7.30 GPa indicated that BGO did not undergo any phase transition. This observation supports the previous report that the material is structurally stable up to the onset of pressure-induced amorphization at about 8.0 GPa (Arora et al., 2004).

The refinement of the data based on the structural model obtained using the program SIR97 (Altomare et al., 1999) produced a structure, with GeO₄ tetrahedra and distorted BiO₆ octahedra (Fig. 1), isotypical with that reported for Bi₄Ge₃O₁₂ (I43d, Z = 4) under ambient conditions (Durif & Averbuch-Pouchot, 1982; Fischer & Waldner, 1982; Milenov et al., 2007). A comparison of the Ge—O bond distances and O—Ge—O angles in Table 1 with those at atmosperic pressure (1.74 Å, and 105.6 and 117.5°, respectively; Durif & Averbuch-Pouchot, 1982; Fischer & Waldner, 1982; Milenov et al., 2007) shows that the GeO₄ tetrahedra are rigid and insensitive to increased pressure. The small bulk modulus of the material can be explained by significant shortening of the three long Bi—O bond distances (2.60 Å at room temperature and ambient pressure) in the octahedral coordination environment around the Bi³⁺ cation, while the other three shorter Bi—O distances (2.16 Å at room temperature and ambient pressure) do not change on compression (Milenov et al., 2007). On the other hand, increased pressure hardly influences the angular distortion of the BiO₆ octahedron (Fig. 2).

The results of this study demonstrate that the structural instability of Bi₄Ge₃O₁₂ at high pressure is due neither to an increased CN of the Bi³⁺ cation nor to the octahedron around the Bi³⁺ cation becoming more regular. However, they do not contradict the argument by Arora et al. (2004) that the pressure-induced amorphization of BGO arises from kinetically hindered decomposition of the material, favourable at all pressures.

S2. Experimental

The crystal was cut from a larger one, synthesized using the Czochralski method in the same batch as that previously investigated by Milenov et al. (2007). High-pressure data at 7.30 GPa were collected in a diamond anvil cell of the Boehler–Almax type (Boehler & de Hantsetters, 2004; Boehler, 2006) at room temperature using a Stoe IPDS 2 T diffractometer with Mo Kα radiation. A 0.25 mm hole was drilled into a stainless steel gasket preindented to a thickness of about 0.08 mm. The intensities were indexed and integrated using X-AREA (Stoe & Cie, 1998). Areas of the images shaded by the diamond anvil cell were masked prior to integration. Corrections for the effects of absorption by the diamond anvil and the crystal were made using the programs ABSORB (Angel, 2006) and X-RED (Stoe & Cie, 1998), respectively. The shape of the crystal was approximated by 20 faces using the program X-SHAPE (Stoe & Cie, 1998). The ruby luminescence method (Mao et al., 1986) was used for pressure calibration, and a mixture of methanol and ethanol was used as the pressure medium. The error in the pressure determination was estimated to be 0.02 GPa.

S3. Refinement

To confirm the absolute structure, the model was refined as an inversion twin. The refined twin fraction corresponds with a Flack parameter of 0.00 (9) (Flack & Bernardinelli, 1999).

Computing details

Data collection: X-AREA (Stoe & Cie, 1998); cell refinement: X-AREA (Stoe & Cie, 1998); data reduction: JANA2006 (Petříček et al., 2006); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: JANA2006 (Petříček et al., 2006); molecular graphics: DIAMOND (Brandenburg, 1999); software used to prepare material for publication: JANA2006 (Petříček et al., 2006).

Figures

	Fig. 1. The crystal structure of Bi4Ge3O12 at 7.30 GPa. The tetrahedra around the Ge atoms are shaded. Thin and thick lines indicate the long and short Bi—O bonds, respectively.
	Fig. 2. The coordination environment around the Bi^3+ cation at two different pressures. Thin and thick lines indicate the long and short Bi—O bonds, respectively.

Tetrabismuth tris(germanate)

Crystal data

Bi4Ge3O12	Dx = 7.868 Mg m^−3
Mr = 1245.7	Mo Kα radiation, λ = 0.71069 Å
Cubic, I43d	Cell parameters from 1616 reflections
Hall symbol: I -4acd;2ab;3	θ = 9.8–56.5°
a = 10.168 (1) Å	µ = 75.24 mm^−1
V = 1051.25 (18) Å^3	T = 293 K
Z = 4	Irregular shape, colourless
F(000) = 2096	0.10 × 0.09 × 0.06 mm

Data collection

Stoe IPDS 2T diffractometer	212 independent reflections
Radiation source: X-ray tube	174 reflections with I > 3σ(I)
Plane graphite monochromator	Rint = 0.085
Detector resolution: 6.67 pixels mm^-1	θmax = 28.5°, θmin = 4.9°
rotation method scans	h = −10→9
Absorption correction: numerical (X-RED; Stoe & Cie, 1998)	k = −13→13
Tmin = 0.009, Tmax = 0.018	l = −11→11
2935 measured reflections

Refinement

Refinement on F	0 constraints
R[F^2 > 2σ(F^2)] = 0.056	Weighting scheme based on measured s.u.'s w = 1/(σ^2(F) + 0.0001F^2)
wR(F^2) = 0.056	(Δ/σ)max = 0.039
S = 2.72	Δρmax = 8.04 (1.58Å from O1) e Å^−3
212 reflections	Δρmin = −5.36 e Å^−3
8 parameters	Absolute structure: Flack (1983), inversion twin model tested
0 restraints	Absolute structure parameter: 0.00 (9)

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

	x	y	z	Uiso*/Ueq
Bi1	0.91972 (14)	0.08028 (14)	0.58028 (14)	0.0121 (6)*
Ge1	0.75	0.375	0.5	0.0100 (16)*
O1	0.8727 (19)	0.2881 (19)	0.578 (2)	0.012 (5)*

Geometric parameters (Å, º)

Bi1—O1	2.167 (19)	Bi1—O1^v	2.167 (19)
Bi1—O1^i	2.499 (19)	Ge1—O1	1.72 (2)
Bi1—O1^ii	2.167 (19)	Ge1—O1^vi	1.72 (2)
Bi1—O1^iii	2.499 (19)	Ge1—O1^vii	1.72 (2)
Bi1—O1^iv	2.499 (19)	Ge1—O1^viii	1.72 (2)
O1—Bi1—O1^i	70.4 (7)	O1^ii—Bi1—O1^v	78.3 (8)
O1—Bi1—O1^ii	78.3 (8)	O1^iii—Bi1—O1^iv	116.7 (7)
O1—Bi1—O1^iii	146.2 (7)	O1^iii—Bi1—O1^v	83.2 (8)
O1—Bi1—O1^iv	83.2 (8)	O1^iv—Bi1—O1^v	70.4 (7)
O1—Bi1—O1^v	78.3 (8)	O1—Ge1—O1^vi	118.3 (9)
O1^i—Bi1—O1^ii	83.2 (8)	O1—Ge1—O1^vii	105.2 (10)
O1^i—Bi1—O1^iii	116.7 (7)	O1—Ge1—O1^viii	105.2 (10)
O1^i—Bi1—O1^iv	116.7 (7)	O1^vi—Ge1—O1^vii	105.2 (10)
O1^i—Bi1—O1^v	146.2 (7)	O1^vi—Ge1—O1^viii	105.2 (10)
O1^ii—Bi1—O1^iii	70.4 (7)	O1^vii—Ge1—O1^viii	118.3 (9)
O1^ii—Bi1—O1^iv	146.2 (7)

Symmetry codes: (i) −x+2, −y+1/2, z; (ii) −z+3/2, −x+1, y+1/2; (iii) −z+3/2, x−1, −y+1; (iv) y+1/2, z−1/2, x−1/2; (v) −y+1, z−1/2, −x+3/2; (vi) −x+3/2, y, −z+1; (vii) z+1/4, −y+3/4, −x+5/4; (viii) −z+5/4, −y+3/4, x−1/4.