A new high-pressure strontium germanate, SrGe₂O₅

The Sr-Ge-O system has an earth-scientific importance as a potentially good low-pressure analog of the Ca-Si-O system, one of the major components in the constituent minerals of the Earth's crust and mantle. However, it is one of the germanate systems that has not yet been fully examined in the phase relations and structural properties. The recent findings that the SrGeO₃ high-pressure perovskite phase is the first Ge-based transparent electronic conductor make the Sr-Ge-O system interesting in the field of materials science. In the present study, we have revealed the existence of a new high-pressure strontium germanate, SrGe₂O₅. Single crystals of this compound crystallized as a co-existent phase with SrGeO₃ perovskite single crystals in the sample recovered in the compression experiment of SrGeO₃ pseudowollastonite conducted at 6 GPa and 1223 K. The crystal structure consists of germanium-oxygen framework layers stacked along [001], with Sr atoms located at the 12-coordinated cubocta­hedral site; the layers are formed by the corner linkages between GeO₆ octa­hedra and between GeO₆ octa­hedra and GeO₄ tetra­hedra. The present SrGe₂O₅ is thus isostructural with the high-pressure phases of SrSi₂O₅ and BaGe₂O₅. Comparison of these three compounds leads to the conclusion that the structural responses of the GeO₆ and GeO₄ polyhedra to cation substitution at the Sr site are much less than that of the SrO₁₂ cubocta­hedron to cation substitution at the Ge sites. Such a difference in the structural response is closely related to the bonding nature.

Computing details

Data collection: RAPID-AUTO (Rigaku, 1998); cell refinement: RAPID-AUTO (Rigaku, 1998); data reduction: RAPID-AUTO (Rigaku, 1998); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015); molecular graphics: ATOMS for Windows (Dowty, 2000); software used to prepare material for publication: publCIF (Westrip, 2010).

Strontium pentaoxidodigermanate

Crystal data

SrGe2O5	Dx = 5.670 Mg m^−3
Mr = 312.80	Mo Kα radiation, λ = 0.71069 Å
Orthorhombic, Cmca	Cell parameters from 2643 reflections
a = 5.4653 (6) Å	θ = 4.2–30.4°
b = 9.7379 (9) Å	µ = 30.70 mm^−1
c = 13.7710 (12) Å	T = 296 K
V = 732.90 (13) Å^3	Fragment, colorless
Z = 8	0.11 × 0.11 × 0.10 mm
F(000) = 1136

Data collection

Rigaku R-AXIS RAPID imaging-plate diffractometer	464 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.041
ω scans	θmax = 30.4°, θmin = 4.2°
Absorption correction: numerical (NUMABS; Higashi, 1998)	h = −7→7
Tmin = 0.095, Tmax = 0.156	k = −13→13
2817 measured reflections	l = −18→19
606 independent reflections

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.023	w = 1/[σ^2(Fo^2) + (0.0285P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.050	(Δ/σ)max < 0.001
S = 1.00	Δρmax = 0.89 e Å^−3
606 reflections	Δρmin = −0.90 e Å^−3
46 parameters	Extinction correction: SHELXL2014 (Sheldrick, 2015), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.00078 (11)

Special details

Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

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

	x	y	z	Uiso*/Ueq
Sr	0.0000	0.33785 (6)	0.41815 (4)	0.01001 (14)
Ge1	0.5000	0.17311 (6)	0.32117 (4)	0.00573 (14)
Ge2	0.5000	0.50467 (6)	0.36359 (3)	0.00711 (15)
O1	0.2500	0.2491 (4)	0.2500	0.0109 (7)
O2	0.5000	0.5042 (4)	0.2379 (2)	0.0072 (7)
O3	0.5000	0.3325 (4)	0.4061 (2)	0.0082 (7)
O4	0.7585 (6)	0.5929 (3)	0.41270 (19)	0.0094 (5)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Sr	0.0085 (3)	0.0093 (3)	0.0122 (2)	0.000	0.000	−0.0006 (2)
Ge1	0.0054 (3)	0.0057 (3)	0.0060 (2)	0.000	0.000	−0.0003 (2)
Ge2	0.0069 (3)	0.0068 (3)	0.0077 (2)	0.000	0.000	0.0006 (2)
O1	0.0090 (19)	0.0100 (17)	0.0136 (17)	0.000	−0.0052 (16)	0.000
O2	0.0098 (19)	0.0064 (16)	0.0054 (15)	0.000	0.000	0.0022 (15)
O3	0.0121 (19)	0.0054 (17)	0.0072 (15)	0.000	0.000	−0.0011 (15)
O4	0.0080 (12)	0.0109 (12)	0.0092 (11)	−0.0034 (12)	0.0008 (10)	−0.0007 (10)

Geometric parameters (Å, º)

Sr—O2^i	2.691 (3)	Ge1—Sr^xiii	3.5274 (9)
Sr—O3	2.7381 (4)	Ge1—Sr^viii	3.5914 (8)
Sr—O3^ii	2.7381 (4)	Ge1—Sr^x	3.6654 (8)
Sr—O4^iii	2.761 (3)	Ge2—O2	1.731 (3)
Sr—O4^iv	2.761 (3)	Ge2—O3	1.776 (4)
Sr—O4^v	2.773 (3)	Ge2—O4	1.787 (3)
Sr—O4^vi	2.773 (3)	Ge2—O4^vii	1.787 (3)
Sr—O4^vii	2.814 (3)	Ge2—Sr^xii	3.2667 (6)
Sr—O4^ii	2.814 (3)	Ge2—Sr^xiv	3.3303 (10)
Sr—O1	2.8241 (12)	O1—Ge1^i	1.8372 (16)
Sr—O1^i	2.8241 (12)	O1—Sr^x	2.8241 (12)
Sr—O3^viii	2.934 (4)	O2—Ge1^xv	1.835 (4)
Ge1—O2^ix	1.835 (4)	O2—Sr^x	2.691 (3)
Ge1—O1^x	1.8372 (16)	O3—Sr^xii	2.7381 (4)
Ge1—O1	1.8372 (16)	O3—Sr^viii	2.934 (4)
Ge1—O3	1.944 (4)	O4—Ge1^xiv	1.985 (3)
Ge1—O4^xi	1.985 (3)	O4—Sr^iii	2.761 (3)
Ge1—O4^vi	1.985 (3)	O4—Sr^xiv	2.773 (3)
Ge1—Sr^xii	3.4387 (5)	O4—Sr^xii	2.814 (3)
O2^i—Sr—O3	87.89 (8)	O4^vi—Ge1—Sr^xii	117.72 (8)
O2^i—Sr—O3^ii	87.89 (8)	O2^ix—Ge1—Sr	126.18 (2)
O3—Sr—O3^ii	172.73 (14)	O1^x—Ge1—Sr	127.61 (11)
O2^i—Sr—O4^iii	121.79 (8)	O1—Ge1—Sr	55.13 (4)
O3—Sr—O4^iii	65.08 (9)	O3—Ge1—Sr	52.676 (12)
O3^ii—Sr—O4^iii	122.19 (9)	O4^xi—Ge1—Sr	117.72 (8)
O2^i—Sr—O4^iv	121.79 (8)	O4^vi—Ge1—Sr	53.74 (8)
O3—Sr—O4^iv	122.19 (9)	Sr^xii—Ge1—Sr	105.25 (2)
O3^ii—Sr—O4^iv	65.08 (9)	O2^ix—Ge1—Sr^xiii	48.57 (11)
O4^iii—Sr—O4^iv	57.11 (12)	O1^x—Ge1—Sr^xiii	125.09 (10)
O2^i—Sr—O4^v	119.74 (8)	O1—Ge1—Sr^xiii	125.09 (10)
O3—Sr—O4^v	119.37 (10)	O3—Ge1—Sr^xiii	120.74 (11)
O3^ii—Sr—O4^v	58.23 (10)	O4^xi—Ge1—Sr^xiii	52.81 (8)
O4^iii—Sr—O4^v	118.45 (5)	O4^vi—Ge1—Sr^xiii	52.81 (8)
O4^iv—Sr—O4^v	89.38 (8)	Sr^xii—Ge1—Sr^xiii	106.543 (15)
O2^i—Sr—O4^vi	119.74 (8)	Sr—Ge1—Sr^xiii	106.543 (15)
O3—Sr—O4^vi	58.23 (10)	O2^ix—Ge1—Sr^viii	114.62 (11)
O3^ii—Sr—O4^vi	119.37 (10)	O1^x—Ge1—Sr^viii	123.04 (3)
O4^iii—Sr—O4^vi	89.38 (8)	O1—Ge1—Sr^viii	123.04 (3)
O4^iv—Sr—O4^vi	118.45 (5)	O3—Ge1—Sr^viii	54.69 (11)
O4^v—Sr—O4^vi	61.27 (12)	O4^xi—Ge1—Sr^viii	49.73 (8)
O2^i—Sr—O4^vii	56.44 (8)	O4^vi—Ge1—Sr^viii	49.73 (8)
O3—Sr—O4^vii	63.08 (10)	Sr^xii—Ge1—Sr^viii	68.015 (12)
O3^ii—Sr—O4^vii	118.90 (10)	Sr—Ge1—Sr^viii	68.015 (12)
O4^iii—Sr—O4^vii	65.34 (10)	Sr^xiii—Ge1—Sr^viii	66.050 (17)
O4^iv—Sr—O4^vii	91.79 (9)	O2^ix—Ge1—Sr^x	89.63 (11)
O4^v—Sr—O4^vii	175.92 (11)	O1^x—Ge1—Sr^x	49.00 (3)
O4^vi—Sr—O4^vii	121.301 (9)	O1—Ge1—Sr^x	49.00 (3)
O2^i—Sr—O4^ii	56.44 (8)	O3—Ge1—Sr^x	101.06 (11)
O3—Sr—O4^ii	118.90 (10)	O4^xi—Ge1—Sr^x	138.00 (9)
O3^ii—Sr—O4^ii	63.08 (10)	O4^vi—Ge1—Sr^x	138.00 (9)
O4^iii—Sr—O4^ii	91.79 (9)	Sr^xii—Ge1—Sr^x	98.339 (18)
O4^iv—Sr—O4^ii	65.34 (10)	Sr—Ge1—Sr^x	98.340 (18)
O4^v—Sr—O4^ii	121.301 (9)	Sr^xiii—Ge1—Sr^x	138.20 (2)
O4^vi—Sr—O4^ii	175.92 (11)	Sr^viii—Ge1—Sr^x	155.747 (18)
O4^vii—Sr—O4^ii	55.94 (12)	O2—Ge2—O3	109.12 (16)
O2^i—Sr—O1	61.93 (8)	O2—Ge2—O4	112.32 (10)
O3—Sr—O1	57.44 (7)	O3—Ge2—O4	109.22 (11)
O3^ii—Sr—O1	115.30 (7)	O2—Ge2—O4^vii	112.31 (10)
O4^iii—Sr—O1	122.36 (6)	O3—Ge2—O4^vii	109.22 (11)
O4^iv—Sr—O1	176.19 (10)	O4—Ge2—O4^vii	104.52 (19)
O4^v—Sr—O1	87.78 (9)	O2—Ge2—Sr	103.23 (6)
O4^vi—Sr—O1	57.87 (8)	O3—Ge2—Sr	56.951 (16)
O4^vii—Sr—O1	91.22 (9)	O4—Ge2—Sr	144.46 (9)
O4^ii—Sr—O1	118.36 (8)	O4^vii—Ge2—Sr	59.38 (9)
O2^i—Sr—O1^i	61.93 (8)	O2—Ge2—Sr^xii	103.23 (6)
O3—Sr—O1^i	115.31 (7)	O3—Ge2—Sr^xii	56.951 (16)
O3^ii—Sr—O1^i	57.44 (7)	O4—Ge2—Sr^xii	59.38 (9)
O4^iii—Sr—O1^i	176.19 (10)	O4^vii—Ge2—Sr^xii	144.46 (9)
O4^iv—Sr—O1^i	122.36 (6)	Sr—Ge2—Sr^xii	113.55 (3)
O4^v—Sr—O1^i	57.87 (8)	O2—Ge2—Sr^xiv	103.18 (12)
O4^vi—Sr—O1^i	87.78 (9)	O3—Ge2—Sr^xiv	147.70 (11)
O4^vii—Sr—O1^i	118.36 (8)	O4—Ge2—Sr^xiv	56.36 (10)
O4^ii—Sr—O1^i	91.22 (9)	O4^vii—Ge2—Sr^xiv	56.36 (10)
O1—Sr—O1^i	57.87 (3)	Sr—Ge2—Sr^xiv	115.631 (15)
O2^i—Sr—O3^viii	177.42 (11)	Sr^xii—Ge2—Sr^xiv	115.632 (15)
O3—Sr—O3^viii	92.24 (8)	Ge1^i—O1—Ge1	132.5 (2)
O3^ii—Sr—O3^viii	92.24 (8)	Ge1^i—O1—Sr^x	92.618 (18)
O4^iii—Sr—O3^viii	56.09 (8)	Ge1—O1—Sr^x	101.588 (16)
O4^iv—Sr—O3^viii	56.09 (8)	Ge1^i—O1—Sr	101.587 (15)
O4^v—Sr—O3^viii	62.36 (8)	Ge1—O1—Sr	92.616 (18)
O4^vi—Sr—O3^viii	62.36 (8)	Sr^x—O1—Sr	144.37 (15)
O4^vii—Sr—O3^viii	121.41 (8)	Ge2—O2—Ge1^xv	116.18 (19)
O4^ii—Sr—O3^viii	121.41 (8)	Ge2—O2—Sr^x	143.13 (18)
O1—Sr—O3^viii	120.22 (8)	Ge1^xv—O2—Sr^x	100.70 (13)
O1^i—Sr—O3^viii	120.22 (8)	Ge2—O3—Ge1	123.72 (19)
O2^ix—Ge1—O1^x	97.16 (12)	Ge2—O3—Sr^xii	90.11 (8)
O2^ix—Ge1—O1	97.16 (12)	Ge1—O3—Sr^xii	92.96 (8)
O1^x—Ge1—O1	96.09 (11)	Ge2—O3—Sr	90.11 (8)
O2^ix—Ge1—O3	169.31 (15)	Ge1—O3—Sr	92.96 (8)
O1^x—Ge1—O3	89.97 (12)	Sr^xii—O3—Sr	172.73 (14)
O1—Ge1—O3	89.97 (12)	Ge2—O3—Sr^viii	143.69 (17)
O2^ix—Ge1—O4^xi	85.93 (11)	Ge1—O3—Sr^viii	92.58 (14)
O1^x—Ge1—O4^xi	90.16 (10)	Sr^xii—O3—Sr^viii	87.76 (8)
O1—Ge1—O4^xi	172.61 (9)	Sr—O3—Sr^viii	87.76 (8)
O3—Ge1—O4^xi	86.09 (11)	Ge2—O4—Ge1^xiv	118.33 (14)
O2^ix—Ge1—O4^vi	85.93 (11)	Ge2—O4—Sr^iii	144.57 (13)
O1^x—Ge1—O4^vi	172.61 (9)	Ge1^xiv—O4—Sr^iii	97.01 (11)
O1—Ge1—O4^vi	90.16 (10)	Ge2—O4—Sr^xiv	91.20 (12)
O3—Ge1—O4^vi	86.09 (11)	Ge1^xiv—O4—Sr^xiv	91.01 (10)
O4^xi—Ge1—O4^vi	83.35 (17)	Sr^iii—O4—Sr^xiv	90.62 (8)
O2^ix—Ge1—Sr^xii	126.18 (2)	Ge2—O4—Sr^xii	87.50 (10)
O1^x—Ge1—Sr^xii	55.13 (4)	Ge1^xiv—O4—Sr^xii	93.00 (11)
O1—Ge1—Sr^xii	127.60 (11)	Sr^iii—O4—Sr^xii	88.21 (9)
O3—Ge1—Sr^xii	52.676 (12)	Sr^xiv—O4—Sr^xii	175.93 (11)
O4^xi—Ge1—Sr^xii	53.74 (8)

Symmetry codes: (i) x−1/2, y, −z+1/2; (ii) x−1, y, z; (iii) −x+1, −y+1, −z+1; (iv) x−1, −y+1, −z+1; (v) −x+1/2, y−1/2, z; (vi) x−1/2, y−1/2, z; (vii) −x+1, y, z; (viii) −x+1/2, −y+1/2, −z+1; (ix) x, y−1/2, −z+1/2; (x) x+1/2, y, −z+1/2; (xi) −x+3/2, y−1/2, z; (xii) x+1, y, z; (xiii) x+1/2, y−1/2, z; (xiv) x+1/2, y+1/2, z; (xv) x, y+1/2, −z+1/2.