Redetermination of β-Ba(PO₃)₂

In comparison with the previous structure determination of the β-modification of barium catena-polyphosphate that was based on Weissenberg film data [Grenier et al. (1967). Bull. Soc. Fr. Minéral. Cristallogr. 90, 24–31], the current CCD-data-based redetermination reveals all atoms with anisotropic displacement parameters, standard uncertainties for the atomic coordinates, and the determination of the absolute structure. Moreover, a much higher accuracy in terms of the bond-length distribution for the polyphosphate chain, with two shorter and two longer P—O distances, was achieved. The structure consists of polyphosphate chains extending parallel to [100] with a periodicity of two PO₄ tetra­hedra. The Ba²⁺ cations are located between the chains and are surrounded by ten O atoms in the form of a distorted coordination polyhedron, with Ba—O distances ranging from 2.765 (3) to 3.143 (3) Å, also reflecting the higher precision of the current redetermination.

Related literature

For polymorphism of Ba(PO₃)₂, see: Grenier & Martin (1975). For the previous structure refinement of β-Ba(PO₃)₂, see: Grenier et al. (1967). For the structure refinement of γ-Ba(PO₃)₂, see: Coing-Boyat et al. (1978). For the crystal chemistry of condensed phosphates, see: Durif (1995). For standardization of structure data, see: Gelato & Parthé (1987).

Experimental

Crystal data

• Ba(PO₃)₂

• M_r = 295.28

• Orthorhombic, P 2₁ 2₁ 2₁

• a = 4.4979 (2) Å

• b = 8.3377 (4) Å

• c = 13.3911 (6) Å

• V = 502.19 (4) ų

• Z = 4

• Mo Kα radiation

• μ = 8.50 mm⁻¹

• T = 293 K

• 0.15 × 0.08 × 0.05 mm

Data collection

• Bruker SMART CCD diffractometer

• Absorption correction: multi-scan (SADABS; Bruker, 2007) T_min = 0.362, T_max = 0.676

• 5836 measured reflections

• 1587 independent reflections

• 1560 reflections with I > 2σ(I)

• R_int = 0.016

Refinement

• R[F² > 2σ(F²)] = 0.020

• wR(F²) = 0.047

• S = 1.13

• 1587 reflections

• 82 parameters

• Δρ_max = 1.09 e Å⁻³

• Δρ_min = −0.75 e Å⁻³

• Absolute structure: Flack (1983), 624 Friedel pairs

• Absolute structure parameter: 0.04 (2)

Table 1
Comparison of the P—O bond lengths (Å) of the two related PO₄ tetra­hedra in the current and the previous (Grenier et al., 1967) refinement of β-Ba(PO₃)₂

current refinement	previous refinement
P1—O3 1.475 (2)^viii	P2—O6 1.427
P1—O1 1.480 (2)^viii	P2—O5 1.540
P1—O6 1.607 (2)^ix	P2—O2 1.590
P1—O4 1.625 (2)^viii	P2—O1 1.652
P2—O2 1.481 (2)	P1—O3 1.476
P2—O5 1.486 (2)^vii	P1—O4 1.540
P2—O6 1.604 (3)^vi	P1—O2 1.559
P2—O4 1.607 (3)^vi	P1—O1 1.621
Symmetry codes: (vi) −x + , −y, z + ; (vii) x − , −y + , −z + 1; (viii) −x, y + , −z + ; (ix) −x + 1, y + , −z + ; (x) x − 1, y, z.

Data collection: SMART (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Supporting information

S1. Comment

Polymorphism of Ba(PO₃)₂ with three modifications has been reported by Grenier & Martin (1975): The stable β-form transforms to the high-temperature α-form at 1058 K, and the γ-form transforms at 978 K to the β-form. Structure determinations were carried out for the γ-form (Coing-Boyat et al., 1978) and for the β-form (Grenier et al., 1967). The crystal structure of α-Ba(PO₃)₂ is yet unknown. Comparative discussions of the structural set-up of the β- and γ-form of Ba(PO₃)₂ and of other divalent long-chain polyphosphates were given by Durif (1995).

During experiments intended to isolate crystals of α-Ba(PO₃)₂ by quenching the reaction product from the re-crystallized melt at temperatures above the indicated transition point, high-quality crystals of β-Ba(PO₃)₂ were obtained instead. Since the first structure refinement of this modification was based on Weissenberg film data and converged with a relatively high residual R = 0.1, with atoms refined only with isotropic displacement factors and without indication of standard uncertainties for the fractional atomic coordinates, a re-refinement of the structure with modern CCD-based data seemed appropriate. The results of this re-refinement are reported here, confirming in principle the results of Grenier et al. (1967), however, achieving bond lengths and angles with much higher accuracy and precision, as exemplified by a comparison of the P—O bond length (Table 1).

The catena-polyphosphate chain has a periodicity of two PO₄ tetrahedra and extends parallel to [100] (Fig. 1). In comparison with the previous structure refinement (Grenier et al., 1967), the determined bond lengths of the present refinement are in much better agreement with the usually observed bond length distribution in such long-chain polyphosphates (Durif, 1995), with two shorter and two longer P—O distances, each with similar values (Table 1).

The Ba²⁺ cation is located between the chains and is surrounded by ten oxygen atoms in an irregular coordination sphere with Ba—O distances in the range from 2.765 (3) to 3.143 (3) Å (Fig. 2).

Related literature

For polymorphism of Ba(PO₃)₂, see: Grenier & Martin (1975). For the previous structure refinement of β-Ba(PO₃)₂, see: Grenier et al. (1967). For the structure refinement of γ-Ba(PO₃)₂, see: Coing-Boyat et al. (1978). For the crystal chemistry of condensed phosphates, see: Durif (1995). For standardization of structure data, see: Gelato & Parthé (1987).

S2. Experimental

Stoichiometric amounts of BaCO₃ and (NH₄)₂HPO₄ (molar ratio 1:2) with a 3% excess of the phosphate precursor were finely ground, heated in a platinum crucible to 1173 K and slowly cooled to 1073 K at a rate of 2 K h^-1. Then the crucible was quenched in a cold water bath. Colourless fragments of the title compound were cut from the clear, transparent reaction product.

S3. Refinement

In contrast to the previous structure refinement (Grenier et al., 1967) with a = 4.510 (2), b = 13.44 (2) c = 8.36 (5) Å, the reduced cell setting was chosen for the current refinement. Structure data were finally standardized with STRUCTURE-TIDY (Gelato & Parthé, 1987). The highest and lowest remaining electron densities are located 0.75 Å from Ba and 1.19 Å from O6, respectively.

Computing details

Data collection: SMART (Bruker, 2007); cell refinement: SAINT (Bruker, 2007); data reduction: SAINT (Bruker, 2007); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. The crystal structure of β-Ba(PO3)2 in a projection approximately along [100]. Ba^2+ cations are shown in blue, PO4 tetrahedra in red with O atoms in white. Displacement ellipsoids are drawn at the 90% probability level. For clarity, no Ba—O bonds are shown.
	Fig. 2. The irregular BaO10 coordination polyhedron in the structure of β-Ba(PO3)2. Displacement ellipsoids are drawn at the 90% probability level. [Symmetry codes: (i) -x + 1/2, -y, z - 1/2; (ii) -x + 1, y - 1/2, -z + 1/2; (iii) x + 1/2, -y + 1/2, -z + 1; (iv) x + 1, y, z; (v) -x, y - 1/2, -z + 1/2; (vi) -x + 1/2, -y, z + 1/2; (vii) x - 1/2, -y + 1/2, -z + 1.]

Barium catena-polyphosphate

Crystal data

Ba(PO3)2	F(000) = 536
Mr = 295.28	Dx = 3.905 Mg m^−3
Orthorhombic, P212121	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: P 2ac 2ab	Cell parameters from 4713 reflections
a = 4.4979 (2) Å	θ = 2.4–31.0°
b = 8.3377 (4) Å	µ = 8.50 mm^−1
c = 13.3911 (6) Å	T = 293 K
V = 502.19 (4) Å^3	Fragment, colourless
Z = 4	0.15 × 0.08 × 0.05 mm

Data collection

Bruker SMART CCD diffractometer	1587 independent reflections
Radiation source: fine-focus sealed tube	1560 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.016
ω scan	θmax = 31.0°, θmin = 2.9°
Absorption correction: multi-scan (SADABS; Bruker, 2007)	h = −6→6
Tmin = 0.362, Tmax = 0.676	k = −12→11
5836 measured reflections	l = −19→19

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)] = 0.020	w = 1/[σ^2(Fo^2) + (0.0223P)^2 + 0.9553P] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.047	(Δ/σ)max = 0.001
S = 1.13	Δρmax = 1.09 e Å^−3
1587 reflections	Δρmin = −0.75 e Å^−3
82 parameters	Absolute structure: Flack (1983), 624 Friedel pairs
0 restraints	Absolute structure parameter: 0.04 (2)

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.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq
Ba	0.49023 (4)	0.05203 (2)	0.375501 (12)	0.01202 (6)
P1	0.03622 (19)	0.67382 (10)	0.34674 (6)	0.00958 (15)
P2	0.04126 (18)	0.03118 (10)	0.60393 (6)	0.01002 (15)
O1	0.0024 (8)	0.1293 (3)	0.25946 (16)	0.0142 (4)
O2	0.0272 (6)	0.1222 (3)	0.69880 (17)	0.0153 (4)
O3	0.0307 (6)	0.3369 (3)	0.11686 (17)	0.0149 (4)
O4	0.1343 (5)	0.0429 (3)	0.08393 (18)	0.0116 (4)
O5	0.4603 (6)	0.3816 (3)	0.48895 (17)	0.0142 (5)
O6	0.6315 (5)	0.1360 (3)	0.11585 (19)	0.0131 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Ba	0.01116 (8)	0.01345 (9)	0.01147 (8)	−0.00007 (7)	−0.00044 (8)	0.00042 (6)
P1	0.0082 (3)	0.0093 (3)	0.0113 (3)	0.0002 (3)	−0.0007 (3)	−0.0001 (3)
P2	0.0087 (3)	0.0103 (3)	0.0111 (3)	0.0000 (3)	−0.0001 (3)	0.0008 (3)
O1	0.0152 (9)	0.0156 (10)	0.0118 (9)	0.0018 (12)	0.0002 (11)	0.0016 (8)
O2	0.0163 (11)	0.0151 (10)	0.0145 (10)	−0.0008 (11)	−0.0010 (10)	−0.0037 (8)
O3	0.0167 (11)	0.0110 (10)	0.0171 (10)	−0.0020 (10)	−0.0019 (12)	0.0017 (8)
O4	0.0076 (9)	0.0135 (11)	0.0137 (11)	0.0021 (9)	−0.0010 (8)	−0.0047 (9)
O5	0.0156 (13)	0.0129 (10)	0.0140 (10)	−0.0012 (10)	0.0022 (10)	0.0041 (8)
O6	0.0072 (9)	0.0118 (10)	0.0203 (12)	−0.0004 (8)	−0.0031 (9)	−0.0014 (10)

Geometric parameters (Å, º)

Ba—O1	2.765 (3)	Ba—O5	3.143 (3)
Ba—O2^i	2.778 (2)	P1—O3^viii	1.475 (2)
Ba—O3^ii	2.806 (2)	P1—O1^viii	1.480 (2)
Ba—O5^iii	2.841 (3)	P1—O6^ix	1.607 (2)
Ba—O1^iv	2.852 (3)	P1—O4^viii	1.625 (2)
Ba—O2^iii	2.897 (2)	P2—O2	1.481 (2)
Ba—O3^v	2.952 (2)	P2—O5^vii	1.486 (2)
Ba—O4^vi	2.955 (2)	P2—O6^vi	1.604 (3)
Ba—O5^vii	3.047 (3)	P2—O4^vi	1.607 (3)
O1—Ba—O2^i	67.71 (8)	O3^ii—Ba—O5	125.05 (7)
O1—Ba—O3^ii	141.17 (7)	O5^iii—Ba—O5	63.43 (6)
O2^i—Ba—O3^ii	73.64 (7)	O1^iv—Ba—O5	95.77 (7)
O1—Ba—O5^iii	154.74 (7)	O2^iii—Ba—O5	49.37 (6)
O2^i—Ba—O5^iii	131.84 (8)	O3^v—Ba—O5	118.72 (7)
O3^ii—Ba—O5^iii	61.94 (7)	O4^vi—Ba—O5	76.68 (7)
O1—Ba—O1^iv	106.39 (7)	O5^vii—Ba—O5	61.30 (6)
O2^i—Ba—O1^iv	71.14 (7)	O3^viii—P1—O1^viii	121.65 (14)
O3^ii—Ba—O1^iv	62.86 (7)	O3^viii—P1—O6^ix	105.50 (14)
O5^iii—Ba—O1^iv	72.72 (7)	O1^viii—P1—O6^ix	111.06 (17)
O1—Ba—O2^iii	68.54 (8)	O3^viii—P1—O4^viii	109.50 (15)
O2^i—Ba—O2^iii	101.50 (3)	O1^viii—P1—O4^viii	108.97 (15)
O3^ii—Ba—O2^iii	124.60 (7)	O6^ix—P1—O4^viii	97.43 (13)
O5^iii—Ba—O2^iii	89.67 (7)	O2—P2—O5^vii	117.17 (15)
O1^iv—Ba—O2^iii	63.57 (7)	O2—P2—O6^vi	109.83 (14)
O1—Ba—O3^v	62.04 (7)	O5^vii—P2—O6^vi	112.95 (15)
O2^i—Ba—O3^v	71.91 (7)	O2—P2—O4^vi	112.26 (15)
O3^ii—Ba—O3^v	102.70 (7)	O5^vii—P2—O4^vi	105.74 (15)
O5^iii—Ba—O3^v	133.37 (7)	O6^vi—P2—O4^vi	97.02 (13)
O1^iv—Ba—O3^v	142.84 (6)	P1^v—O1—Ba	133.76 (18)
O2^iii—Ba—O3^v	128.82 (7)	P1^v—O1—Ba^x	119.03 (18)
O1—Ba—O4^vi	116.29 (7)	Ba—O1—Ba^x	106.39 (7)
O2^i—Ba—O4^vi	131.24 (7)	P2—O2—Ba^vi	117.64 (13)
O3^ii—Ba—O4^vi	86.55 (7)	P2—O2—Ba^vii	100.85 (12)
O5^iii—Ba—O4^vi	65.78 (7)	Ba^vi—O2—Ba^vii	141.37 (9)
O1^iv—Ba—O4^vi	136.74 (7)	P1^v—O3—Ba^ix	137.20 (14)
O2^iii—Ba—O4^vi	125.91 (7)	P1^v—O3—Ba^viii	112.75 (13)
O3^v—Ba—O4^vi	69.76 (7)	Ba^ix—O3—Ba^viii	102.70 (7)
O1—Ba—O5^vii	70.83 (6)	P2^i—O4—P1^v	126.27 (15)
O2^i—Ba—O5^vii	125.48 (8)	P2^i—O4—Ba^i	103.09 (11)
O3^ii—Ba—O5^vii	134.05 (7)	P1^v—O4—Ba^i	129.06 (12)
O5^iii—Ba—O5^vii	99.57 (7)	P2^iii—O5—Ba^vii	128.45 (15)
O1^iv—Ba—O5^vii	156.26 (7)	P2^iii—O5—Ba^iii	102.60 (13)
O2^iii—Ba—O5^vii	94.54 (7)	Ba^vii—O5—Ba^iii	99.57 (7)
O3^v—Ba—O5^vii	57.92 (7)	P2^iii—O5—Ba	90.73 (11)
O4^vi—Ba—O5^vii	48.51 (6)	Ba^vii—O5—Ba	120.73 (9)
O1—Ba—O5	91.95 (7)	Ba^iii—O5—Ba	114.41 (9)
O2^i—Ba—O5	150.21 (7)	P2^i—O6—P1^ii	130.68 (17)

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