The high-temperature modification of zinc catena-polyphosphate, β-Zn(PO₃)₂

Single crystals of the high-temperature modification of zinc catena-polyphosphate, β-Zn(PO₃)₂, were grown from a melt and quenched from 1093 K to room temperature. The structure was solved from single-crystal X-ray diffraction data and is built of corrugated (PO₃)_∞ polyphosphate chains, which extend along the c direction with an eight-tetrahedra repeat. Slightly distorted [ZnO₄] tetrahedra link the polyphos­phate chains into a three-dimensional network.

S1. Comment

Interest in materials with open-framework architectures has increased enormously in the past few years; in particular, solids based on organically templated phosphate networks have been investigated thoroughly (Cheetham et al., 1999). Among the open-framework materials, zinc phosphates have an exceptional position because their structural variety results in different physico- chemical properties (Neeraj & Natarajan, 2001). Additionally, some of these zinc phosphates form analogous aluminosilicate zeolite structures as a result of the formation of [ZnO₄] tetrahedral units, which make these materials very promising for application on a large industrial scale. Frequently, these materials have to be treated thermally to expel the template molecules and, at higher temperatures, thermolysis results in the formation of inorganic zinc phosphates, which also exhibit a diverse and interesting crystal chemistry.

Phase equilibria in the ZnO-P₂O₅ system have been investigated for the first time by Katnack & Hummel (1958), who showed that three congruently melting phases exist, viz. Zn₃(PO₄)₂, Zn₂P₂O₇ and Zn(PO₃)₂, all of which are polymorphic. Two polymorphic forms of zinc ultraphosphate with a Zn:P ratio of 1:4 have also been reported in the Zn—P₂O₅ system. Since ultraphosphates are difficult to prepare by conventional ceramic methods, no phases with this composition were observed during Katnack & Hummel's investigation. Up to now, the following anhydrous zinc phosphate structures have been reported (the coordination numbers (CN)of the unique Zn atoms are given in parentheses): three zinc orthophosphates α-Zn₃(PO₄)₂ (2x4; Calvo, 1965a), β-Zn₃(PO₄)₂ (2x5, 1x6; Stevens & Calvo, 1967), γ-Zn₃(PO₄)₂ (1x5, 1x6) (Calvo, 1963), three diphosphates, α-Zn₂P₂O₇ (2x5, 1x6; Robertson & Calvo, 1970), β-Zn₂P₂O₇ (1x6; Calvo, 1965b), γ-Zn₂P₂O₇ (2x5; Bataille et al., 1998), two modifications of the ultraphosphate ZnP₄O₁₁, one with a 16–4-membered (1x6; Baez-Doelle et al., 1993) and one with a 10–10-membered phosphate framework (2x6; Weil & Glaum, 1998), and four phosphates with a Zn:P ratio of 1:2. For the latter, three different phases are reported, viz. a cyclotetrametaphosphate, Zn₂P₄O₁₂, which is isotypic with the corresponding M₂P₄O₁₂ structures [M = Ni, Mg, Cu, Co, Mn and Fe; CN(M) = 6; Beucher & Grenier, 1968; Bagieu-Beucher et al., 1976], and two long-chain polyphosphates, Zn(PO₃)₂, viz. one of the low-temperature α form [CN(Zn) = 6; Averbuch-Pouchot et al., 1983] and one of the high-temperature β form. Although the thermal transformations of these Zn tetrameta- and polyphosphates have been the subject of several investigations (Thilo & Grunze, 1957; Nirsha et al., 1982; Trojan, 1990; Takenaka & Matsuda, 2002), no structural details for β-Zn(PO₃)₂ have been reported previously. For this latter phase, only lattice parameters have been reported (Schultz, 1974) and, therefore, single-crystal growth experiments for subsequent structure determination were started.

The high-temperature polymorph β-Zn(PO₃)₂ crystallizes in a new structure type and shows no close relationship with the low-temperature α polymorph. In the α polymorph, chains of distorted edge-sharing [ZnO₆] octahedra extend along the c direction, and the (PO₃)_∞ chains run parallel to the cationic chains, with a period of two PO₄ tetrahedra. This compact arrangement results in a very regular layered organization, and the density of α-Zn(PO₃)₂ (3.641 Mg m⁻³; volume of 16.97 ų per O atom) is much higher than that of β-Zn(PO₃)₂ (3.129 Mg m⁻³; 19.75 ų). The high-temperature polymorph contains two Zn, four P and 12 O atoms in the asymmetric unit. The Zn atoms are tetrahedrally coordinated, and each [ZnO₄] tetrahedron shares its four corners with four adjacent PO₄ tetrahedra, resulting in a layered assembly parallel to the (110) plane (Fig. 1). The (PO₃)_∞ polyphosphate chains cross the unit cell parallel to the c direction, with a period of eight PO₄ tetrahedra (P4, P1, P2, P3, P4', P1', P2' and P3'; Fig. 2). The distorted PO₄ tetrahedra display the bond-length distribution observed for various polyphosphate structures (Durif, 1995), with two significantly longer bridging P—O bonds (mean 1.586 Å) and two shorter terminal P—O bonds (mean 1.474 Å). The mean P—O—P angle [134.9 °] is also in the characteristic range for long-chain polyphosphates. Both [ZnO₄] tetrahedra are considerably distorted, with Zn—O distances in the range 1.904 (2)–1.955 (2) Å and an overall mean of 1.929 Å, which is in good agreement with the values found for other structures that contain tetrahedral [ZnO₄] units. The coordination number of all O atoms is two. Except for the bridging O3, O5, O11 and O12 atoms of the polyphosphate chain, all O atoms are bonded to one Zn and one P atom.

Results from the bond-valence calculations, using the parameters of Brese & O'Keeffe (1991), are in accordance with the expected values and reflect the small distortions in the [ZnO₄] and PO₄ tetrahedra (Zn1 2.19, Zn2 2.18, P1 4.98, P2 4.95, P3 4.96, P4 4.88, O1 1.88, O2 2.03, O3 2.08, O4 1.99, O5 2.15, O6 1.93, O7 2.03, O8 1.91, O9 1.95, O10 2.03, O11 2.08, O12 2.10).

S2. Experimental

For the preparation of pure zinc polyphosphate, a slight excess of the phosphate source (about 2%) is recommended (Thilo & Grunze, 1957). In a typical experiment, ZnO (0.364 g, Aldrich, p·A.) and (NH₄)₂HPO₄ (1.204 g, Merck, p·A.) were mixed thoroughly and milled in an agate mortar. The powder was then placed in a platinum crucible and in a conventional laboratory furnace [room temperature → 1163 K (3 h) → 1093 K (17 h)]. The platinum crucible then was removed from the furnace and immediately quenched to room temperature using a cold-water bath. The resulting solidified melt was crushed and examined under a polarizing microscope. Colourless crystals of the title compound, of an unspecific habit and up to 0.3 mm in length, were isolated. Powder X-ray diffraction of the bulk material revealed β-Zn(PO₃)₂ as a single-phase product. The powder data are consistent with those given by Katnack & Hummel (1957) and the lattice parameters are in very good agreement with those reported by Schultz (1974).

S3. Refinement

Systematic absences revealed the possible space groups Cc and C2/c. For the latter, no reasonable structure solution was obtained using either direct methods or a Patterson synthesis with the SHELX97 program package (Sheldrick, 1997). For the structure determination in the Cc space group, the Zn- and P-atom positions were obtained from direct methods and the O-atom positions from subsequent Fourier syntheses. Analysis of the refined atomic coordinates with the PLATON program (Spek, 2003) showed no higher symmetry. Moreover, the Flack parameter (Flack & Bernardinelli, 1999) gives a clear indication of the absence of a centre of symmetry.

Computing details

Data collection: CAD-4 (Enraf–Nonius, 1989); cell refinement: CAD-4 (Enraf–Nonius, 1989); data reduction: HELENA in PLATON (Spek, 2003); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ATOMS for Windows (Dowty, 2000); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. Projection of the β-Zn(PO3)2 crystal structure along [110]. The polyphosphate chains are dark grey and the [ZnO4] tetraheda are light grey.
	Fig. 2. A plot of the corrugated polyphosphate chain with attached [ZnO4] tetrahedra. The anisotropic displacement ellipsoids are drawn at the 74% probability level. Symmetry operators refer to Table 1.

Zinc catena-polyphosphate

Crystal data

Zn(PO3)2	F(000) = 864
Mr = 223.31	Dx = 3.129 Mg m^−3
Monoclinic, Cc	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: C -2yc	Cell parameters from 25 reflections
a = 7.6353 (8) Å	θ = 12.0–16.6°
b = 7.6077 (8) Å	µ = 5.80 mm^−1
c = 16.335 (2) Å	T = 293 K
β = 92.190 (9)°	Fragment, colourless
V = 948.17 (19) Å^3	0.20 × 0.16 × 0.14 mm
Z = 8

Data collection

Enraf–Nonius CAD-4 diffractometer	3242 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.025
Graphite monochromator	θmax = 32.5°, θmin = 2.5°
ω/2θ scans	h = −11→11
Absorption correction: numerical The crystal shape was optimized by minimizing the R-value of 10 selected ψ-scanned reflections using the program HABITUS (Herrendorf, 1993-1997). The habit so derived was used for the numerical absorption correction.	k = −11→11
Tmin = 0.454, Tmax = 0.538	l = −24→24
6620 measured reflections	3 standard reflections every 300 min
3424 independent reflections	intensity decay: none

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	w = 1/[σ^2(Fo^2) + (0.0214P)^2] where P = (Fo^2 + 2Fc^2)/3
R[F^2 > 2σ(F^2)] = 0.020	(Δ/σ)max = 0.002
wR(F^2) = 0.043	Δρmax = 0.45 e Å^−3
S = 1.03	Δρmin = −0.48 e Å^−3
3424 reflections	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
164 parameters	Extinction coefficient: 0.00160 (13)
2 restraints	Absolute structure: Flack & Bernardinelli (1999), 1714 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: −0.001 (6)

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
Zn1	0.54201 (3)	0.59896 (3)	0.402327 (16)	0.01320 (6)
Zn2	0.63572 (3)	0.30933 (4)	0.119393 (16)	0.01399 (6)
P1	0.22087 (7)	0.21549 (7)	0.14693 (3)	0.01053 (9)
P2	0.23113 (7)	0.52016 (7)	0.25942 (3)	0.01064 (9)
P3	−0.04783 (7)	0.51759 (7)	0.37625 (3)	0.01099 (10)
P4	−0.03902 (7)	0.76957 (7)	0.51140 (3)	0.01011 (9)
O1	−0.0001 (2)	0.9594 (2)	0.50005 (10)	0.0168 (3)
O2	0.0132 (3)	0.3472 (2)	0.40846 (13)	0.0281 (4)
O3	0.0805 (2)	0.3043 (2)	0.08498 (11)	0.0201 (3)
O4	0.1673 (3)	0.0356 (2)	0.16829 (11)	0.0184 (3)
O5	0.1938 (3)	0.3280 (2)	0.22632 (11)	0.0219 (4)
O6	0.2611 (2)	0.6424 (2)	0.19131 (11)	0.0208 (3)
O7	−0.2358 (2)	0.5431 (3)	0.35899 (11)	0.0223 (4)
O8	−0.2220 (2)	0.7130 (2)	0.52380 (10)	0.0167 (3)
O9	0.3634 (2)	0.5038 (3)	0.32699 (11)	0.0259 (4)
O10	0.3945 (2)	0.2413 (3)	0.11371 (13)	0.0278 (4)
O11	0.0322 (2)	0.6663 (2)	0.43542 (10)	0.0184 (3)
O12	0.0455 (2)	0.5675 (3)	0.29458 (10)	0.0182 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.01138 (10)	0.01338 (11)	0.01484 (11)	0.00002 (9)	0.00051 (8)	0.00254 (10)
Zn2	0.01241 (11)	0.01345 (11)	0.01644 (12)	−0.00139 (9)	0.00473 (9)	−0.00234 (9)
P1	0.0108 (2)	0.0103 (2)	0.0104 (2)	0.00002 (19)	−0.00077 (17)	−0.00152 (18)
P2	0.0104 (2)	0.0115 (2)	0.0100 (2)	−0.00009 (19)	0.00035 (17)	−0.00128 (18)
P3	0.0105 (2)	0.0119 (2)	0.0106 (2)	0.00111 (19)	0.00066 (17)	−0.00079 (19)
P4	0.0095 (2)	0.0112 (2)	0.0096 (2)	−0.00171 (18)	0.00031 (16)	−0.00040 (18)
O1	0.0235 (8)	0.0124 (7)	0.0144 (7)	−0.0049 (6)	0.0001 (6)	0.0016 (6)
O2	0.0384 (12)	0.0148 (8)	0.0309 (10)	0.0044 (8)	0.0009 (9)	0.0044 (7)
O3	0.0242 (8)	0.0139 (8)	0.0212 (8)	0.0008 (7)	−0.0107 (7)	−0.0026 (6)
O4	0.0286 (9)	0.0106 (7)	0.0158 (7)	−0.0014 (7)	−0.0009 (6)	−0.0007 (6)
O5	0.0388 (11)	0.0145 (8)	0.0125 (8)	−0.0028 (8)	0.0020 (7)	−0.0051 (6)
O6	0.0207 (8)	0.0190 (8)	0.0233 (8)	0.0026 (7)	0.0071 (7)	0.0074 (7)
O7	0.0107 (7)	0.0378 (11)	0.0183 (8)	0.0003 (7)	0.0003 (6)	−0.0052 (7)
O8	0.0114 (6)	0.0257 (9)	0.0133 (7)	−0.0048 (6)	0.0019 (5)	−0.0016 (6)
O9	0.0179 (8)	0.0376 (12)	0.0215 (8)	−0.0037 (8)	−0.0093 (7)	−0.0008 (8)
O10	0.0114 (8)	0.0380 (11)	0.0343 (10)	−0.0055 (7)	0.0052 (7)	−0.0031 (9)
O11	0.0139 (7)	0.0220 (8)	0.0198 (8)	−0.0038 (6)	0.0049 (6)	−0.0108 (6)
O12	0.0146 (7)	0.0257 (9)	0.0145 (7)	0.0057 (7)	0.0048 (6)	0.0026 (7)

Geometric parameters (Å, º)

Zn1—O2^i	1.9044 (19)	P3—O7	1.4648 (18)
Zn1—O7^ii	1.9110 (18)	P3—O2	1.468 (2)
Zn1—O9	1.9418 (18)	P3—O12	1.5829 (17)
Zn1—O1^iii	1.9541 (17)	P3—O11	1.5946 (17)
Zn2—O4^i	1.9088 (17)	P4—O8	1.4834 (17)
Zn2—O10	1.9120 (18)	P4—O1	1.4872 (17)
Zn2—O8^iv	1.9433 (16)	P4—O11	1.5827 (17)
Zn2—O6^iii	1.9553 (18)	P4—O3^v	1.5842 (17)
P1—O10	1.4648 (18)	O1—Zn1^vi	1.9541 (17)
P1—O4	1.4743 (18)	O2—Zn1^vii	1.9044 (19)
P1—O5	1.5741 (18)	O3—P4^viii	1.5842 (17)
P1—O3	1.5954 (18)	O4—Zn2^vii	1.9088 (17)
P2—O9	1.4725 (18)	O6—Zn2^vi	1.9553 (18)
P2—O6	1.4745 (18)	O7—Zn1^ix	1.9110 (18)
P2—O5	1.5807 (19)	O8—Zn2^x	1.9433 (16)
P2—O12	1.5906 (18)
O2^i—Zn1—O7^ii	110.24 (10)	O7—P3—O2	118.76 (13)
O2^i—Zn1—O9	108.86 (10)	O7—P3—O12	105.99 (11)
O7^ii—Zn1—O9	107.12 (8)	O2—P3—O12	111.54 (11)
O2^i—Zn1—O1^iii	118.23 (8)	O7—P3—O11	111.88 (10)
O7^ii—Zn1—O1^iii	110.96 (8)	O2—P3—O11	107.47 (12)
O9—Zn1—O1^iii	100.47 (8)	O12—P3—O11	99.55 (10)
O4^i—Zn2—O10	111.74 (9)	O8—P4—O1	119.53 (11)
O4^i—Zn2—O8^iv	110.45 (8)	O8—P4—O11	108.47 (10)
O10—Zn2—O8^iv	120.28 (8)	O1—P4—O11	107.93 (10)
O4^i—Zn2—O6^iii	106.45 (8)	O8—P4—O3^v	108.17 (10)
O10—Zn2—O6^iii	107.57 (9)	O1—P4—O3^v	109.06 (10)
O8^iv—Zn2—O6^iii	98.65 (7)	O11—P4—O3^v	102.33 (10)
O10—P1—O4	118.38 (13)	P4—O1—Zn1^vi	132.01 (11)
O10—P1—O5	112.43 (12)	P3—O2—Zn1^vii	153.18 (15)
O4—P1—O5	105.29 (10)	P4^viii—O3—P1	133.22 (12)
O10—P1—O3	107.68 (12)	P1—O4—Zn2^vii	140.35 (12)
O4—P1—O3	110.94 (10)	P1—O5—P2	139.21 (13)
O5—P1—O3	100.74 (10)	P2—O6—Zn2^vi	140.95 (12)
O9—P2—O6	119.83 (12)	P3—O7—Zn1^ix	146.65 (12)
O9—P2—O5	106.72 (12)	P4—O8—Zn2^x	133.90 (10)
O6—P2—O5	110.87 (10)	P2—O9—Zn1	152.67 (15)
O9—P2—O12	110.08 (11)	P1—O10—Zn2	154.43 (14)
O6—P2—O12	107.18 (10)	P4—O11—P3	133.80 (11)
O5—P2—O12	100.46 (11)	P3—O12—P2	133.32 (12)

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