The new lead vanadylphosphate Pb₂VO(PO₄)₂

The structure of dilead vanadium oxide bis­(phosphate) contains corrugated layers formed by VO₅ square pyramids oriented in opposite directions in a chessboard fashion. The pyramids are connected by tetra­hedral PO₄ groups. The layers are separated by the Pb atoms and isolated PO₄ tetra­hedra.

S1. Comment

Reduced vanadates and vanadylphosphates (with V atoms in oxidation state less than +5), often have different low-dimensional units such as chains, ribbons or layers in their structures. Such compounds attract the particular attention of researchers since they exhibit a variety of specific magnetic properties induced by highly correlated d-electrons (Ueda, 1998). Compounds with the general formula A₂VO(PO₄)₂ (where A = Sr and Ba) contain chains of V⁺⁴O₆ octahedra or V⁺⁴O₅ square pyramids in their structures (Wadewitz & Mueller-Buschbaum, 1996a,b). The chains may be either isolated or linked in layers. Quite often these compounds are isostructural with the respective vanadylvanadates with the same stoichiometry. Recently, Mentre et al. (1999) have reported the synthesis of the ternary oxide Pb₂V₃O₉ with a structure similar to that of Sr₂VO(PO₄)₂. The former structure is more distorted owing to the presence of the Pb²⁺ lone pair. Although structures of A₂VO(PO₄)₂ (A = Sr and Ba) vanadylphosphates were described some time ago, the corresponding lead compound has not yet been reported. Recently, we synthesized Pb₂VO(PO₄)₂ and found that it exhibits interesting and quite unusual magnetic properties (Kaul et al., 2004).

Three projections of the Pb₂VO(PO₄)₂ structure are shown in Fig.1. It contains corrugated layers formed by VO₅ square pyramids oriented in opposite directions in chess fashion. The pyramids are connected by tetrahedral PO₄ groups. The waves in the layers propagate along the b axis. The square bases of the equally oriented pyramids are located approximately at the same level. The [VOPO₄] layers are separated by Pb atoms and isolated PO₄ tetrahedra. The V atom is coordinated by five O atoms forming a distorted square pyramid (Fig. 2c). A short V1—O9 distance [1.558 (13) Å] indicates the existence of a vanadyl bond. The basal distances are in the range 1.975 (14) to 2.02 (14) Å. The V atom polyhedron may also be considered as a distorted octahedron with a long V1—O2 distance of 2.484 (17) Å. Although Schindler et al. (2000) suggested considering this bond length as trans to the vanadyl bond in a square-bipyramidal coordination, a pyramidal representation in our case is more reasonable. This is supported by the τ-criteria calculation (Addison et al., 1984), resulting in a value of τ = 0.06 corresponding to a square-pyramidal coordination. Additionally, the O9—V1—O2 angle is 170.3 (7)°, while in a square–bipyramidal or octahedral coordination this angle is closer to 180°.

The coordination polyhedron of atom Pb1 (Fig. 2a) has a flat base formed by five O atoms, and two additional atoms, O3ⁱ and O8ⁱⁱⁱ atoms, are located above the base. The Pb1—O3ⁱ and Pb1—O8ⁱⁱⁱ distances are noticeably shorter [2.468 (15) and 2.335 (17) Å, respectively; see Table 1 for symmetry codes] than those for the basal atoms. This arrangement appears to be due to the presence of a lone pair, which should be located under the pentagonal basis opposite to atoms O3ⁱ and O8ⁱⁱⁱ. The Pb2 coordination polyhedron looks more complicated (Fig. 2b). Nevertheless, one can note that in this case the lone pair should be situated oppositely to atom O2.

Sr₂VO(PO₄)₂ is isotypical with Sr₂VO(VO₄)₂ and Pb₂VO(VO₄)₂ vanadates in spite of distortions induced by the lone pair of the Pb atom. One may expect that the same or a similar structural motif would be realised in the structure of Pb₂VO(PO₄)₂. However, the Pb₂VO(PO₄)₂ structure is completely different. This may be explained by the presence of the Pb²⁺ lone pair, leading to a strongly asymmetric coordination of these cations, together with the reduced size of the tetrahedral groups (the average distances are 1.55 Å in PO₄ versus. 1.65 Å in VO₄). The fact that mixed vanadylphosphates AMVO(PO₄)₂ (A = Sr and Ba; M = Zn and Cd) (Mueller-Buschbaum & Meyer, 1997; Mueller-Buschbaum et al., 1997) have crystal structures very similar to that of Pb₂VO(PO₄)₂ allows us to make some conclusions about the role of the Pb²⁺ lone pair and thus explain the differences between Pb₂VO(PO₄)₂ and Pb₂VO(VO₄)₂. A noticeable difference in the A and M ionic radii {Zn²⁺[coordination number (CN) 4] = 0.74 Å, Sr²⁺(CN 8) = 1.39 Å and Ba²⁺(CN 8) = 1.56 Å} results in different coordination arrangements for these two cations. Thus, alkali-earth atoms are coordinated by eight O atoms and Zn atoms are tetrahedrally coordinated, while in the title structure both Pb atoms [the ionic radius of Pb²⁺(CN 8) is 1.45 Å] are surrounded by seven O atoms. The V atoms in AMVO(PO₄)₂ structures are in octahedral coordinations with much shorter trans bonds, viz. 2.188 and 2.391 Å for the Sr and Ba phases, respectively. Note that even in the case of BaZnVO(PO₄)₂ the length of the trans bond (2.43 Å) is shorter than that in Pb₂VO(PO₄)₂ [2.484 (17) Å]. The larger the difference of the ionic radius of the A cation, the larger is the average displacement of the V atom from the equatorial plane of the square pyramid, viz. 0.12 and 0.25 Å for Sr- and Ba-containing structures, respectively. However, it it still less than that in the Pb-containing structure (0.35 Å). Additionally, in both Zn-containing vanadylphosphates, the O_vanadyl—V—O_trans angle is 175.7° in contrast to the value of 170.3 (7)° found in the studied structure. From this consideration, one may conclude that the effect of the lone pair in the Pb₂VO(PO₄)₂ structure is somewhat similar to the result of ordering of two A cations with markedly different atomic radii.

The most likely reason for corrugation of the layers is the presence of the extra tetrahedra between them (Fig. 3). Thus, in both Pb₂VO(PO₄)₂ and BaZnVO(PO₄)₂ (Figs. 3a and 3b, respectively) extra PO₄ tetrahedra are located between the [VOPO₄] layers. In the Li₂VOSiO₄ structure (Millet & Satto, 1998), adjacent [VOSiO₄] layers with a similar type of polyhedral linkage are flat, since they are separated only by Li⁺ cations (Fig. 3c). The same type of [(VO)₂(PO₄)₂] layer was found in Pb(VO)₂(PO₄)₂·4H₂O (Kang et al., 1992) (Fig. 3d), in which the layers are separated by Pb atoms and water molecules. In contrast to the [VOPO₄] layers, in the title compound the [VO(PO₄)] (X = P andSi) layers in Li₂VOSiO₄ and Pb(VO)₂(PO₄)₂·4H₂O are almost flat.

S2. Experimental

Green plate-like crystals of Pb₂VO(PO₄)₂ for the single-crystal experiment were obtained by slow cooling of a melted stoichiometric mixture of Pb₂P₂O₇ and VO₂ from 1073 K to 973 K in dynamic vacuum followed by furnace cooling.

S3. Refinement

Our preliminary studies indicated the presence of superstructure reflections for some crystals. Therefore, the crystals under investigation were carefully checked for the presence of superstructure, but no additional superstructure reflections were found. Nevertheless, a strong positive maximum exists in the difference Fourier map near atom Pb1 (0.66 Å). This maximum, as well as the relatively high displacement parameters for O atoms, may indicate that some domains in the crystal have modulated structure. Additional electron microscopy and X-ray structural studies are needed to clarify the type and the origin of the superstructure.

Computing details

Program(s) used to solve structure: CSD (Akselrud et al., 1993); program(s) used to refine structure: (JANA2000; Petricek and Dusek, 2000); software used to prepare material for publication: (JANA2000; Petricek and Dusek, 2000).

Figures

	Fig. 1. The crystal structure of Pb2VO(PO4)2.
	Fig. 2. Coordination polyhedra for (a) Pb1, (b) Pb2 and (c) V1 atoms.
	Fig. 3. The [VOXO4] layers (X = P and Si) formed by VO5 square pyramids and tetrahedral XO4 groups in (a) Pb2VO(PO4)2, (b) BaZnVO(PO4)2, (c) LiVOSiO4 and (d) Pb(VO)2(PO4)2·4H2O. (In the latter structure, large circles represent Pb atoms, medium circles O atoms and small circles H atoms.)

dilead vanadium oxide bis(phosphate)

Crystal data

Pb2VO(PO4)2	F(000) = 1156
Mr = 671.3	Dx = 5.837 Mg m^−3
Monoclinic, P21/a	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -P 2yab	Cell parameters from 23 reflections
a = 8.747 (4) Å	θ = 17.0–20.7°
b = 9.016 (5) Å	µ = 45.63 mm^−1
c = 9.863 (9) Å	T = 293 K
β = 100.96 (4)°	Plate, green
V = 763.6 (9) Å^3	0.24 × 0.18 × 0.08 mm
Z = 4

Data collection

CAD-4 diffractometer	Rint = 0.062
θ scans	θmax = 34.1°, θmin = 2.1°
Absorption correction: gaussian (CAD-4 User's Manual; Enraf–Nonius, 1988)	h = 0→13
Tmin = 0.004, Tmax = 0.024	k = −14→14
4932 measured reflections	l = −15→15
2703 independent reflections	2 standard reflections every 0 reflections
2020 reflections with I > 3σ(I)	intensity decay: 7.9%

Refinement

Refinement on F	Weighting scheme based on measured s.u.'s w = 1/[σ^2(F) + 0.000625F^2]
R[F^2 > 2σ(F^2)] = 0.032	(Δ/σ)max = 0.009
wR(F^2) = 0.045	Δρmax = 5.11 e Å^−3
S = 1.62	Δρmin = −1.35 e Å^−3
2020 reflections	Extinction correction: B-C type 1 Lorentzian isotropic (Becker & Coppens, 1974)
128 parameters	Extinction coefficient: 0.18 (2)

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

	x	y	z	Uiso*/Ueq
Pb1	0.48092 (8)	0.19226 (8)	0.35765 (6)	0.0342 (2)
Pb2	0.89553 (8)	0.14650 (8)	0.71406 (7)	0.0337 (2)
V1	0.2558 (3)	0.4989 (3)	0.0623 (3)	0.0282 (6)
P1	0.2682 (5)	0.5482 (5)	0.4276 (5)	0.0304 (10)
P2	0.0052 (5)	0.2494 (5)	0.0337 (4)	0.0267 (9)
O1	−0.0752 (17)	0.1486 (14)	0.1264 (13)	0.033 (3)
O2	0.3381 (18)	0.590 (2)	0.3030 (17)	0.045 (5)
O3	0.2541 (17)	0.3813 (15)	0.443 (2)	0.045 (5)
O4	0.1018 (16)	0.6158 (16)	0.4057 (15)	0.037 (4)
O5	0.1128 (15)	0.3487 (15)	0.1263 (14)	0.033 (3)
O6	−0.1089 (15)	0.3369 (16)	−0.0721 (11)	0.032 (3)
O7	0.0825 (17)	0.1485 (17)	−0.0625 (12)	0.035 (3)
O8	0.373 (2)	0.5999 (19)	0.557 (2)	0.053 (6)
O9	0.2205 (17)	0.4648 (17)	−0.0954 (13)	0.037 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Pb1	0.0394 (4)	0.0344 (4)	0.0291 (3)	−0.0031 (2)	0.0070 (2)	0.0006 (2)
Pb2	0.0356 (3)	0.0355 (4)	0.0306 (3)	−0.0008 (2)	0.0077 (2)	0.0002 (2)
V1	0.0297 (10)	0.0273 (11)	0.0287 (10)	0.0002 (9)	0.0082 (9)	0.0007 (8)
P1	0.0314 (18)	0.0284 (18)	0.0320 (16)	0.0000 (15)	0.0074 (14)	0.0022 (14)
P2	0.0304 (16)	0.0240 (17)	0.0268 (13)	0.0010 (14)	0.0080 (13)	−0.0011 (12)
O1	0.041 (6)	0.025 (5)	0.034 (5)	−0.002 (5)	0.013 (5)	0.003 (4)
O2	0.037 (6)	0.047 (8)	0.053 (8)	0.003 (6)	0.017 (6)	0.020 (7)
O3	0.033 (6)	0.022 (5)	0.079 (11)	−0.004 (5)	0.005 (7)	−0.002 (6)
O4	0.034 (6)	0.036 (6)	0.043 (6)	0.001 (5)	0.015 (5)	−0.006 (5)
O5	0.032 (5)	0.029 (5)	0.041 (6)	−0.008 (4)	0.014 (5)	−0.011 (5)
O6	0.035 (5)	0.041 (7)	0.021 (4)	0.004 (5)	0.007 (4)	−0.001 (4)
O7	0.036 (6)	0.045 (7)	0.024 (4)	0.004 (5)	0.003 (4)	−0.004 (5)
O8	0.045 (8)	0.034 (7)	0.079 (12)	0.000 (7)	0.012 (8)	0.008 (8)
O9	0.042 (6)	0.043 (7)	0.026 (4)	−0.001 (6)	0.009 (5)	−0.009 (5)

Geometric parameters (Å, º)

Pb1—O1^i	2.660 (13)	V1—O1^i	1.999 (14)
Pb1—O3	2.860 (16)	V1—O2	2.484 (17)
Pb1—O3^i	2.468 (15)	V1—O5	2.024 (14)
Pb1—O4^ii	2.663 (16)	V1—O6^vii	1.975 (14)
Pb1—O4^i	2.978 (14)	V1—O7^viii	1.954 (15)
Pb1—O5^i	2.771 (15)	V1—O9	1.558 (13)
Pb1—O8^iii	2.335 (17)	P1—O2	1.519 (19)
Pb2—O2^iv	2.423 (16)	P1—O3	1.519 (15)
Pb2—O3^i	2.737 (18)	P1—O4	1.554 (15)
Pb2—O4^iii	2.449 (15)	P1—O8	1.50 (2)
Pb2—O6^v	2.725 (13)	P2—O1	1.550 (15)
Pb2—O7^v	2.482 (12)	P2—O5	1.482 (13)
Pb2—O8^i	2.693 (19)	P2—O6	1.520 (13)
Pb2—O9^vi	2.825 (15)	P2—O7	1.558 (15)
O1^i—V1—O2	81.6 (5)	O7^viii—V1—O9	98.3 (7)
O1^i—V1—O5	85.4 (6)	O2—P1—O3	112.0 (11)
O1^i—V1—O6^vii	158.8 (5)	O2—P1—O4	107.5 (9)
O1^i—V1—O7^viii	88.1 (6)	O2—P1—O8	109.9 (10)
O1^i—V1—O9	100.3 (7)	O3—P1—O4	108.1 (8)
O2—V1—O5	90.3 (6)	O3—P1—O8	105.8 (10)
O2—V1—O6^vii	77.3 (5)	O4—P1—O8	113.6 (9)
O2—V1—O7^viii	72.2 (5)	O1—P2—O5	107.3 (8)
O2—V1—O9	170.3 (7)	O1—P2—O6	113.4 (8)
O5—V1—O6^vii	92.3 (6)	O1—P2—O7	108.4 (8)
O5—V1—O7^viii	162.1 (5)	O5—P2—O6	111.5 (8)
O5—V1—O9	99.3 (7)	O5—P2—O7	115.4 (8)
O6^vii—V1—O7^viii	87.7 (6)	O6—P2—O7	100.9 (7)
O6^vii—V1—O9	100.9 (6)

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