TlPO₃ and its comparison with isotypic RbPO₃ and CsPO₃

The crystal structure of thallium(I) catena-polyphosphate, TlPO₃, contains a polyphosphate chain extending parallel to [010] with a repeating unit of two phosphate tetra­hedra. The Tl^I atom is located in-between the polyphosphate chains and is bonded by oxygen atoms in a distorted [6 + 1] coordination in the form of a monocapped prism, with the longest Tl—O bond to the bridging O atom of the polyphosphate chain. A qu­anti­tative structural comparison with isotypic RbPO₃ and CsPO₃ reveals that the usually pronounced stereoactivity of the 6s² lone pair at the Tl^I atom is not apparent in the case of TlPO₃.

1. Chemical context

The crystal chemistry of inorganic thallium(I) (or thallous) compounds is dominated by the presence of the 6s² electron lone pair that, in the majority of cases, is stereochemically active (Galy et al., 1975). Therefore, crystal structures comprising a Tl^I atom mostly have unique structures, and isotypism with analogous phases where the Tl^I site is replaced by a metal cation of comparable size or by ammonium is comparatively rare. One of these cases pertains to the catena-polyphosphate series MPO₃ (M = Tl, Rb, Cs) for which isotypism of TlPO₃ with the alkali polyphosphates was reported on basis of indexed powder X-ray diffraction data (El Horr, 1991). Although single crystals were available, a refinement of the crystal structure was not performed at that time.

With the intention of obtaining detailed structure data for TlPO₃ for a qu­anti­tative structural comparison with isotypic RbPO₃ and CsPO₃, single crystals of the thallium polyphos­phate phase were grown and the crystal structure refined using single-crystal X-ray data.

2. Structural commentary

The asymmetric unit of TlPO₃ comprises one Tl, one P and three O sites, all on general positions. The crystal structure of TlPO₃ is made up from a polyphosphate chain with a repeating unit of two phosphate tetra­hedra propagating along the [010] direction. Two polyphosphate chains with different orientations cross the unit cell (Fig. 1). The bond-length distribution (Table 1) within a PO₄ tetra­hedron is characteristic of a polyphosphate chain (Durif, 1995), i.e. two long bonds to the bridging atoms O1 and O1^vii [mean 1.600 (8) Å; for symmetry code see Table 1] and two short bonds to the terminal O2 and O3 atoms [mean 1.483 (19) Å] are observed. The Tl^I atoms are situated between the chains and are coordinated to seven oxygen atoms. As has been done for thallium(I) oxoarsenates (Schroffenegger et al., 2020), it is useful to classify the corresponding Tl^I—O bonds into `short' bonds less than 3.0 Å, and `long' bonds greater than this threshold up to the maximum bond length of 3.50 Å for the first coordination sphere. The resulting [6 + 1] polyhedron can be derived from a monocapped trigonal prism where the capping O atom is that with the longest Tl—O bond (Fig. 2). This atom (O1) represents the bridging oxygen atom of the polyphosphate chain. Next to the Tl and two P atoms, atom O1 has no further coordination partners. The terminal O2 and O3 atoms of the polyphosphate chain each are bonded to one P and to three Tl atoms in the form of a distorted tetra­hedron.

Table 1
Comparison of bond lengths in the isotypic MPO₃ polyphosphates (M = Tl, Rb, Cs)

(a) This work; (b) a = 12.123 (2), b = 4.228 (2), c = 6.479 (2) Å, β = 96.33 (33)° (Corbridge, 1956); (c) a = 12.6162 (11), b = 4.2932 (4), c = 6.7575 (6) Å, β = 96.068 (5)° (Weil & Stöger, 2020).

	M = Tl^a	M = Rb^b	M = Cs^c
M—O3^i	2.867 (5)	2.920	3.1097 (18)
M—O3	2.889 (4)	2.971	3.0980 (14)
M—O3^ii	2.935 (4)	2.948	3.0983 (14)
M—O2^iii	2.943 (5)	2.973	3.0981 (14)
M—O2^iv	2.963 (4)	2.905	3.0431 (15)
M—O2^v	2.997 (4)	3.024	3.1455 (15)
M—O1^vi	3.216 (4)	3.196	3.3649 (14)
P—O2	1.469 (5)	1.474	1.4793 (16)
P—O3	1.497(5	1.438	1.4919 (16)
P—O1	1.595 (4)	1.621	1.6134 (16)
P—O1^vii	1.606 (4)	1.624	1.6183 (16)
Symmetry codes (i) −x + , y − , −z + ; (ii) x, y − 1, z; (iii) x, y, z + 1; (iv) −x, −y, −z + 1; (v) −x, −y + 1, −z + 1; (vi) x, y − 1, z + 1; (vii) −x + , y − , −z + .

Figure 1 The crystal structure of TlPO3 in a projection along [00]. Displacement ellipsoids are drawn at the 75% probability level. The symmetry code refers to Table 1.

Figure 2 The [TlO6 + 1] monocapped prism in the crystal structure of TlPO3, with the long Tl—O bond shown in yellow. Displacement ellipsoids are drawn at the 75% probability level. Symmetry codes refer to Table 1.

Bond-valence-sum (BVS) calculations (Brown, 2002) for TlPO₃ were carried out with the values provided by Locock & Burns (2009) for Tl^I—O bonds, and by Brown & Altermatt (1985) for P—O bonds. The obtained BVS values (in valence units) of 0.88 for Tl1, 4.97 for P1, 2.17 for O1, 1.97 for O2 and 1.82 for O3 differ somewhat from the idealized values for atoms with a formal charge of +1, +5 and −2, respectively. In consequence, the global instability index GII (Salinas-Sanchez et al., 1992) of 0.14 valence units is rather high and indicates a stable but strained structure (GII values < 0.1 valence units are typical for unstrained structures, GII values between 0.1 and 0.2 are characteristic of structures with lattice-induced strain, and GII values > 0.2 indicate unstable structures).

Apparently, the usually observed stereochemical activity of the 6s² electron lone pair at the Tl^I atom is not very pronounced in the case of TlPO₃, and a crystal structure isotypic with those of the room-temperature forms of RbPO₃ and CsPO₃ is realized (Table 1). This may be due to the comparable ionic radii for monovalent Tl, Rb⁺ and Cs⁺ cations of 1.50, 1.52 and 1.67 Å, respectively, using a coordination number of six (Shannon, 1976; values for a coordination number of seven were not listed for Tl and Cs).

Whereas the isotypic polyphosphates RbPO₃ and CsPO₃ show structural phase transitions to two (Holst et al., 1994) and to one high-temperature phases (Chudinova et al., 1989), a structural phase transition at higher temperatures has not been reported for TlPO₃. On the contrary, the tetra­metaphosphate Tl₄P₄O₁₂ converts at 690 K to the title polyphosphate (Dostál et al., 1969) that therefore represents the high-temperature form of a phosphate with a Tl:P ratio of 1:1.

For a qu­anti­tative structural comparison of the three isotypic MPO₃ (M = Tl, Rb, Cs) structures, the program compstru (de la Flor et al., 2016) available at the Bilbao Crystallographic Server (Aroyo et al., 2006) was used. Numerical details of the comparison with the TlPO₃ structure as the reference are collated in Table 2. The low values for the degree of lattice distortion (S), the similarity index Δ and the arithmetic mean distance of paired atoms (d_av) indicate very similar structures, with the highest absolute displacement of atom O3 in each case.

Table 2
Absolute atomic displacements (Å) of isotypic RbPO₃ and CsPO₃ relative to TlPO₃, as well as lattice distortion (S), arithmetic mean distance d_av (Å) and measure of similarity (Δ)

	Rb	Cs
M1	0.0656	0.0703
P1	0.0505	0.0595
O1	0.0421	0.0494
O2	0.0996	0.0735
O3	0.1332	0.1793
S	0.0099	0.0245
dav	0.0782	0.0864
Δ	0.060	0.039

3. Synthesis and crystallization

A mass of 0.50 g Tl₂CO₃ was immersed in 3 ml of concentrated phospho­ric acid (85%_wt) in a glass carbon crucible. The mixture was heated within six hours to 573 K, kept at that temperature for ten hours and slowly cooled to room temperature over the course of twelve hours. The obtained highly viscous phosphate flux was leached with a mixture of water and methanol (v/v = 1:4). After separation of the liquid phase through suction filtration, colourless crystals of TlPO₃, mostly with a platy form, were obtained.

4. Refinement

Crystal data, data collection and structure refinement details are summarized in Table 3.

Table 3
Experimental details

Crystal data
Chemical formula	TlPO3
Mr	283.34
Crystal system, space group	Monoclinic, P21/n
Temperature (K)	296
a, b, c (Å)	12.2315 (12), 4.2432 (7), 6.3039 (1)
β (°)	96.727 (7)
V (Å^3)	324.92 (6)
Z	4
Radiation type	Mo Kα
μ (mm^−1)	49.99
Crystal size (mm)	0.09 × 0.08 × 0.01
Data collection
Diffractometer	Bruker APEXII CCD
Absorption correction	Numerical (HABITUS; Herrendorf, 1997)
Tmin, Tmax	0.110, 0.536
No. of measured, independent and observed [I > 2σ(I)] reflections	6570, 1189, 912
Rint	0.059
(sin θ/λ)max (Å^−1)	0.758
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.027, 0.054, 0.99
No. of reflections	1189
No. of parameters	46
Δρmax, Δρmin (e Å^−3)	1.85, −1.64
Structure solution: isomorphous replacement. Computer programs: APEX3 and SAINT (Bruker, 2018), SHELXL2018/3 (Sheldrick, 2015), ATOMS (Dowty, 2006) and publCIF (Westrip, 2010).

The lattice parameters determined in the current study are in good agreement with those of the previous report [a = 12.270 (7), b = 4.263 (2), c = 6.328 (4) Å, β = 96.72 (3)°, V = 328.7 ų; El Horr, 1991], however with higher precision.

For better comparison with the two isotypic structures of RbPO₃ and CsPO₃, the setting of the unit cell (cell choice 2 of space group No. 14), starting coordinates and atom labelling were adapted from RbPO₃ (Corbridge, 1956). The maximum and minimum electron densities in the final difference-Fourier synthesis are located 0.65 and 0.74 Å, respectively, from the Tl1 site.

Supporting information

Computing details

Data collection: APEX3 (Bruker, 2018); cell refinement: SAINT (Bruker, 2018); data reduction: SAINT (Bruker, 2018); program(s) used to solve structure: isomorphous replacement; program(s) used to refine structure: SHELXL2018/3 (Sheldrick, 2015); molecular graphics: ATOMS (Dowty, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Thallium(I) catena-polyphosphate

Crystal data

TlPO3	F(000) = 480
Mr = 283.34	Dx = 5.792 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 12.2315 (12) Å	Cell parameters from 2009 reflections
b = 4.2432 (7) Å	θ = 3.4–29.2°
c = 6.3039 (1) Å	µ = 49.99 mm^−1
β = 96.727 (7)°	T = 296 K
V = 324.92 (6) Å^3	Plate, colourless
Z = 4	0.09 × 0.08 × 0.01 mm

Data collection

Bruker APEXII CCD diffractometer	912 reflections with I > 2σ(I)
Radiation source: fine-sealed tube	Rint = 0.059
ω– and φ–scans	θmax = 32.6°, θmin = 3.4°
Absorption correction: numerical (HABITUS; Herrendorf, 1997)	h = −18→18
Tmin = 0.110, Tmax = 0.536	k = −6→6
6570 measured reflections	l = −9→9
1189 independent reflections

Refinement

Refinement on F^2	46 parameters
Least-squares matrix: full	0 restraints
R[F^2 > 2σ(F^2)] = 0.027	w = 1/[σ^2(Fo^2) + (0.0219P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.054	(Δ/σ)max < 0.001
S = 0.99	Δρmax = 1.85 e Å^−3
1189 reflections	Δρmin = −1.64 e Å^−3

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
Tl1	0.08906 (2)	0.14776 (5)	0.77279 (4)	0.02917 (9)
P1	0.18222 (13)	0.4944 (3)	0.3224 (2)	0.0183 (3)
O1	0.2184 (4)	0.7447 (9)	0.1551 (6)	0.0202 (9)
O2	0.0814 (4)	0.3466 (9)	0.2180 (8)	0.0331 (11)
O3	0.1858 (4)	0.6404 (9)	0.5393 (7)	0.0316 (11)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Tl1	0.02974 (15)	0.02897 (13)	0.02893 (13)	−0.00135 (13)	0.00402 (10)	0.00170 (11)
P1	0.0196 (8)	0.0134 (6)	0.0227 (7)	−0.0015 (6)	0.0060 (6)	−0.0018 (5)
O1	0.024 (2)	0.0119 (15)	0.025 (2)	−0.0029 (16)	0.0039 (18)	0.0000 (15)
O2	0.021 (2)	0.026 (2)	0.052 (3)	−0.004 (2)	0.002 (2)	−0.005 (2)
O3	0.046 (3)	0.025 (2)	0.026 (2)	0.001 (2)	0.011 (2)	−0.0035 (18)

Geometric parameters (Å, º)

Tl1—O3^i	2.867 (5)	Tl1—O1^vi	3.216 (4)
Tl1—O3	2.889 (4)	P1—O2	1.469 (5)
Tl1—O3^ii	2.935 (4)	P1—O3	1.497 (5)
Tl1—O2^iii	2.943 (5)	P1—O1	1.595 (4)
Tl1—O2^iv	2.963 (4)	P1—O1^vii	1.606 (4)
Tl1—O2^v	2.997 (4)
O3^i—Tl1—O3	77.73 (13)	Tl1^viii—O1—Tl1^ix	75.05 (8)
O3^i—Tl1—O3^ii	77.00 (13)	P1—O1—Tl1^x	129.7 (2)
O3—Tl1—O3^ii	93.54 (13)	P1^x—O1—Tl1^x	66.97 (15)
O3^i—Tl1—O2^iii	75.25 (13)	Tl1^viii—O1—Tl1^x	79.47 (8)
O3—Tl1—O2^iii	109.83 (12)	Tl1^ix—O1—Tl1^x	73.65 (7)
O3^ii—Tl1—O2^iii	138.47 (13)	P1—O1—Tl1^v	69.65 (16)
O3^i—Tl1—O2^iv	128.90 (12)	P1^x—O1—Tl1^v	130.4 (2)
O3—Tl1—O2^iv	148.59 (14)	Tl1^viii—O1—Tl1^v	68.02 (8)
O3^ii—Tl1—O2^iv	79.42 (13)	Tl1^ix—O1—Tl1^v	79.10 (8)
O2^iii—Tl1—O2^iv	94.78 (12)	Tl1^x—O1—Tl1^v	141.92 (11)
O3^i—Tl1—O2^v	129.52 (12)	P1—O1—Tl1^xi	61.69 (14)
O3—Tl1—O2^v	79.59 (13)	P1^x—O1—Tl1^xi	68.71 (14)
O3^ii—Tl1—O2^v	149.02 (14)	Tl1^viii—O1—Tl1^xi	153.65 (12)
O2^iii—Tl1—O2^v	71.11 (16)	Tl1^ix—O1—Tl1^xi	131.30 (10)
O2^iv—Tl1—O2^v	90.79 (13)	Tl1^x—O1—Tl1^xi	105.89 (10)
O3^i—Tl1—O1^vi	47.18 (11)	Tl1^v—O1—Tl1^xi	112.06 (10)
O3—Tl1—O1^vi	124.87 (13)	P1—O2—Tl1^ix	115.5 (3)
O3^ii—Tl1—O1^vi	78.11 (12)	P1—O2—Tl1^iv	148.1 (3)
O2^iii—Tl1—O1^vi	60.37 (11)	Tl1^ix—O2—Tl1^iv	85.22 (12)
O2^iv—Tl1—O1^vi	83.93 (12)	P1—O2—Tl1^v	103.6 (2)
O2^v—Tl1—O1^vi	130.42 (13)	Tl1^ix—O2—Tl1^v	108.89 (16)
O3^i—Tl1—P1	87.23 (9)	Tl1^iv—O2—Tl1^v	90.79 (13)
O3—Tl1—P1	24.76 (9)	P1—O2—Tl1	74.9 (2)
O3^ii—Tl1—P1	73.32 (9)	Tl1^ix—O2—Tl1	149.54 (14)
O2^iii—Tl1—P1	134.59 (9)	Tl1^iv—O2—Tl1	75.68 (11)
O2^iv—Tl1—P1	127.54 (11)	Tl1^v—O2—Tl1	95.07 (13)
O2^v—Tl1—P1	90.46 (10)	P1—O2—Tl1^viii	82.93 (19)
O1^vi—Tl1—P1	130.72 (8)	Tl1^ix—O2—Tl1^viii	66.99 (9)
O3^i—Tl1—O2	108.64 (12)	Tl1^iv—O2—Tl1^viii	128.67 (15)
O3—Tl1—O2	45.47 (11)	Tl1^v—O2—Tl1^viii	61.83 (8)
O3^ii—Tl1—O2	69.49 (11)	Tl1—O2—Tl1^viii	143.17 (12)
O2^iii—Tl1—O2	149.54 (14)	P1—O3—Tl1^xi	107.8 (3)
O2^iv—Tl1—O2	104.32 (11)	P1—O3—Tl1	101.3 (2)
O2^v—Tl1—O2	84.93 (13)	Tl1^xi—O3—Tl1	103.19 (15)
O1^vi—Tl1—O2	144.09 (10)	P1—O3—Tl1^xii	142.3 (3)
P1—Tl1—O2	23.87 (8)	Tl1^xi—O3—Tl1^xii	102.06 (14)
O2—P1—O3	121.3 (3)	Tl1—O3—Tl1^xii	93.54 (13)
O2—P1—O1	105.7 (3)	P1—O3—Tl1^v	72.3 (2)
O3—P1—O1	110.3 (2)	Tl1^xi—O3—Tl1^v	163.77 (15)
O2—P1—O1^vii	110.2 (2)	Tl1—O3—Tl1^v	92.54 (13)
O3—P1—O1^vii	104.6 (3)	Tl1^xii—O3—Tl1^v	72.64 (11)
O1—P1—O1^vii	103.41 (12)

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