Na₃MP₃O₁₀·12H₂O (M = Co, Ni): Crystal Structure and IR Spectroscopy

Abstract

Two new dodecahydrate trisodium triphosphates Na₃MP₃O₁₀·12H₂O (M = Co (1), Ni (2)) were synthesized using a wet chemistry route and characterized by X-ray diffraction and FT-IR spectroscopy. They are isotypic, monoclinic (P21/𝑛, 𝑍=2), with pseudoorthorhombic unit cell parameters (Ǻ,°): (𝑎=14,9906 (4), 𝑏=9,1628 (2), 𝑐=14,6660 (4), 𝛽=90,098 (2), for (1) and 𝑎=15,0236 (3), 𝑏=9,1972 (2), 𝑐=14,6654 (3), 𝛽=90,0492 (16) for (2)). Values of 𝑅/𝑅𝑤 are 0.0267/0.0738 and 0.0284/0.0907, respectively, for (1) and (2). Both compounds were found to be systematically twinned by 180° rotation around 𝑎. Their frameworks are made by slabs parallel to 𝑎𝑏 plane, resulting from the cohesion of two kinds of metallic chains. IR spectrum confirms the presence of characteristic bands from P₃O₁₀ phosphate group.

1. Introduction

Phosphates containing polyanions P_𝑛O_3𝑛+1^(𝑛+2)− (𝑛=1 and 2) have been intensively investigated during the last few decades because of their applications in different fields: solid electrolytes for energy stockage, Li-ion batteries, ceramics, luminescence, and magnetism [1]. Kanonerovite, Na₃MnP₃O₁₀·12H₂O, is a natural triphosphate mineral [2]; few other synthetic triphosphates have been reported in the literature: Na₃CuP₃O₁₀·12H₂O [3], LiZn₂P₃O₁₀·8H₂O [4], NaZn₂P₃O₁₀·9H₂O [5], and Na₃CdP₃O₁₀·12H₂O [6]. Anhydrous triphosphates have been also studied: LiFe₂P₃O₁₀ [7], LiM₂P₃O₁₀, M = Co and Ni [8, 9], RbBe₂P₃O₁₀ [10], Cs₂MP₃O₁₀ (M = Ga, Al, Cr) [11]. These latter ternary phosphates, with the frameworks consisting in associated [PO₄] and [MO₆], are efficiently used for waste storage hosts [12]. We deal in the present work with the synthesis and structural study of two new hydrated phases: Na₃MP₃O₁₀·12H₂O, M = Co (1) and Ni (2).

2. Experimental

2.1. Synthesis

The title compounds Na₃M(P₃O₁₀)·12H₂O (M = Co (1), Ni (2)) were obtained by mixing two solutions made of following reactants, each dissolved in 10 mL distilled water: Na₅P₃O₁₀ (0.3219 g) and, respectively, CoCl₂·6H₂O (0.2378 g) (1) and NiCl₂·6H₂O (0.2375 g) (2). The mixtures were stirred for 1 h and then allowed to stand at room temperature. After 2 weeks pink crystals of compound (1) and light green crystals of compound (2) were collected by filtration. The crystals were washed with a water-ethanol (80 : 20 v/v) mixture.

2.2. Single Crystal Study

The X-ray diffraction data for (1) and (2) were collected in a four-circles diffractometer Gemini of Oxford Diffraction (now Agilent Technologies), using graphite monochromatized CuKα radiation (𝜆=1.54184 Å) collimated by mirrors, detector Atlas. The intensity data were corrected for Lorentz and polarization effects. A numerical absorption correction based on the crystal shape was carried out with the program CrysAlis RED [13]. The structures were solved by Superflip program [14] and refined by a full-matrix least-squares technique based on 𝐹² with Jana2006 [15]. Both compounds were found to be systematically twinned by 180° rotation around 𝑎. The refined volume fraction of the major twin domain was 0.628(1) for both samples, confirming exactly the same kind of twinning. Only the major twin domain reflections were used in the refinement, using the twinning matrix for different scaling of the fully overlapped reflections. Influence of possible partial overlaps was negligible, without any need to use hklf5 approach. All nonhydrogen atoms were refined with anisotropic displacement parameters. Positions of hydrogen atoms belonging to water molecules were found in the difference Fourier map and refined with a restrain on their distance to the parent oxygen atom. Their isotropic temperature parameters were calculated as 1.2∗𝑈_eq of the parent oxygen. Table 1 reports the crystallographic data and experimental details about data collection and structure refinements, while selected bond distances are in Table 2. The structural graphics were created using DIAMOND program [16].

Table 1 

Crystal data and details of collection and structure refinement for Na₃MP₃O₁₀·12H₂O M = Co (1) and Ni (2).

Table 2 

Interatomic distances (Å) and angles (°) in Na₃MP₃O₁₀·12H₂O, M = Co, Ni.

Supplementary tables of crystal structures and refinements, notably atomic positions, full list of bond lengths and angles, and anisotropic thermal parameters, have been deposited with the Inorganic Crystal Structure Database, FIZ, Hermann von Helmholtz Platz 1, 76344 EggensteinLeopoldshafen, Germany; fax: [phone]; email: [email protected]. CCDC deposition numbers are, respectively, 884535 for Na₃Co(P₃O₁₀)·12H₂O and 884536 for Na₃Ni(P₃O₁₀)·12H₂O.

2.3. IR Measurements

The infrared measurements were performed in transmission geometry using an FTIR Biorad spectrometer FTS-40A with dynamic alignment and a spectral resolution of 2 cm⁻¹ in the range 400–4000 cm⁻¹. 0.2 mg sample per 200 mg KBr has been pressed into pellets.

3. Results and Discussions

Na₃MP₃O₁₀·12H₂O [M = Co (1), Ni (2)] are isostructural, made by slabs parallel to 𝑎𝑏 plane (Figure 1). A slab results from the cohesion of two kinds of metallic chains, C1 and C2 (Figure 2). The chain C1 consists of [Na(2)O₆] octahedra sharing the edge O15-O16 (water molecules) with strongly deformed tetragonal pyramid [Na(3)O₅]. Such dimmers share a corner O5 (oxygen of P2). In the chain C2, [Na(1)O₆] and [MO₆] octahedra share the face O6 (oxygen of P2), O12 (water), and O19 (water), and they are further connected by a corner O11 (water). The details of the connection between the chains C1 and C2 through O–P–O bridges from the P₃O₁₀ phosphate groups are shown in Figure 3. Two parallel sheets are connected by hydrogen bonds, the most pronounced ones are reported in Table 3. The octahedra [MO₆] (M = Co, Ni) are not directly connected in the structures, the shortest Co–Co and Ni–Ni distances are almost the same, respectively, 5.069(6), and 5.068(7)  Ǻ.

Table 3 

Hydrogen bonding parameters (Å, °) for (1) and (2).

Figure 1 

Projection of the crystal structures of Na3MP3O10·12H2O along 𝑏, here the case of (2).

Figure 2 

A slab in the crystal structures of the title compounds.

Figure 3 

Details of the connection between the chains C1 and C2 inside a slab.

Phosphorous (P^VI) atoms occupy three symmetrically independent tetrahedrally coordinated positions. The [PO₄] share an apex to build up the triphosphate polyanion P₃O₁₀. Average distances P–O in P(1)O₄, P(2)O₄, and P(3)O₄ are, respectively, 1.5292, 1.5404, and 1.5423 Ǻ  (see Table 2 for details), which are of the same magnitude as the values reported in Na₃Cu(P₃O₁₀)·12H₂O [3] and Na₃Cd(P₃O₁₀)·12H₂O [6]. P₃O₁₀ could be viewed as a combination of two P₂O₇ groups, characterized by the P–O–P angle 133.61(14) and 132.02(13) in (1) and 129.08(14) and 131.36(15) in (2), which is also comparable with the already mentioned known phases. The phosphate group P₃O₁₀ acts differently towards metals, being, respectively, monodentate and tridentate with respect to the coordination of Na and M.

The Infrared spectra of (1) and (2) are almost identical, they were recorded in the spectral zone 4000–400 cm⁻¹. Figure 4 depicts the spectrum from the Co phase. Its interpretation can be made on the basis of characteristic vibrations of PO₃ group, POP bridge, and H₂O molecules. Band assignments (Table 4) have been obtained by comparing the results of data reported for known compounds [1]. The asymmetric and symmetric terminal stretching vibrations of the PO₃ groups usually occur in the region 1250–975 cm⁻¹. The bands observed at 1138.75 and 1113.68 cm⁻¹ and 1168.84 cm⁻¹ are assigned to the asymmetric terminal stretching vibration of the PO₃ group 𝜈_as(PO₃).

Table 4 

I-R band assignments for (1) and (2).

Figure 4 

IR spectrum of Na3CoP3O10·12H2O.

The POP bridge vibrations always occur at lower frequency than the terminal P–O stretching modes. In the IR spectra, the asymmetric mode 𝜈_as(POP) gives rise to a band at 872.63 cm⁻¹. Symmetric bridge vibrations 𝜈_s(POP) appear in the IR spectra at 767.52 cm⁻¹. The broad bands in the region (3450–3000 cm⁻¹) are assigned to the stretching of water molecules 𝜈(H₂O). The bands appearing in the spectral region 2900–2000 cm⁻¹ are assigned to the O–H vibrations. Several other weak and medium intensity bands are due to the presence of multiple hydrogen bonds and the field correlation effect. The doublet near 1630 cm⁻¹ corresponds to the bending of water molecules 𝛿(H₂O).

4. Conclusion

Crystals of the title compounds have been synthesized using a wet chemistry route. Na₃MP₃O₁₀·12H₂O (M = Co, Ni) are isostructural and their framework can be described as two kinds of chains connected through O–P–O of P₃O₁₀ phosphate groups into slabs parallel with 𝑎𝑏. The slabs are connected only by hydrogen bonds. IR spectra of the two compounds reveal typical P₃O₁₀ bands known for phosphates.

Acknowledgment

The authors would like to thank the Grant Agency of the Czech Republic, Grant P204/11/0809.