What Is the Crystal Structure of Ammonium Metatungstate?

Ammonium metatungstate (AMT) produced by CTIA GROUP LTD is a crucial polynuclear tungstate compound with a complex yet highly ordered crystal structure. Centered around a Keggin-type polyoxoanion, it combines with ammonium cations and crystal water to form a stable lattice. This unique structure imparts AMT with exceptional physicochemical properties, making it widely applicable in fields such as catalysts, tungsten materials, and nanotechnology.

The core of AMT from CTIA GROUP LTD is the Keggin-type polyoxoanion [H₂W₁₂O₄₀]⁶⁻, composed of 12 tungsten-oxygen octahedra (WO₆) connected through shared oxygen atoms. This anion exhibits near-tetrahedral symmetry, belonging to the α-Keggin configuration. The 12 tungsten atoms are positioned at the vertices of the cluster, each coordinated with six oxygen atoms to form an octahedral unit. The oxygen atoms are categorized into four types:

Terminal oxygen (W=O): Forms double bonds with tungsten atoms.

Bridging oxygen (W-O-W): Includes edge-sharing and corner-sharing oxygen atoms connecting tungsten octahedra.

Central oxygen: Located inside the cluster, stabilizing the structure.

Protonated oxygen: Bonded to two hydrogen atoms, characteristic of the [H₂W₁₂O₄₀]⁶⁻ anion.

The Keggin anion carries a 6⁻ charge, which is balanced by six ammonium cations (NH₄⁺), ensuring overall electroneutrality of the molecule.

Crystallographically, AMT belongs to the monoclinic crystal system, commonly with the space group P2₁/c. Its unit cell parameters vary slightly depending on the number of crystal water molecules (typically 3-4), and the cell contains multiple AMT molecular units. X-ray diffraction (XRD) analysis reveals characteristic diffraction peaks at 2θ angles of approximately 10°, 18°, and 27°, indicating a highly ordered arrangement of Keggin anions within the lattice. This regular crystal structure is a key foundation for AMT’s stability.

Ammonium cations interact with the Keggin anion through electrostatic forces and hydrogen bonds, distributed around the anion to fill lattice voids. They not only balance the charge but also form N-H…O hydrogen bonds with the terminal or bridging oxygen atoms of the Keggin anion. This hydrogen bond network significantly enhances the structural rigidity of the crystal, enabling AMT to maintain a stable form at room temperature, suitable for various processing and application scenarios.

As a hydrate, AMT typically contains 3-4 crystal water molecules, existing as coordinated or lattice water. These water molecules connect to the Keggin anion and ammonium cations via hydrogen bonds, filling lattice gaps and further stabilizing the three-dimensional network. When heated to 100-150°C, the crystal water begins to be lost, causing slight lattice contraction, but the Keggin framework remains intact. The hydrophilicity of these water molecules contributes to AMT’s high water solubility, providing an advantage in solution-based preparations.

The structural features of AMT can be verified through various spectroscopic techniques:

Fourier Transform Infrared Spectroscopy (FT-IR): Shows a stretching vibration peak for W=O terminal bonds at 950-960 cm⁻¹ and W-O-W bridging bond vibrations at 800-900 cm⁻¹, confirming the presence of the polynuclear tungsten-oxygen framework.

Raman Spectroscopy: Further validates the symmetry of the Keggin structure, displaying similar W-O bond vibration characteristics.

Nuclear Magnetic Resonance (NMR): ¹H and ¹⁷O NMR detect the chemical environments of protonated oxygen and crystal water, verifying the integrity of the [H₂W₁₂O₄₀]⁶⁻ structure.

These spectroscopic data provide reliable evidence for the structural analysis of AMT.

The Keggin structure endows AMT with excellent chemical and thermal stability. At room temperature, the crystal resists decomposition, and even in aqueous solutions, it maintains the integrity of its cluster structure. These properties make AMT an ideal precursor for preparing high-purity tungsten materials, catalysts, and nanomaterials.

In catalyst preparation, the regular pore structure of the Keggin anion serves as a carrier for active sites. In tungsten powder production, its ordered thermal decomposition ensures uniform particle size in the resulting products.

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