Ammonium Metatungstate, (NH<sub>4</sub>)<sub>6</sub>[H<sub>2</sub>W<sub>12</sub>O<sub>40</sub>]: Crystallization and Thermal Behavior of Various Hydrous Species
Felix Eder, Matthias Weil, Wolf‐Dieter Schubert, Christian Gierl‐Mayer
Abstract
High Resolution Image Download MS PowerPoint Slide Various hydrous ammonium metatungstate phases (NH 4 ) 6 [H 2 W 12 O 40 ]· X H 2 O (AMT- X ) have been obtained in the form of single crystals, using supersaturated solutions, antisolvent crystallization, and partial dehydration as strategies for crystal growth. On the basis of laboratory X-ray diffraction data, the crystal structures of the hydrous phases with X = 22, 12.5, 9.5, 8.5, 6, 4, and 2 as well of the cocrystals with X = 12-ethanol and X = 4-2acetone have been determined for the first time. AMT-22 forms the solid solution series (NH 4 ) 6– x K x [H 2 W 12 O 40 ]·22H 2 O with the potassium salt (PMT-22) without a miscibility gap. Its tetragonal crystal structure shows a distorted cubic close-packed arrangement of the spherical α-Keggin-type [H 2 W 12 O 40 ] 6– anions with disordered ammonium N and water O atoms in the voids. The crystal structures of the other, less-hydrated AMT phases can be derived from distorted hexagonal rod packings of the α-Keggin anions, likewise with the cations and crystal water molecules in the voids. The thermal behavior of AMT-22 crystals reveals a quick dehydration, with AMT-9.5 as an intermediate and AMT-4 as a stable hydrate phase under ambient conditions. Upon further heating, the material decomposes with stepwise formation of AMT-2, AMT-1, AMT-0, and an amorphous phase, before orthorhombic WO 3 forms above 400 °C. One of the commercially available AMT phases, “(NH 4 ) 6 [H 2 W 12 O 40 ]· X H 2 O” (with X = unspecified) has a water content of X = 4 but crystallizes in a structure other than AMT-4. Consequently, the 4-hydrate shows polymorphic behavior.