Encyclopedia of Crystallographic Prototypes

12-phosphotungstic acid [H$_{3}$PW$_{12}$O$_{40}\cdot$5H$_{2}$O] ($H4_{16}$) Structure: A5B40CD12_cP116_224_bd_e3k_a_k-001

• This structure is a bit problematic. (Brown, 1977) offers evidence that the true structure of the hydrated phosphotungstic acid is H$_{3}$PW$_{12}$O$_{40}$ · 6H$_{2}$O, rather than 5H$_{2}$O as deduced by (Keggin, 1934). The presence of the extra water molecule does change the structure somewhat, and Keggin does not give the positions of the hydrogen atoms not attached to water molecules. Ordinarily we would mark this structure obsolete, but it is possible to remove water molecules from this structure and still have a molecule that is recognizably related to the acid, as in H$_{3}$PW$_{12}$O$_{40}$ · 3H$_{2}$O. In addition, the lattice constant reported by Keggin is 12.141Å compared to Brown's 12.506Å, a change consistent with the loss of a water molecule. Given this, we will not depreciate this five Water molecule structure.
• (Gottfried, 1937) does not give the positions of the water molecules, and they reverse the $x$ and $z$ coordinates for O-IV and W.
• This structure is a partially dehydrated form of H$_{3}$PW$_{12}$O$_{40}$ · 29H$_{2}$O ($H4_{21}$). Further dehydration produces the H$_{3}$PW$_{12}$O$_{40}$ · 3H$_{2}$O structure.
• The positions of the three hydrogen atoms are unknown. If we follow (Marosi, 2000), then the 'free' hydrogens are likely to be bound to some of the water molecules, giving the unusual stoichiometry in the prototype.
• The ICSD entry for (Keggin, 1934) does not list the positions of the water molecules.

Simple Cubic primitive vectors

References

• J. F. Keggin, The structure and formula of 12-phosphotungstic acid, Proc. Royal Soc. London A 144, 75–100 (1934), doi:10.1098/rspa.1934.0035.
• C. Gottfried and F. Schossberger, eds., Strukturbericht Band III 1933-1935 (Akademische Verlagsgesellschaft M. B. H., Leipzig, 1937).
• L. Marosi, E. E. Platero, J. Cifre, and C. O. Areána, Thermal dehydration of H$_{3+x}$PV$_{x}$M${12−x}$O$_{40}$$\cdot$yH$_{2}$O Keggin type heteropolyacids; formation, thermal stability and structure of the anhydrous acids H$_{3}$PM$_{12}$O$_{40}$, of the corresponding anhydrides PM$_{12}$O$_{38.5}$ and of a novel trihydrate H$_{3}$PW$_{12}$O$_{40}$$\cdot$3H$_{2}$O, Journal of Materials Chemistry 10, 1949–1955 (2000), doi:10.1039/b001476l.

Found in

• G. M. Brown, M.-R. Noe-Spirlet, W. R. Busing, and H. A. Levy, Dodecatungstophosphoric acid hexahydrate, (H$_{5}$O$_{2}^+$)$_{3}$(PW$_{12}$O$_{40}^{3-}$). The true structure of Keggin's 'pentahydrate' from single-crystal X-ray and neutron diffraction data, Acta Crystallogr. Sect. B 33, 1038–1046 (1977), doi:10.1107/S0567740877005330.

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