The crystal structure of H 3 PW 12 O 40 . 29H 2 O

Abstract

The special interest of this structure lies in the peculiar manner in which the anions (PW 12 O 40 ) -3 are linked together by the water molecules. The remarkable number (29) of water molecules associated with each anion is accounted for by symmetry considerations. Alternating with the anions on a body-centred cubic lattice, seventeen molecules of water are linked together in tetrahedral symmetry. These comprise a central molecule in contact with four molecules at the corners of a tetrahedron, and twelve molecules strung out in pairs along fourfold cube axes, to link together six anions. Midway between two blocks of seventeen waters and independent of them, six water molecules form a puckered hexagonal ring, which links together six anions, lying symmetrically about a threefold axis. For each anion there are two hexagonal rings and one block of seventeen molecules. This accounts for the total of 29 molecules per anion. The environment of the water molecules is in striking contrast with that of the oxygen atoms in the anions. The latter are close-packed, being held together by the phosphorus and tungsten atoms. The water molecules are more open, and only six of the 29 have more than four nearest neighbours. The interatomic distances of the water molecules are very consistent, all lying between 2·8 A and 2·9 A.