Unique cation surroundings in the structure of Ag_3PW_(12)O_(40) salt

摘要

The Ag_3PW_(12)O_(40).3H_2O salt forms two different structures: the first one, existing at temperature up to 473 K, contains one water molecule hydrating each silver cation, whereas the other one appears above 573 K after the loss of the water molecule. It results in the shortening of the lattice parameter a from 1193 to 11.70 A. This dehydration-hydration process of silver salt is reversible. In the structure at 473 K, the [Ag(H_2O)]~+ complex is centered in the octahedral space between the Keggin anions. Each silver cation is bonded to two terminal oxygens of different Keggin anions while water molecule forms weak hydrogen bonds with other two Keggin anions. Unexpectedly, in dehydrated structure existing at 573 K, Ag~+ cations are not centered in the octahedral space between Keggin anions and are connected to two terminal oxygens of Keggin anions only. Probably, the formation of two various structures of silver tungstophosphate influences its catalytic activity with changing temperature.