Topotaxy and structural mechanisms of solid state reactions
Abstract The system PbO-P 2 O 5 -H 2 O bears in it a good model for investigation of mechanisms of solid state reactions on an atomic scale. Thermal dehydration of Pb 2 P 4 O 12 ·4 H 2 O and Pb ( H 2 PO 4 ) 2 lead via a number of well defined intermediate compounds to the final product lead polyphosphate [ Pb ( PO 3 ) 2 ] x . Several of these reactions proceed crystallographically oriented. The following reactions were examined in detail: Pb 2 P 4 O 12 ·4 H 2 O → Pb 2 P 4 O 12 ·2 H 2 O +2 H 2 O ; Pb 2 P 4 O 12 ·2 H 2 O → Pb 2 HP 3 O 10 + H 3 PO 4 ; Pb 2 HP 3 O 10 + H 3 PO 4 → Pb 2 P 4 O 12 +2 H 2 O ; Pb(H 2 PO 4 ) 2 →PbHPO 4 +H 3 PO 4 . Conclusions on structural mech anisms were derived by comparisons of crystal structures of the respective starting compound and the corresponding reaction product in the observed orientation relation. In case of the first and fourth reactions these conceptions on mechanisms were confirmed by mass spectroscopy and chemical experiments, respectively. The courses of topotactic reactions in the system PbO-P 2 O 5 -H 2 O are essentially determined by one- or two-dimensionally infinite arrangements of PbO x -polyhedra. Extended parts of these structural units are taken over from the starting compound into the product structure without changing their orientation and construction.