VIBRATIONAL-SPECTRA OF ALPO4 AS A STRUCTURE MODEL FOR SILICA

The vibrational spectra of various crystalline forms of SiO2 and AlPO4 are compared. Both compounds are characterized by a linked network of tetrahedra with Si-O-Si and Al-O-P oxygen-atom bridges, respectively. For the vibrational spectra band assignments, two types of model mole cules, XO4 and X2O, are proposed. The tetrahedral model explains well the spectra of aluminium phosphate, whereas in the case of SiO2 the Si 2O unit model is more useful. It has been shown that in geometrical te rms the structures of SiO2 and AlPO4 are similar, whereas their spectr a, which are due to the electronic structures (chemical bonds) are dif ferent.