An X-ray Diffraction Study of the Structure of Vitreous P 2 O 5

Recently, the lengths of the two P-O bonds in the PO 4 tetrahedron were obtained by neutron diffraction of high real-space resolution. By use of the present X-ray diffraction experiments, the P-P distance belonging to pairs of corner-linked PO 4 units is determined. Using this length of (294 ± 2) pm and taking into account the P-0 bond distance to the bridging oxygen atom of 158 pm, a mean P-O-P angle of 137° ± 3° is calculated. The reverse Monte Carlo simulations fit the neutron and X-ray structure factors. The P-O-P angle distribution obtained this way possesses a mean angle of 141°. An interpretation of the first scattering peaks is presented by analysing the occupancy and the distances of various co-ordination shells by use of model configurations. The low occupancy of the first shells allows the application of the schematic hole model of Dixmier. The first X-ray diffraction peak at 13 nm -1 is related to the P-P 2nd shell, the shoulder at 20 nm -1 arises from the P-O 2nd shell. The most similar crystalline structure with vitreous P 2 O 5 is the orthorhombic P 2 O 5 , form II. But it has more effectively orientated terminal oxygen atoms and, thus, a higher packing than the glass.