Conduction properties of condensed lanthanum phosphates: La(PO3)3 and LaP5O14

Single crystals of LaP3O9·3H2O were prepared using a classical method of aqueous chemistry whereas those of the ultraphosphate LaP5O14 were obtained by flux method. Thermal treatment of LaP3O9·3H2O up to 700 °C allowed us to obtain the lanthanum polyphosphate La(PO3)3. The conductivity as well as the vibrational study of the lanthanum ultraphosphate and polyphosphate were analysed in order to explain the mechanism of conduction. Activation energies of 0.45 and 0.84 eV, deduced from the Arrhenius relation, were respectively found for the polyphosphate and the ultraphosphate. Taking into account the structural features of these materials we linked the origin of their electrical properties to the incorporation of atmospheric humidity. Thus, it was a proton conduction phenomenon which took place as a result of the establishment of hydrogen bonds as well as the reorganisation of water molecules already incorporated into these structures. Referring to the Grotthuss mechanism, a proton transfer model inside the polyphosphate's structure was suggested.