Vapour pressure measurements on lanthanum polyphosphate and ultraphosphate by the transpiration method

► The vapour pressure of P4O10 over LaP3O9 and LaP5O14 was measured. ► The standard thermodynamic quantities of LaP3O9 and LaP5O14 were derived by the second-law method. ► The validity of the thermodynamic data of LaP3O9 and P4O6 (g) in the literature was discussed. ► The maximum operating temperatures for the LaP3O9 and LaP5O14 electrolytes for use in fuel cells were estimated. ► In the presence of water vapour or hydrogen gas, decomposition of LaP3O9 and LaP5O14 will be accelerated.