Cubic alkali orthophosphates with high ionic conductivity

M₃PO₄ (M = Li, Na) polymorphs with a face centered cubic structure at room temperature, have been prepared by heat-treatment either in a humid or hydrogen containing atmosphere. A cubic structure at room temperature has also been prepared by heat-treatment of K₃PO₄ mixed with Al₂O₃. The lattice parameters are 7.28 ± 0.02 A ̊ for Li₃PO₄, 7.405 ± 0.005 A ̊ for Na₃PO₄, and 4.435 A ̊ ± 0.005 A ̊ for K₃PO₄1bAl₂O₃. Infrared studies on these cubic phases indicate the existence of hydrogen bonding within the crystal structures, which could be important for stabilizing the cubic structure to room temperature. The ionic conductivities were found to be much higher for the cubic phases than for other polymorphs. Preliminary studies showed that proton conduction occurs in these materials.