Room temperature synthesis and phase transformation of lithium phosphate Li₃PO₄ as solid electrolyte

We successfully prepared single-phase sample of the low-temperature beta-type form of lithium phosphate Li3PO4 by a stoichiometric mixing of LiOH solution and NH4H2PO4 powder at room temperature without heating. The orthorhombic lattice parameters with the space group of Pmn2(1) for beta-Li3PO4 were refined to be a = 6.1281(6) angstrom, b = 5.2674(5) angstrom, c = 4.8923(2) angstrom, and V = 157.92(2) angstrom(3). We also prepared the single-phase sample of high-temperature gamma-Li3PO4 by heating the beta-Li3PO4 at 650 degrees C. The orthorhombic lattice parameters with the space group of Pmnb for gamma-Li3PO4 were refined to be a = 6.12569(5) angstrom, b = 10.48730(9) angstrom, c = 4.92957(4) angstrom, and V = 316.685(4) angstrom(3). The structure transformation from beta- to gamma-type phase was examined utilizing the XRD measurements. The XRD data confirmed the continuous phase change in the temperature range between 25 degrees C and 650 degrees C. The lattice contraction for the beta-Li3PO4 phase was observed between 150 degrees C and 350 degrees C. The phase transformation from beta- to gamma-Li3PO4 was observed around 450 degrees C. The spherical primary particle size was ca. 20-30 nm order for both beta- and gamma-Li3PO4 samples. Accordingly, the nano-sized particle Li3PO4 sample can be easily prepared using the present synthetic technique at room temperature.