Solid-State NMR Studies of Lithium Phosphorus Oxynitride Films Prepared by Nitrogen Ion Beam-Assisted Deposition

The chemical stability of phosphate glasses can be improved through the incorporation of nitrogen into the structure. In nitrided amorphous Li3PO4 thin films (UPON), the exceptional stability and enhanced Li+ conductivity make them particularly attractive for microbattery applications. Reported here are 7Li, 31P, and 15N solid-state nuclear magnetic resonance (NMR) studies of UPON thin films fabricated by an ion beam-assisted deposition (IBAD) process. Variable temperature measurements of the 7Li resonance yield an activation energy of about 0.2 eV, which is smaller than values obtained from ionic conductivities. 31P spectra provide direct evidence for at least three phosphate environments: two resonances associated with pO4(2-) and pO4(3-) structures and a third anomalous peak assigned to a nitrided tetrahedral phosphate structural unit of charge -3. Additional evidence exists for other nitrided phosphate structures of charge -2. 15N spectra show evidence for P-N=P type units and molecular N2 with relative intensities that vary with the nitrogen ion beam gun voltage. No 15N peak associated with trigonal NP3 units is observed. Additional evidence for molecular N2 is provided by Fourier transform infrared measurements. A structural model of the phosphorus oxynitride host is presented.