Li super ionic conducting glasses and glass ceramics in the Li1+xGaxGe2-xP3O12 system: Solid state nuclear magnetic resonance and electrical conductivity study

Glassy precursors in the Li1+xGaxGe2-xP3O12 system are prepared by melting and casting. The glasses are annealed at the crystallization peak temperatures to obtain glass ceramics with Na-superionic conducting structure. The ionic conductivity can be significantly increased by substituting Ge with Ga and Li. The highest ionic conductivity of the glass ceramics is 9.74 x 10(-5) S cm(-1) at 20 degrees C. Multiple magic angle spinning nuclear magnetic resonance (MAS NMR) techniques are used to characterize the structural transitions from glasses to their glass ceramic counterparts. Ga-71 and P-31 MAS NMR spectra have shown that the local structures of the glasses are distinctly modified after crystallization. The second coordinated shells of Ga3+ ions are consisted of phosphorus in both glasses and glass ceramics although their coordination numbers change. The glassy precursors show homonuclear segregations such as P-O-P and Ge-O-Ge linkages. However, the glass ceramics are homogeneous, in which per phosphorus tetrahedron connects with total four Ga and Ge, forming four different phosphorus species (Q((4-n)Ge, nGa)(0)(n = 0, 1, 2, and 3)). A random distribution model can be used to quantitatively predict the population of every Q((4-n)Ge), (nGa0) species, if the compositions are known. A correlation between local structure and ionic conductivity is proposed.