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

Abstract

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 × 10−5 S cm−1 at 20 °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. 71Ga and 31P 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 (Q0(4-n)Ge, nGa (n = 0, 1, 2, and 3)). A random distribution model can be used to quantitatively predict the population of every Q0(4-n)Ge, nGa species, if the compositions are known. A correlation between local structure and ionic conductivity is proposed.