Large enhancement of electrical conductivity in magnesium phosphate glasses by ion implantation of proton

Abstract

Large enhancement of electrical conductivity has been observed in Mg (PO3)2 glasses by ion implantation of protons at 120 keV to a fluence of 1 × 1018 cm-2. Upon implantation d.c. conductivity at 300 K was drastically increased from∼10-15 S·cm-1 to 5×10-4 S·cm-1 and the activation energy of the conduction was greatly reduced from∼1.3 eV to 0.18 eV. Observation of absorption current and the positive sign of Seebeck coefficient indicate that the primary charge carrier in H+-implanted Mg (PO3)2 glasses is a cation. The same conductivity enhancement was observed in D+-implanted Mg (PO3)2 glasses, but no significant change was seen in the He+-implanted glasses. These results strongly suggest that fast protonic conduction occurs in the H+-implanted Mg (PO3)2 glasses. No such a high conductivity was observed for H+-implanted SiO2 glasses. It was found that implanted protons occur in the form of Si-OH and Si-H in SiO2 but exist as POH groups and molecular water H2O in Mg(PO3)2 glasses. A quantitative discussion on the proton conductivity led to a hypothesis that the coexistence of acidic groups such as POH and molecular water H2O is a structural requirement for the emergence of fast proton transport in oxide glasses.