Small-polaron conduction in ${\mathrm{Y}}_2$${\mathrm{Ti}}_2$${\mathrm{O}}_7$

An activated low electron mobility in single-crystal ${\mathrm{Y}}_2$ ${\mathrm{Ti}}_2$ ${\mathrm{O}}_7$ has been interpreted in terms of small-polaron conduction. Formation of these polarons is attributed to strong short-range forces and to a large oxygen-vacancy concentration, which are related to the large site volume of one of the oxygen ions. Strong temperature dependence of the optical band gap and a large coupling constant are evidence for the strong electron-phonon interaction. An increase in hopping energy upon second ionization of the vacancy donor is indicative of the role of defects in polaron formation.