Defects and Transport in Langasite II: Donor-doped (La₃Ga_4.75Nb_0.25SiO₁₄)

Abstract

The electrical conductivity of Nb doped langasite (La₃Ga_4.75Nb_0.25SiO₁₄) was examined as a function of temperature and oxygen partial pressure by complex impedance spectroscopy. A pO₂-independent regime was found at high pO₂ followed by a pO₂^{− 1/6} dependent regime at low pO₂. A defect model consistent with these results was derived in which the electron density n is fixed by the density of ionized Nb donors at high pO₂ and by the generation of oxygen vacancies at low pO₂. The temperature and oxygen partial pressure dependence of the electron density was obtained independently by thermoelectric power measurements. The Nb donor ionization energy was determined to be 1.52 ± 0.06 eV, confirming Nb to be a deep donor in langasite. By combining conductivity and thermoelectric power data, an expression for the electron mobility given by μ _e = 1.1× 10^{− 2}exp(${-\frac{0.15\pm 0.01eV}{kT}}$) was obtained. After evaluating the temperature dependent conductivity data under reducing conditions, in light of the defect model, a value for the reduction enthalpy (E _r = 6.57 ± 0.24 eV) was derived.