Defects and transport in langasite I:: Acceptor-doped (La3Ga5SiO14)

Both nominally undoped and 1%Sr-doped langasite were found to exhibit acceptor behavior and correspondingly mixed ionic-electronic conductivity under experimentally accessible conditions. At high pO(2), the conductivity of nominally undoped langasite was pO(2)-independent (ionic behavior) but became increasingly pO(2)-dependent (n-type behavior) under reducing conditions. The substitution of strontium on lanthanum sites, increased the ionic and introduced a p-type electronic conductivity behavior at the highest pO(2)'s, while completely depressing the n-type electronic conductivity - observations consistent with predictions of the proposed defect model based on oxygen vacancy formation in response to negatively charged acceptor impurities. Sr-doped langasite was found to have a higher activation energy for oxygen ion transport (1.27 +/- 0.02 eV) than nominally undoped langasite (0.91 +/- 0.01 eV). This difference could not be successfully explained by applying a simple defect association model but required the assumption of long range defect interactions. Using the defect model, a number of key equilibrium constants (reduction (5.7 +/- 0.06 eV), oxidation (2.18 +/- 0.08 eV), electron-hole generation (3.94 +/- 0.07 eV), and defect mobilities (oxygen vacancies and holes) were derived and summarized.