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.
The electrical and gravimetric properties of langasite, La 3 Ga 5 SiO 14 , are related to its underlying defect and transport processes via previously developed predictive defect and transport models. These models are used here to calculate the dependence of the partial ionic and electronic conductivities and the mass change for langasite as functions of temperature, dopant type and level and pO 2 . Doping strategies devised for minimizing conductivity in langasite based on use conditions are described. For example, the required dopant level to achieve minimum conductivity and thus minimum electrical losses in acceptor-doped langasite is shown to depend on the operating pO 2 . Likewise intrinsic mass changes in langasite, dependent on dopant level, pO 2 and temperatures, if high enough, can mask mass changes induced in active layers applied to langasite when used as a microbalance. For example, the model predicts that the dopant level in donor-doped langasite has less of an impact on intrinsic mass change due to external environmental changes when compared to acceptor-doped langasite. The models are also applied in defining acceptable operating limits needed to achieve and/or the design of properties for desired levels of microbalance resolution and sensitivity.
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