Point Defects in Optical Ceramics: High‐Temperature Absorption Processes in Lanthana‐Strengthened Yttria

The optical transmission of transparent polycrystaliine La 2 O 3 strengthened Y 2 O 3 was measured in both the near‐ultraviolet and infrared regions at temperatures between 20° and 1400°C. The absorption remains low until about 900°C, but rises almost exponentially thereafter. The magnitude of this increase is a function of the oxygen partial pressure (Po 2 ) in the ambient atmosphere. The temperature‐dependent absorption is lowest when the Po 2 is between 10 −11 and 10 −8 atm (1 atm = 10 5 Pa), representing the range in which the concentration of free carriers (conduction band electrons and valence band holes), generated by stoichiometty‐related point defects (oxygen interstitiais and vacancies), is minimized. The temperature dependence is significantly greater in the uitraviolet than in the infrared, but the optimal Po 2 range is the same. The absorption behavior can be described in terms of a simple phenomenological model involving carriers thermally liberated from defect states within the band gap of the material. Various aspects of the model and their experimental implications are discussed.