Point Defects in Ceramics for Detector Applications

Abstract

This thesis examines point defects in REAlO₃ perovskites, RE₃Al₅O₁₂ garnets (where RE is a rare earth ion or yttrium), zinc oxide (ZnO) and yttrium oxide (Y₂O₃). Areas of application include medicine, industrial manufacturing, the security sector and high-energy physics research. Point defects, both intrinsic and extrinsic, are known to directly affect the optical efficiency of materials.

The perovskite ceramics YAlO₃ (YAP), LaAlO₃ (LaAP), LuAlO₃ (LuAP) and the garnet ceramics Y₃Al₅O₁₂ (YAG) and Lu₃Al₅O₁₂ (LuAG) are used as scintillators when doped with rare earth ions. The influence of crystal structure on defect properties and defect structure were investigated in these materials using pair potential based simulations. These simulations were further used to consider defect reactions and volume changes in non-stoichiometric compositions. Trends in defect properties across structure types were considered as a function of constituent ion radii. To support the simulation studies, the optical float zone method was used to grow single crystal samples of rare earth aluminate perovskites and rare earth hafnates. Laser excited spectroscopy, optical and IR absorption measurements were taken on SmAP:Er samples.

Pair potential simulations have been employed to predict the energetically preferred defect mechanism of MO₂ incorporation in Y₂O₃, to form non-stoichiometric materials, where M = Ti, Zr, Hf or Ce. Ionic radii and equilibrium bond length were used as predictors of site preference and critically compared to the simulation results. Finally, quantum mechanical simulations using Density Functional Theory (DFT) have been used to investigate nickel doping in ZnO. Predicted bulk volume changes induced by nickel impurities were compared to experimental data.