Copper and iron precipitates in thermochemically reduced yttria-stabilized zirconia crystals

Thermochemical reduction (TCR) at 21-73 K has been performed on nominally pure yttria-stabilized zirconia crystals. After TCR the crystals become opaque owing to a very intense optical absorption band centred at 2.64 eV. Analytical transmission electron microscopy studies have shown that both copper and iron impurity precipitates are present. Colloids with different sizes and shapes are inhomogeneously distributed and both large and small precipitates are observed. Most of the large precipitates are elongated and irregular shaped, with an average dimension of 130 nm × 30 nm; they all contain copper. Exclusively copper and exclusively iron contents can be found in different parts of the same precipitate, an observation that is compatible with the fact that copper and iron are inmiscible. Smaller precipitates, with an average size of about 5 nm, are usually iron rich or copper rich.