If Ca impurities are added to tio2 and form substitutional defects on the ti sites, Two point defect might form to charge compensate for them and in 1:1 ratio.
Given that calcium is on a titanium site, where titanium has a +4 charge and calcium only has a +2 charge, calcium will take on an effect charge of -2. Vacant oxygen sites effectively have a +2 charge because they are anticipating a -2 charge. These two point defects will therefore balance each other out 1:1.
In particular, perovskite-structured orthorhombic crystals of calcium titanate are produced. The Ca2+ centers in this motif occupy a cage of 12 oxygen centers, and the Ti(IV) centers are octahedral. A lot of useful materials adopt similar structures, like barium titanate, or variations of those structures, like yttrium barium copper oxide.
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