Defects Vs Non-stoichiometry
The cuprate superconductors highlight the concept of "defect" structures, which is related to non-stoichiometry. YBa2Cu3O7−x can be viewed as a variant of the perovskite family of materials, which have idealized stoichiometry ABO3. For the cuprates, Y + Ba occupy "A sites" whereas Cu occupies the "B sites". The non-defect material would have the stoichiometry YBa2Cu3O9. Using this way of describing a structure, W40O118 is said to be a defect variant of WO3.
Read more about this topic: Non-stoichiometric Compound
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