128. Non-stoichiometric cuprous oxide, Cu₂O can be prepared in laboratory. In this oxide, copper to oxygen ratio is slightly less than 2 :1. Can you account for the fact that this substance is a p-type semi-conductor ?

In the sample of cuprous oxide, Cu₂O prepared in laboratory, copper to oxygen ratio is less than 2 : 1 and this deficiency of copper causes the metal deficiency defects. In these defects the positive ions are less in number as compared to anions. These defects are caused in two ways:
(i) By Cation Vacancies : Some Cu⁺ may be missing from lattice sites and their positive charges are balanced by presence of extra charge on adjacent cations i.e., some Cu²⁺ are present and some cation vacancies are present.
(ii) By presence of extra anions at interstitial sites: The type of defect can arise where some oxide ion, O^2– are present at lattice sites and their charge is balanced by neighbouring cations in higher oxidation states i.e., some Cu²⁺ ions are present instead of Cu²⁺ ions.

Metal deficiency defect due cation vacancy.

Metal deficiency defect due to presence of extra anions.