What is a non-stoichiometric compound?

A non-stoichiometric compound is characterized by its variable composition, which distinguishes it from stoichiometric compounds that have a fixed ratio of elements. In a non-stoichiometric compound, the proportions of the constituent elements can fluctuate due to the presence of vacancies, interstitials, or defects in the crystal structure. This variability often occurs in solid compounds, particularly in materials like transition metal oxides or certain alloys, where the actual ratio of elements deviates from the idealized formula.

For example, in the case of some metal oxides, the presence of different oxidation states can lead to a non-stoichiometric nature where the actual metal-to-oxygen ratio is not fixed but can change based on factors like temperature or partial pressure of oxygen. Understanding the significance of non-stoichiometric compounds is crucial in materials science and solid-state chemistry, as their unique properties can be exploited in various applications, such as catalysis or semiconductors.

A substance with fixed composition defines stoichiometric compounds, while a pure compound without impurities and a mixture of different metals do not capture the essence of the varied ratio that characterizes non-stoichiometric compounds.