<p>Formula of nickel oxide with metal deficiency defect in its crystal is&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>N</mi><msub><mi>i</mi><mrow><mn>0</mn><mo>.</mo><mn>98</mn></mrow></msub></math>O. The crystal contains&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>N</mi><msup><mi>i</mi><mrow><mn>2</mn><mo>+</mo></mrow></msup></math>&nbsp;and&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>N</mi><msup><mi>i</mi><mrow><mn>3</mn><mo>+</mo></mrow></msup></math>&nbsp;ions. The fraction of nickel existing as&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mi>N</mi><msup><mi>i</mi><mrow><mn>2</mn><mo>+</mo></mrow></msup></math>&nbsp;ions in the crystal is:</p>