1、Definition： Non-stoichiometry means there is an excess ordeficit of either cations or anions, the ratio of cations and anionsis not integer or fixed.Non-stoichiometric compound is essentially the substitutionalsolid solution between the high and low valence ions for the sameelements.Taking Fe0.95O f
2、or an example, the ratio of iron to oxygen is0.95:1, it means a part of Fe2+ convert into Fe3+. So Fe0.95O can beregard as some Fe3+ replace Fe2+ and form a substitutional solidsolution.2.4 Non-stoichiometric compounds Non-stoichiometric compounds are mainly divided into four categories:Cation inter
3、stitial typeAnion interstitial typeCation vacancy typeAnion vacancy typeTiO2-x is a typical non-stoichiometric compound with oxygendeficiency. It can be regarded as the solid solution of Ti4+ and Ti3+oxide.Ti O Ti O OTi O Ti V O O2 4 2 312 2 Ti4+e Ti3+ , this electron does not belong to a fixed Ti4+
4、 andis bound around the oxygen vacancy. Because oxygen vacancyhas two effective positive charges, the free electrons canmigrate and form electron conduction under electric field. Wedefinite the defect formed by the free electron trapped at or nearthe vacancy as F-color centers.The formation of F-col
5、or centers：1. anion interstitial type TiO2-x(1-1)F-color centersFigure 2.5 F-color centersTiO2-x structure defects, the electrons on the F-color centers can absorb a certain wavelength of light, so that titanium oxide changes from yellow to blue until gray black.Relationship between oxygen partial p
6、ressure and vacancy concentration： 212 2 O OO V O eV PO O 2 16e PO216Conductivity decreases with the increase of oxygen partial pressure.平衡常数假定 又 K V P eOO V eK V PO OOO OO O22 1 , 2 4 12 23 12Equilibrium constantHypothesisorAccording to the law of mass action The interstitial Zinc ions concentratio
7、n is proportional to the 1/3 power of zinc vapor pressure. iZnO Zn e O g.2212( ).( ) 2 iZn g Zn e . 2KZn ePiZnZn Pi Zn . 1/3e PO 2162. cation interstitial type Zn1+xOA typical example of this type is ZnO which excess Zn2+ ions enter to interstitial sites and the electros to neighboring interstitial
8、sites.3. anion interstitial typeOnly UO2+x can be regarded as solid solution of U3O8 in UO2.Due to the interstitial anions in the structure, there are electronholes to maintain the electric neutrality, and the reactionformula as follows:2 U O U O OU O U i Op-type semiconductorAccording to chemical e
9、quilibrium:4. cation vacancy type Fe1-xO , Cu2-xOIn order to maintain the electric neutrality, the electron holes arecaptured around the positive ion vacancy. The defects reactionequation as follows:, 2Fe2+ are replaced by Fe3+The formation of Fe2+ vacancy has two effective negativecharges. In order to maintain the electric neutrality, the electronholes are captured or rotate and transition around the positive ionvacancy forming the V-color centers.hFigure 2.6 the formation of V-color centers
