Influence of vacancies and mixed valence on the transport processes in solid solutions with the spinel structure

Abstract

In the spinel compounds of ACr₂ X ₄ type, and in the spinel series of Zn_1−xGa_2x/3Cr₂Se₄, ZnCr_2-xIn_xSe₄, Zn_1−xCu_xCr₂Se₄ and Li₁+_xTi_2-x,O₄, the influence of vacancies and mixed valence on the transport processes is considered. For this purpose, a procedure of calculation of the ion packing coefficients was used and calculations of the fulfilment coefficient, the difference of the Pauling electronegativities and the vacancy parameter were introduced. For the spinel compounds of ACr₂ X ₄ type (where A=Zn, Cd, Mn; X=O, S, Se) the vacancy parameter strongly increases with an increase of the anion radius. The vacancy parameter was found to decrease most with an increase of the imperfection degree of the spinel structure in Zn_1−xGa_2x/3Cr₂Se₄. For the spinel series ZnCr_2-x, In_XSe₄, the vacancy parameter remains almost constant with an increase of the In concentration. For the spinel series Zn_1−xCu_xCr₂Se₄, the appearance of the Cr mixed valence with an increase of x makes the carrier transport easier, leading to the metallic state. In the superconducting spinel system Li_1+xTi_2-x,O₄, the disappearance of the Ti mixed valence with an increase of x worsens the electrical conductivity, leading to the insulating state.