Influence of composition on the emission properties of impurities in solids: Case study of Mg_1-xZn_xAl₂O₄:Cr³⁺ with the spinel structure

The importance of structural disorder of crystal lattice for the formation of the optical properties of impurity ions is studied by evaluating the optical properties of Cr³⁺ impurity ions in Mg_1-xZn_xAl₂O₄:Cr³⁺ (x = 0; 0.25; 0.50; 0.75; 1.0) spinel solid solutions. It is demonstrated that the gradual replacement of Zn by Mg, which transforms the normal spinel ZnAl₂O₄ into the inverse spinel MgAl₂O₄, leads to an enhancement of the Cr³⁺ red emission. In addition, the Cr³⁺ decay rate (inverse to the emitting excited state lifetime) increases linearly when Zn is gradually replaced by Mg. These observations are explained by the lifting of the parity selection rule due to the random distribution of the Zn and Mg cations in the second coordination sphere around the emitting ion, which lowers the local symmetry at the impurity ion site. The strategy of deliberately creating disorder in the crystalline lattice is a successful way of increasing brightness and decreasing the excited state lifetime of the impurity ions, which can be applied for improving emission properties of the phosphor materials used in solid state lighting.