Abstract
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 Mg1-xZnxAl2O4: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.
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