Anomalous discrete disorder response of high-symmetry impurity centers spectra in garnet solid solutions

Abstract : The zero-phonon electronic transitions in Cr³⁺ and Ce³⁺ impurity ions in a series of Lu_3xY_3−3xAl₅O₁₂ (0 < x < 1) garnet solid solution crystals were studied experimentally. It was observed that in contrast to the case of Ce³⁺, the modification of zero-phonon R-line (²E–⁴A₂) fluorescence spectra of Cr³⁺ ions with changing x occurs in a discrete fashion and is not accompanied by strong inhomogeneous broadening, as usually happens in solid solutions. The effect is ascribed to the high C_3i symmetry of the Cr³⁺(Al³⁺) sites that allows only a limited number of non-equivalent Cr³⁺ centers in a mixed environment. The energies and radiative lifetimes of the ²E states of locally identical Cr³⁺ centers inside different mixed garnet matrices were studied and the observed dependences on the Lu content x are discussed in terms of lattice compression and dilation.