Inhomogeneous broadening of Cr3+ luminescence in doped LiTaO3

A theoretical model of the effect of inhomogeneous broadening on the luminescence lineshape of Cr³⁺ is developed and applied to the disordered system Cr³⁺:LiTaO₃. At ambient pressure, Cr³⁺ occupies primarily low-field sites in LiTaO₃ and exhibits broad ⁴T₂-->⁴A₂ emission. The induction of a low-field --> high-field transition with pressure led to the stabilization of narrow, well-resolved ²E-->⁴A₂ emission and permitted us to determine that Cr³⁺ occupies two crystallographically distinct low-field sites at ambient pressure. These two sites were incorporated into a comprehensive electronic-structure model for Cr³⁺ that included the effects of spin-orbit and vibronic coupling to assess the influence of inhomogeneous broadening on the spectral lineshape. The model predicts that each of the two low-field sites is best described in terms of a Gaussian distribution of ⁴T₂ and ²E energies. The model further indicates that the ⁴T₂ and ²E energies within each Gaussian distribution are inversely correlated. The model successfully describes the emission lineshape, linewidth, and intensity of Cr³⁺ over a wide range of temperature and pressure, including during the low-field to high-field electronic transformation. A tentative molecular description of the different Cr³⁺ sites is also presented.