Comparative Systematic Study of Covalent Effects for the Cr3+-, Mn4+- and Ni2+-Doped Optical Materials
In the present work we review the spectroscopic properties of the Cr3+, Mn4+ and Ni2+ ions in a number of solids. The crystal field strength Dq, Racah parameters B, C and position of the first excited state were all tabulated.
In an attempt to quantify the collected information, we used the model recently developed by us (1-3), which allows to link together the energy of the spin-forbidden 2Eg-4A2g transition of the Cr3+ and Mn4+ ions or the 1Eg-3A2g transition of the Ni2+ ions and covalent effects in the considered crystals. To rationalize the variation of energy of these spin-forbidden transitions, we introduced a non-dimensional parameter
ß1=((B/B0)2+α(C/C0)2)1/2 , (1)
where the subscript “0” refers to the values of the Racah parameters for a free ion, and B and C are the Racah parameters for the same ion in a given crystal. The value of α was 1 for the Mn4+ and Ni2+ ions, and 1.5 for the Cr3+ ions. An advantage of our model is that it takes into account reduction of both Racah parameters B and C.
Fig. 1 shows that the energy of the Ni2+ 1Eg→3A2g transition is linearly related to β1 value (3). This indicates that any description of the nephelauxetic effect for the Ni2+ containing hosts must consider both Racah parameters B and C; the dashed lines in Fig. 1 mark the ±489 cm-1 corridor from the central fit line (this value is the rms deviation of the data points).
Figs. 2-3 depict dependence of the Cr3+ and Mn4+ 2Eg→4A2g transition on the β1 parameter, respectively Again, a linear trend can be seen clearly. The ±σ range (where σ stands for the rms deviation) is shown in both figures. The reason for some data points to deviate from the straight line can be related to the difficulties of unambiguous experimental identification of the zero-phonon lines, which can be hidden by prominent vibronic progressions. Still the maximal difference between the fitting line and mostly deviated data points is a few hundred wave numbers, which is within a typical range of the phonon frequencies for the majority of optical materials.
Further studies of the transition metal doped materials are under way; the main aim of these studies is to elucidate the common and different features in the behavior of the spin-forbidden transitions and covalency effects experienced by the transition metal ions in solids.
1. A.M. Srivastava, M.G. Brik, J. Lumin. 132, 579, (2012).
2. M.G. Brik, A.M. Srivastava, ECS J. Solid State Sci. Technol. 2, R148, (2013).