Comparative Systematic Study of Covalent Effects for the Cr3+-, Mn4+- and Ni2+-Doped Optical Materials

The Cr³⁺, Mn⁴⁺ and Ni²⁺ ions are widely used transition metal dopants. Laser generation was obtained on the ²E_g-⁴A_2g spin-forbidden emission transition in a number of crystals and ceramics containing trivalent chromium; the same transition of the Mn⁴⁺ ions is used for getting red emission; the ¹E_g-³A_2g spin-forbidden transition of the Ni²⁺ ions is also used for lasing in the infrared region.

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 ²E_g-⁴A_2g transition of the Cr³⁺ and Mn⁴⁺ ions or the ¹E_g-³A_2g 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 Ni^{2+ 1}E_g→³A_2g 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 Mn^{4+  2}E_g→⁴A_2g 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.

REFERENCES

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).

3. M.G. Brik, A.M. Srivastava, N.M. Avram, A. Suchocki, J. Lumin. 148, 338, (2014).