Research on the thermal shifts and electron–phonon coupling parameters of R-line for Cr3+ and Mn4+ ions in YAlO3 crystals

The thermal shifts of R-line for isoelectronic 3d3 ions Cr3+ and Mn4+ in YAlO3 crystal are studied by using a new theoretical expression including both the static contribution due to lattice thermal expansion and the vibrational contribution due to electron–phonon interaction. The new expression in form is almost the same as that containing only the vibrational contribution (which is widely-used in the previous papers) except the change of coefficient from α to α′+A (where α, obtained from the old expression, and α′, obtained from the new expression, are the apparent and true electron–phonon coupling parameters, respectively. A is a parameter concerning the static contribution). By using the new expression and the analysis of pressure- and temperature-dependences of R-line, it is found that the static contribution to the thermal shift of R-line in sign is contrary to and in magnitude is about 36% of the vibrational one. Thus, the electron–phonon coupling parameters α′ increase about 36% compared with the corresponding parameters α for both Cr3+ and Mn4+ in YAlO3 crystal. So, for the reasonable explanation of thermal shifts of spectral lines and the determination of the true electron–phonon coupling parameter, both the static and vibrational contributions to thermal shift should be taken into account. The reason that the electron–phonon coupling parameter |α′| of Mn4+ is larger than that of Cr3+ in YAlO3 crystal is discussed.