Relative importance of static contribution to the thermal shifts of spectral lines in Nd3+-doped Y3Al5O12 laser crystals

By means of the observed pressure and temperature dependences of several spectral lines in yttrium aluminum garnet Y3Al5O12 (YAG) : Nd3+ laser crystals, the static contribution due to lattice thermal expansion and the vibrational contribution due to electron-phonon interaction to the thermal shifts of these spectral lines are studied. It is found that (i) the importance of static contribution of a spectral line depends on the pressure dependence of this line, (ii) the static contribution always leads the spectral lines to shift to the blue (i. e. higher energy) with increasing temperature, while the vibrational one leads the lines to shift to the red, and (iii) the relative importance of static contribution changes not only from crystal to crystal (as suggested in the previous papers for 'hard' or 'soft' crystals), but also from spectral line to spectral line in the same crystal. So, the general statement in previous studies that the static contribution to thermal shift is negligibly small in a 'hard' crystal is inappropriate.