Temperature shift of the zero-phonon $f$–$f$ lines of Er$^{3+}$ luminescence in quantum paraelectric KTaO$_{3}$

In КТаО$_{3}$ : Er crystals, we observed a temperature shift, unusual for rare earth impurities, for narrow zero-phonon luminescence lines, due to the $^{4}$S$_{3/2}\to{}^{4}$I$_{13/2}$, $^{4}$F$_{9/2}\to{}^{4}$I$_{15/2}$, and $^{4}$S$_{3/2}\to{}^{4}$I$_{15/2}$ transitions in Er$^{3+}$ impurity ions. The magnitude of the shifts turned out to be comparable with the large shifts of the $R$ lines of the Cr$^{3+}$ luminescence in SrTiO$_3$ : Cr and KTaO$_{3}$ : Cr crystals, that is, substantially larger than that for the three-charged rare earth ions usually observed in the $f$–$f$ luminescence spectra in ionic crystals.