High-resolution spectroscopy of the ErCrO$_3$ crystal: a new phase transition?

Infrared absorption spectra of the ErCrO$_3$ crystal in the region of $f{-}f$ transitions in the Er$^{3+}$ ion are recorded for the first time. An analysis of high-resolution temperature-dependent spectra reveals a step at $T = 47$ K on the temperature dependences of the characteristics of spectral lines, in addition to the features at temperatures of magnetic ordering $T_N = 133$ K and spin-reorientation transition $T_{SR} = 9.3$ K. This feature can be associated with either a previously unknown phase transition or with local changes in the crystal structure. The shape of the lines at liquid helium temperatures indicates the presence of additional positions for Er$^{3+}$ ions in the ErCrO$_3$ crystal. Presumably, these are positions near uncontrolled impurities that enter the crystal during its growth by the solution–melt method and form regions with a distorted structure responsible for the occurrence of polarization.