Optical spectroscopy of ErFe3(BO3)4: detection of phase transitions and crystal-field levels of the Er3+ ground multipletDmitry Erofeev, Artjoms Jablunovskis and Elena ChukalinaEPJ Web Conf., 185 (2018) 07002DOI: https://doi.org/10.1051/epjconf/201818507002