Holmium iron borate: high-resolution spectroscopy and crystal-field parameters D.A. Erofeev, E.P. Chukalina, M.N. Popova, B.Z. Malkin, L.N. Bezmaternykh and I.A. Gudim EPJ Web Conf., 132 (2017) 03013 Published online: 13 December 2016 DOI: 10.1051/epjconf/201713203013