Holmium iron borate: high-resolution spectroscopy and crystal-field parametersD.A. Erofeev, E.P. Chukalina, M.N. Popova, B.Z. Malkin, L.N. Bezmaternykh and I.A. GudimEPJ Web Conf., 132 (2017) 03013DOI: https://doi.org/10.1051/epjconf/201713203013