Rare earth elements as a source of impurities in doped chalcogenide glasses

•Impurities in the samples of Ce, Pr, Nd, Tb, and Dy used to prepare REE-doped chalcogenide glasses are determined.•The main impurity elements in REEs are W, Mo, Fe, Mn, Ti, F, O and H.•In doped Ge-Ga-Sb-Se glasses, REEs are the main sources of impurities of d-transition metals, Si, F, H, O, and heterogeneous inclusions.•Impurities of hydrogen, oxygen, d-transition metals, and REE analogues can affect the rate of non-radiative relaxation of the excited state of REE.