Comment on "A universal approach for calculating the Judd-Ofelt parameters of RE3+ in powdered phosphors and its application for the β-NaYF4:Er3+/Yb3+ phosphor derived from auto-combustion-assisted fluoridation" by B. Chen, S. Xu et al., Phys. Chem. Chem. Phys., 2018, 20, 15876.