Refractive indices of congruent crystals codoped with and MgO were measured at room temperature in a wavelength range from using critical angle measurement and minimum deviation methods. For reference, measurements by the critical angle method were also performed on a congruent and a congruent MgO . The measured data are compared with previously reported theoretical and/or experimental results of pure and MgO-doped , especially with the results predicted by the MgO content-related Sellmeier equation (SE) reported by Schlarb and Betzler. It is shown that the Schlarb and Betzler SE consistently predicts a lower index, by as much as , than what is actually measured by the authors for both congruent and for , whether it is codoped with Er or not. To check the Er doping effect, measurements by the critical angle method were also performed on some congruent crystals singly doped with . The results show that Er doping has no measurable effect. It is concluded that there may be a systematic error as one uses the Schlarb and Betzler SE to predict the index of the undoped, only MgO-doped, and -codoped crystals. The Schlarb and Betzler SE parameters are refined to fit our experimental data. The corrected version of the SE suitable for with or without Er doping is given.
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