looks like no one has measured it... In the following paper an attempt of estimation of refractive index of Neodymium Oxide is made based on measured data for other rare earth oxides and crystal structure of Nd2O3 (formulas for calculating refractive index at different wavelength are also given)
if you find other information, or calculate Nd2O3 refractive indexes for different wavelength using the formulas from Medenbach's paper could you please let me know? I would like to include that data into the database along with other rare earth oxides from the paper