Hello Diego,

You can find the calculated refractive index for CuAl₂O₄ in this paper https://faculty.uobasrah.edu.iq/uploads/publications/1724623366.pdf. I don't know what specific measurements you need to make, but for many of them refractive index not needed at all e.g. intensity size distribution. Remember that in DLS measurements you give the optical properties of the material for the wavelength of the laser used in the instrument.