Source of information of elemental properties

In the pymatgen.core.periodic_table module of pymatgen.core.package, what is the source of the elemental properties such as velocity_of_sound, reflectivity, refractive index, Poisson’s ratio, etc. For example, the atomic radius of the elementals has been taken from Atomic radii of the elements (data page) - Wikipedia. But I can’t find the source of information of some other elemental properties that I have mentioned above