I've just read this article about meta-materials with 0-refractive index, which potentially may "permit" light with super-luminous rate /under rate is meant the propagation of light's phase, which doesn't transmit information => there is no valuation of the laws of physics/.
Generally speaking, the refractive index of materials depends on the (dominant) molecular interactions within them, on their morphous structure, etc. As you know, these molecular properties govern lots of other chemical features like the viscosity, the density, to some extend the aggregate state, and so on... On this account, it should not come as a surprise that there exists a clear correlation between the refractive index and the properties, mentioned above - e.g. fluids of higher density tend to have higher refractive properties; same goes for viscosity; and so on... Based on those remarks one is able (at least) to guess how they can improve / worsen the refractive index of a given fluid - discussed for instance in this research on aerosols.
My question, though, is how does the refractive index of alloys, polymers, crystals, and solids in general relate to their other chemical properties?
Furthermore, is there a way at least to guess what materials one should potentially try to use in order to create / synthesize a meta-material, like the one from the article, that breaks the limits?