Well solid can difuse to a liquid by add ingredients heat or melting something and liquid can go back to solid by freezing or letting something cool i don’t really know but if your asking if solid can difuse solid Yes it is possible that, the solid diffuses into solid . If we write on the blackboard and leave it uncleaned for a longer time we will observe that it is difficult to clean the blackboard afterwards. This is because of the diffusion of chalk particles into the blackboard also that part i looked up but makes sense