In case of indirect transition in an indirect semiconductor material, momentum as well as energy changes. I got from the book "solid state electronics devices" by Streetmen & Banerjee that in such kind of transition "the energy is generally given up as heat to latice rather than as an emitted photon". Then my query is what is the basic difference between Heat to latice & photon emmission. Beacause as per my knowledge heat is infrared & infrared emmission is nothing but low energy photon emmission. Then how the heat & photon emission are different.