A sub-stoichiometric oxide, nitride or oxynitride layer in an optical stack, alone or in direct contact with one or two stabilizing layers, stabilizes the optical properties of the stack. The stabilizing layer(s) can stabilize the chemistry and optical properties of the sub-stoichiometric layer during heating. The change in optical characteristics of the sub-stoichiometric layer upon heating can counter the change in optical characteristics of the rest of the optical stack.