The oxynitrides are a group of inorganic compounds containing oxygen and nitrogen not bound to each other, instead combined with other non-metallic or metallic elements. Some of these are with oxygen replaced by nitrogen. Some of these compounds do not have a fixed oxygen to nitrogen ratio, but instead form ceramics with a range of compositions. They are in the class of mixed anion compounds. There may not be a definite ratio of nitrogen to oxygen, and also nitrogen and oxygen may be disordered, swapping places at random. Compared to oxides, the oxynitrides have a smaller band gap.