Recently a customer requested information about the correct storage procedures for Barium and Cerium:

Unfortunately both barium and cerium are highly reactive with the environment at room temperature. Barium has an oxidation state of +2 and reacts readily with oxygen in the air. It should be stored either in oil (mineral spirits) or in a vacuum dissector if the material is not to be sputtered within a short period of time after being produced in elemental form such as a sputtering target. Barium also reacts with water to produce hydrogen gas so care should also be taken to keep it away from water or moist air.

Cerium is also highly reactive in air. It is highly pyrophoric and can cause a fire, especially small particle sizes such as scrapings from the chamber wall or shutter assemblies in the vacuum chamber. Care should be taken when cleaning the system. Placing aluminum foil within the chamber in all areas where stray cerium atoms may be deposited (away from the substrate itself) and then disposing of the foil after each run is generally a good idea to avoid any accidental ignition. Cerium reacts slowly in air forming CeO2, therefore, like barium, precautions need to be taken when storing cerium sputtering targets over long intervals and between runs.