Which materials are classified as rare earth phosphors?

Rare earth phosphors are materials that exhibit luminescence and typically include certain compounds that contain rare earth elements. These elements are found in the lanthanide series of the periodic table and are known for their ability to efficiently convert energy into light.

Among the options given, gadolinium oxysulfide falls under the category of rare earth phosphors because it contains gadolinium, a rare earth element, and is known for its efficiency in luminescence, especially in application settings like scintillators and displays.

Lanthanum oxybromide, while it does contain a rare earth element, is less commonly classified specifically as a phosphor compared to gadolinium oxysulfide. Cesium iodide, on the other hand, does not contain rare earth elements and is not categorized as a rare earth phosphor.

Thus, the classification of gadolinium oxysulfide as a rare earth phosphor makes the selection of this answer appropriate, as it satisfies the criteria of containing a rare earth element and possessing phosphorescent properties. Choosing just gadolinium oxysulfide aligns closely with the definitions utilized in the context of rare earth phosphors.