RbAP (Rubidium Biphthalate) is a type of inorganic crystal material that is used in X-ray spectroscopy and diffraction applications. RbAP is a hygroscopic crystal, meaning that it readily absorbs moisture from the air, and is relatively soft and brittle. These properties make it particularly useful in X-ray powder diffraction and high-resolution X-ray spectroscopy applications where its high diffraction efficiency and good chemical and thermal stability are beneficial. Despite its softness, RbAP has good mechanical stability, making it possible to obtain well-defined X-ray diffraction patterns even with low quality samples.