Systematics of the phase behavior in lanthanide sesquioxides

Abstract

The crystal structures of the lanthanide (Ln) sesquioxides, Ln 2O 3, are systematized according to the radius ratio η between the lanthanide ion crystal radius and the “equivalent formula unit radius” which is derived from the unit cell volume of the sesquioxide. New criteria for the stability range of each structure are given quantitatively. These criteria were used to correlate the phase transformations of lanthanide sesquioxides with two critical values of η: B → A occurs at η = 2.26; C → B occurs at η = 2.42. The C → B phase transformation pressure in Eu 2O 3 was estimated. The polymorphism of lanthanide sesquioxides at any temperature and pressure can be estimated using these new systematic critical values.