High pressure phase transistions in the lanthanide sesquioxides

Crystallizing into 5 known polymorphic forms, the rare-Earth sesquioxides RE 2 O 3 represent a family of 4-f electron compounds that have been the subject of study for many years. Aspects of the systematics in the structural phase transitions for RE 2 O 3 (RE=La, Sm, Eu, Gd, Y, Er, Yb, Lu) under pressure were studied using diamond anvil cells (DACs). As a part of this study, detailed equation of state (EOS) data for all these compounds were obtained through the use of angle-dispersive x-ray diffraction (XRD) where anomalies in compression data led to the detailed investigation into one particular rare-Earth sesquioxide, La 2 O 3 . I show evidence that a long established sequence of high pressure phase transformations in rare-Earth sesquioxides involve more than the five currently known phases. In XRD patterns acquired by imaging detectors at pressures above 7.5 GPa, new reflections appear at low 2[straight theta] angle, implying that a super lattice forms. High resolution XRD scans reveal peak splittings consistent with a monoclinic distortion of the A-type structure. A model of the distorted A-type structure is derived using group-subgroup relations to predict peak splittings and unit cells refined based on the observed XRD patterns. Super lattice formation is further supported by Raman spectra acquired for separate samples which show a doubling of the A 1g stretching mode of the A-type phase above 4 GPa. IR absorption and Raman scattering data are used together in a normal coordinate analysis where calculated mode frequencies model the observed spectra up to 18 GPa with reasonable refined values for all force constants. Until this study, there have been no reported pressure induced structural phase transitions outside the 5 known rare earth sesquioxide polymorphs