Spectroscopic properties and martensitic phase transition of Y₄Al₂O₉:Ce single crystals under high pressure

High pressure studies of monoclinic yttrium aluminum oxide single crystals (Y₄Al₂O₉ -YAM) doped with Ce³⁺ ions grown by micro-pulling-down method are reported. The results of absorption measurements in the mid-infrared prove the existence of four different sites in which Ce³⁺ ions substitute Y ions, in accordance with the crystallographic structure of YAM. The lowest 5d level of Ce³⁺ is located very close to the bottom of the conduction band for two of four Ce³⁺ related centers at ambient pressure, which results in strong temperature quenching of their luminescence. Two other Ce³⁺ centers do not emit at ambient pressure since their 5d levels are resonant with the conduction band. Application of high pressure above 11 GPa restores their luminescence. Consequences of such energy structure of various Ce³⁺ sites in YAM for the Dorenbos theory are discussed. The bandgap energy of YAM is found to be 5.93 eV, which is the smallest out of the three yttrium aluminum oxides. High-pressure synchrotron angle dispersive x-ray diffraction and Raman scattering measurements identify a phase transition occurring at pressures between 8 and 11 GPa, which has martensitic character. The second phase transition to another structure (likely with hexagonal symmetry) occurs at a pressure around 16 GPa.