Higher coordinated Erbium in Er2Ti2O7 under high-pressure

Abstract

In this article, we report on the structural stability of Er₂Ti₂O₇ cubic pyrochlore under pressure, investigated using x-ray diffraction, Raman spectroscopy, photoluminescence, x-ray absorption spectroscopy (XAS), and ab-initio calculations. Our studies reveal a phase transition from the ambient cubic phase to a high-pressure orthorhombic (cotunnite) phase, initiated at around 40 GPa. The transformation is gradual and does not complete even at the highest pressure studied (∼60.0 GPa). This is further corroborated by first-principles calculations, which indicate that the cotunnite phase becomes energetically more stable than the cubic phase above ∼53 GPa. Upon the complete release of pressure, the high-pressure cotunnite phase is retained, while the untransformed pyrochlore phase partially becomes amorphous. Additionally, XAS data from the recovered sample, taken after pressure cycling at the L₃ edge of Er³⁺ ions, show an increase in the cation coordination number during the structural transition. EXAFS analysis suggests that the high-pressure phase has an average Erbium coordination number between 9 and 10. The structural transformation mechanism is attributed to the accumulation of cation antisite defects, which cause subsequent disordering of the cations and anions within their respective sublattices. The amorphization of the pyrochlore phase upon pressure release is interpreted as the result of the inability to accommodate the point defects formed during compression at ambient conditions.