A2TiO5 (A = Dy, Gd, Er, Yb) at High Pressure

The structural evolution of lanthanide A 2 TiO 5 (A = Dy, Gd, Yb, Er) at high pressure is investigated using synchrotron X-ray diffraction. The effects of A-site cation size and of the initial structure are systematically examined by varying the composition of the isostructural lanthanide titanates and the structure of dysprosium titanate polymorphs (orthorhombic, hexagonal, and cubic), respectively. All samples undergo irreversible high-pressure phase transformations, but with different onset pressures depending on the initial structure. While each individual phase exhibits different phase transformation histories, all samples commonly experience a sluggish transformation to a defect cotunnite-like (Pnma) phase for a certain pressure range. Orthorhombic Dy 2 TiO 5 and Gd 2 TiO 5 form P2 1 am at pressures below 9 GPa and Pnma above 13 GPa. Pyrochlore-type Dy 2 TiO 5 and Er 2 TiO 5 as well as defect-fluorite-type Yb 2 TiO 5 form Pnma at ∼21 GPa, followed by Im3̅m. Hexagonal Dy 2 TiO 5 forms Pnma directly, although a small amount of remnants of hexagonal Dy 2 TiO 5 is observed even at the highest pressure (∼55 GPa) reached, indicating kinetic limitations in the hexagonal Dy 2 TiO 5 phase transformations at high pressure. Decompression of these materials leads to different metastable phases. Most interestingly, a high-pressure cubic X-type phase (Im3̅m) is confirmed using high-resolution transmission electron microscopy on recovered pyrochlore-type Er 2 TiO 5 . The kinetic constraints on this metastable phase yield a mixture of both the X-type phase and amorphous domains upon pressure release. This is the first observation of an X-type phase for an A 2 BO 5 composition at high pressure.