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

The structural evolution of lanthanide ATiO (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 DyTiO and GdTiO form P2am at pressures below 9 GPa and Pnma above 13 GPa. Pyrochlore-type DyTiO and ErTiO as well as defect-fluorite-type YbTiO form Pnma at ∼21 GPa, followed by Im3̅m. Hexagonal DyTiO forms Pnma directly, although a small amount of remnants of hexagonal DyTiO is observed even at the highest pressure (∼55 GPa) reached, indicating kinetic limitations in the hexagonal DyTiO 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 ErTiO. 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 ABO composition at high pressure.