Pressure-induced structural transformations in lanthanide titanates: La2TiO5 and Nd2TiO5

The structure of orthorhombic rare earth titanates of La2TiO5 and Nd2TiO5, where Ti cations are in five-fold coordination with oxygen, has been studied at high pressures by X-ray diffraction (XRD), Raman scattering measurements, and quantum mechanical calculations. Both XRD and Raman results indicated two pressure-induced phase transitions during the process. An orthorhombic super cell (a×b×2c) formed at a pressure between 6 and 10 GPa, and then transformed to a hexagonal high-pressure phase accompanied by partial decomposition. The hexagonal high-pressure phase is quenchable. Detailed structural analysis indicated that the five-coordinated TiO5 polyhedra remain during the formation of super cell, but the orthorhombic-to-hexagonal phase transition at high pressures is a reconstructive process, and the five-fold Ti–O coordination increased to more than 6. This phase transition sequence was verified by quantum mechanical calculations.

Graphical abstractAt high pressures, La2TiO5 and Nd2TiO5 transform from the orthorhombic phase to an a×b×2c superlattice of the orthorhombic structure and then to a hexagonal high-pressure phase.Figure optionsDownload full-size imageDownload as PowerPoint slide