Thermal expansion and phase transformation in the rare earth di-titanate (R₂Ti₂O₇) system

Characterization of the thermal expansion in the rare earth di-titanates is important for their use in high-temperature structural and dielectric applications. Powder samples of the rare earth di-titanates R₂Ti₂O₇ (or R₂O₃.2TiO₂), where R = La, Pr, Nd, Sm, Gd, Dy, Er, Yb, Y, which crystallize in either the monoclinic or cubic phases, were synthesized for the first time by the solution-based steric entrapment method. The three-dimensional thermal expansions of these polycrystalline powder samples were measured by in situ synchrotron powder diffraction from 25^◦C to 1600^◦C in air, nearly 600^◦C higher than other in situ thermal expansion studies. The high temperatures in synchrotron experiments were achieved with a quadrupole lamp furnace. Neutron powder diffraction measured the monoclinic phases from 25^◦C to 1150^◦C. The La₂Ti₂O₇ member of the rare earth di-titanates undergoes a monoclinic to orthorhombic displacive transition on heating, as shown by synchrotron diffraction in air at 885^◦C (864^◦C–904^◦C) and neutron diffraction at 874^◦C (841^◦C–894^◦C).