Thermal expansion and phase transformation behavior in the rare-earth titanate system

In this work, the thermal expansion behavior in the Ln2TiO5 system was explored, and mechanisms behind such behavior were described. The components of the thermal expansion tensor were calculated from the refined d-spacings using the program CTEAS for the cubic, orthorhombic, and hexagonal phases. In the cubic phase, the thermal expansion behavior was found to increase as expected with an increase in thermal vibrations. The orthorhombic phase exhibited an interesting shift from an expanding c-axis to one which contracted. Subsequent analysis of the crystallographic information of the orthorhombic phase revealed that the mechanism behind this behavior was the result of the strained trigonal bipyramidal structure overcoming an energy barrier to become more ideal and relaxed. The thermal expansion behavior in the hexagonal phase was found to be caused by the movement of the axial oxygen in the trigonal bipyramidal structure towards the central Ti cation with increasing temperature. This information was paired with insights into the volume expansion, structural elements, and geometric units between the orthorhombic and hexagonal phases to describe a potential pathway between two crystallographic cells which have no group-subgroup relationship. The novel pairing of information to describe a reconstructive transformation in this manner is unique and may be a new method to describe such transformations where few tools currently exist today. Additionally, a new experimental technique was developed to study the phase transformation kinetics between the orthorhombic and hexagonal phases in situ. The activation energy of this transformation was found to be 149 kJ/mol. This new technique avoids complications which arise from the study of transformations at high temperatures using thermal analysis methods, and provides increased time resolution of the data improving the calculation of the activation energy