Investigation of ferroelastic phase transformation and thermo-physical properties in pure and substituted rare earth orthoniobates
Abstract
Rare earth orthoniobates (RENbO<sub>4</sub>) are an attractive family of materials for many applications including, proton conducting solid oxide fuel cells (PC-SOFCs), shape memory ceramics, large force actuators, and toughening of brittle composites. For these applications, a better understanding and control of the RENbO<sub>4</sub> phase transformation and thermal expansion properties is needed. The ternary nature of these oxides provides ample crystallographic design space to tailor these properties. Aliovalent substitution of the rare-earth cation sites in RENbO<sub>4</sub> materials has been investigated extensively in the last decade, however, the effects on the thermo-physical properties were not well understood. Co-substitution of the RE<sup>3+</sup> and the Nb<sup>5+</sup> sites with Zr<sup>4+</sup> is a novel approach towards controlling the phase transformation behavior in these materials; this has not been previously explored. This research fills the crucial gaps in our understanding of the effect of doping on the thermo-physical properties through investigation of atomic level changes, studied using <I>in situ</I> methods. For RENbO<sub>4</sub> materials, the transformation behavior is primarily dictated by the average size of the RE<sup>3+</sup> and Nb<sup>5+</sup> sites within the unit cell. In the case of aliovalent doping for the La<sup>3+</sup> cation in LaNbO<sub>4</sub>, the transformation temperature (T<sub>Tr</sub>) increased even as the unit cell volume increased due to doping. However, in DyNbO<sub>4</sub> extensive doping of the Dy<sup>3+</sup> position with Mg<sup>2+</sup> resulted in no change of T<sub>Tr</sub> and inconsequential changes to the thermal expansion behavior. In contrast, co-substitution of RE<sup>3+</sup> and Nb<sup>5+</sup> with Zr<sup>4+</sup> resulted in unit cell volume reduction with increasing Zr<sup>4+</sup> concentration with notable reduction of T<sub>Tr</sub>. Co-substitution of DyNbO<sub>4</sub> and YNbO<sub>4</sub> with Zr<sup>4+</sup> had the additional benefit of stabilizing the high temperature tetragonal phase below T<sub>Tr</sub> with significant fractions stabilized to room temperature. Rigorous analysis of crystallographic changes due to Zr<sup>4+</sup> co-substitution were correlated with the lattice parameters from which the preferred Zr<sup>4+</sup> substitution sites were identified. Overall, this study improves our fundamental understanding of the ferroelastic transformation in RENbO<sub>4</sub> materials, and the role of cationic substitution in defining the thermo-physical properties. It provides the necessary framework for future studies on the design and development of novel materials based on RENbO<sub>4</sub> compositions for enhanced proton conductivity and for ferroelastic transformation toughening.
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- OSU Dissertations [11222]