Investigation of ferroelastic phase transformation and thermo-physical properties in pure and substituted rare earth orthoniobates

Abstract

Rare earth orthoniobates (RENbO₄) 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 RENbO₄ 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 RENbO₄ 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 RE³⁺ and the Nb⁵⁺ sites with Zr⁴⁺ 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 RENbO₄ materials, the transformation behavior is primarily dictated by the average size of the RE³⁺ and Nb⁵⁺ sites within the unit cell. In the case of aliovalent doping for the La³⁺ cation in LaNbO₄, the transformation temperature (T_Tr) increased even as the unit cell volume increased due to doping. However, in DyNbO₄ extensive doping of the Dy³⁺ position with Mg²⁺ resulted in no change of T_Tr and inconsequential changes to the thermal expansion behavior. In contrast, co-substitution of RE³⁺ and Nb⁵⁺ with Zr⁴⁺ resulted in unit cell volume reduction with increasing Zr⁴⁺ concentration with notable reduction of T_Tr. Co-substitution of DyNbO₄ and YNbO₄ with Zr⁴⁺ had the additional benefit of stabilizing the high temperature tetragonal phase below T_Tr with significant fractions stabilized to room temperature. Rigorous analysis of crystallographic changes due to Zr⁴⁺ co-substitution were correlated with the lattice parameters from which the preferred Zr⁴⁺ substitution sites were identified. Overall, this study improves our fundamental understanding of the ferroelastic transformation in RENbO₄ 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 RENbO₄ compositions for enhanced proton conductivity and for ferroelastic transformation toughening.