Rare Earth Niobates, RENbO4: How Does the RE3+ Size Impact Structural Evolution and Thermophysical Behavior?

Abstract

Rare earth niobates (RENbO4) are structurally fascinating utilitarian compounds for high temperature applications. The pursuit of understanding the implications of RE3+‐ionic size on its wide‐ranging properties motivated the synthesis and investigation of RENbO4 (RE: La, Nd, Sm, Gd, Dy, and Y) by diffraction, spectroscopic, luminescence, and thermomechanical studies. Single‐phasic fergusonite (I2/c) structure with lattice parameters proportional to RE3+‐radius, transform to tetragonal scheelite (I41/a) at transition temperatures (Tc), inversely proportional to RE3+‐size. Various crystal chemistry parameters such as monoclinic distortions, landau order parameters (𝜂) and spontaneous strain (εs) decreased with temperature as the symmetrical scheelite structure is approached, but strain parameters behave rather differently. Longitudinal strain decreases but shear strain increases with RE3+‐size, and hence the εs exhibits a maximum. The lattice thermal expansion coefficients (LTE) showed anisotropic behavior along different crystallographic axes and decrease with decrease in RE3+‐size. Continuity in Δl/l versus T, but a change in slope at Tc in thermomechanical studies, indicates the second‐order nature of fergusonite–scheelite phase transition, which is also supported by variation of εs versus 𝜂. Bulk thermal expansion coefficients for monoclinic structures exhibited a decrease from La to Y (14.5 ppm to 10.1 ppm), but a slight increase (9.6 ppm to 10.4 ppm) for the tetragonal. The knowledge of dependence of critical functionalities of RENbO4 on RE‐size provides a key to design functionality‐specific compounds.