Rare Earth Niobates, RENbO ₄ : How Does the RE ³⁺ Size Impact Structural Evolution and Thermophysical Behavior?

ABSTRACT

Rare earth niobates (RENbO ₄ ) are structurally fascinating utilitarian compounds for high temperature applications. The pursuit of understanding the implications of RE ³⁺ ‐ionic size on its wide‐ranging properties motivated the synthesis and investigation of RENbO ₄ (RE: La, Nd, Sm, Gd, Dy, and Y) by diffraction, spectroscopic, luminescence, and thermomechanical studies. Single‐phasic fergusonite ( I 2/ c ) structure with lattice parameters proportional to RE ³⁺ ‐radius, transform to tetragonal scheelite ( I 4 ₁ / a ) at transition temperatures ( T _c ), inversely proportional to RE ³⁺ ‐size. Various crystal chemistry parameters such as monoclinic distortions, landau order parameters (𝜂) and spontaneous strain ( ε _s ) decreased with temperature as the symmetrical scheelite structure is approached, but strain parameters behave rather differently. Longitudinal strain decreases but shear strain increases with RE ³⁺ ‐size, and hence the ε _s exhibits a maximum. The lattice thermal expansion coefficients (LTE) showed anisotropic behavior along different crystallographic axes and decrease with decrease in RE ³⁺ ‐size. Continuity in Δl/l versus T , but a change in slope at T _c in thermomechanical studies, indicates the second‐order nature of fergusonite–scheelite phase transition, which is also supported by variation of ε _s 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 RENbO ₄ on RE‐size provides a key to design functionality‐specific compounds.