Study of Structural Evolution and Dielectric Properties of Solid Solutions A₂(Sm_1-xRE_x)Ti₂Nb₃O₁₅ (A = Sr, Ba; RE = Nd, Sm, Gd, La)

Tetragonal tungsten bronzes (TTBs) represent an important class of functional ceramics, known for their high dielectric properties and thermal stability. Tailoring the properties of these materials through ionic substitution enables modulation of the crystal structure and, consequently, their electrical performance. In this context, the study of solid solutions A₂(Sm₁₋ₓREₓ)Ti₂Nb₃O₁₅ (A = Sr, Ba; RE = Nd, Sm, Gd, La) provides an ideal framework to examine the influence of rare-earth ionic radii on the TTB structure and its dielectric properties, offering key insights for the development of high-performance ceramics.

The compounds A₂(Sm₁₋ₓREₓ)Ti₂Nb₃O₁₅ (A = Sr, Ba; RE = Nd, Sm, Gd, La) were synthesized via solid-state reaction. The resulting ceramics were characterized using various techniques, including X-ray diffraction (XRD), scanning electron microscopy (SEM), and dielectric measurements. Substitution with different rare-earth elements in the TTB compounds leads to a gradual decrease in lattice parameters as the ionic radius decreases (from La to Gd), accompanied by a space group transition from P4bm for La, Sm, and Nd to P4/mbm for Gd. Dielectric properties show significant variation depending on the rare-earth element, with high dielectric constants and low losses, reflecting the influence of local polarization and structural distortions. SEM observations reveal changes in grain morphology and size consistent with the effects induced by ionic substitution. Overall, these results highlight a direct correlation between the RE ionic radius, lattice distortions, and dielectric properties, confirming the crucial role of substitution chemistry in tuning the properties of tetragonal tungsten bronzes.